LinuxLists.cc - [PATCH v11 0/3] cachestat: a new syscall for page cache state of files

2023-03-08 03:28:01

Subject: [PATCH v11 0/3] cachestat: a new syscall for page cache state of files

Changelog:
v11:
* Clean up code and comments/documentation.
(patch 1 and 2) (suggested by Matthew Wilcox)
* Drop support for hugetlbfs (patch 2)
(from discussion with Johannes Weiner and Matthew Wilcox).
v10:
* Reorder the arguments for archs with alignment requirements.
(patch 2) (suggested by Arnd Bergmann)
v9:
* Remove syscall from all the architectures syscall table except x86
(patch 2)
* API change: handle different cases for offset and add compat syscall.
(patch 2) (suggested by Johannes Weiner and Arnd Bergmann)
v8:
* Add syscall to mips syscall tables (detected by kernel test robot)
(patch 2)
* Add a missing return (suggested by Yu Zhao) (patch 2)
v7:
* Fix and use lru_gen_test_recent (suggested by Brian Foster)
(patch 2)
* Small formatting and organizational fixes
v6:
* Add a missing fdput() (suggested by Brian Foster) (patch 2)
* Replace cstat_size with cstat_version (suggested by Brian Foster)
(patch 2)
* Add conditional resched to the xas walk. (suggested by Hillf Danton)
(patch 2)
v5:
* Separate first patch into its own series.
(suggested by Andrew Morton)
* Expose filemap_cachestat() to non-syscall usage
(patch 2) (suggested by Brian Foster).
* Fix some build errors from last version.
(patch 2)
* Explain eviction and recent eviction in the draft man page and
documentation (suggested by Andrew Morton).
(patch 2)
v4:
* Refactor cachestat and move it to mm/filemap.c (patch 3)
(suggested by Brian Foster)
* Remove redundant checks (!folio, access_ok)
(patch 3) (suggested by Matthew Wilcox and Al Viro)
* Fix a bug in handling multipages folio.
(patch 3) (suggested by Matthew Wilcox)
* Add a selftest for shmem files, which can be used to test huge
pages (patch 4) (suggested by Johannes Weiner)
v3:
* Fix some minor formatting issues and build errors.
* Add the new syscall entry to missing architecture syscall tables.
(patch 3).
* Add flags argument for the syscall. (patch 3).
* Clean up the recency refactoring (patch 2) (suggested by Yu Zhao)
* Add the new Kconfig (CONFIG_CACHESTAT) to disable the syscall.
(patch 3) (suggested by Josh Triplett)
v2:
* len == 0 means query to EOF. len < 0 is invalid.
(patch 3) (suggested by Brian Foster)
* Make cachestat extensible by adding the `cstat_size` argument in the
syscall (patch 3)

There is currently no good way to query the page cache state of large
file sets and directory trees. There is mincore(), but it scales poorly:
the kernel writes out a lot of bitmap data that userspace has to
aggregate, when the user really doesn not care about per-page information
in that case. The user also needs to mmap and unmap each file as it goes
along, which can be quite slow as well.

This series of patches introduces a new system call, cachestat, that
summarizes the page cache statistics (number of cached pages, dirty
pages, pages marked for writeback, evicted pages etc.) of a file, in a
specified range of bytes. It also include a selftest suite that tests some
typical usage. Currently, the syscall is only wired in for x86
architecture.

This interface is inspired by past discussion and concerns with fincore,
which has a similar design (and as a result, issues) as mincore.
Relevant links:

https://lkml.indiana.edu/hypermail/linux/kernel/1302.1/04207.html
https://lkml.indiana.edu/hypermail/linux/kernel/1302.1/04209.html

For comparison with mincore, I ran both syscalls on a 2TB sparse file:

Using mincore:
real 0m37.510s
user 0m2.934s
sys 0m34.558s

Using cachestat:
real 0m0.009s
user 0m0.000s
sys 0m0.009s

This series should be applied on top of:

workingset: fix confusion around eviction vs refault container
https://lkml.org/lkml/2023/1/4/1066

This series consist of 3 patches:

Nhat Pham (3):
workingset: refactor LRU refault to expose refault recency check
cachestat: implement cachestat syscall
selftests: Add selftests for cachestat

MAINTAINERS | 7 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
include/linux/compat.h | 5 +-
include/linux/swap.h | 1 +
include/linux/syscalls.h | 3 +
include/uapi/asm-generic/unistd.h | 5 +-
include/uapi/linux/mman.h | 9 +
init/Kconfig | 10 +
kernel/sys_ni.c | 1 +
mm/filemap.c | 166 +++++++++++
mm/workingset.c | 145 ++++++----
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/cachestat/.gitignore | 2 +
tools/testing/selftests/cachestat/Makefile | 8 +
.../selftests/cachestat/test_cachestat.c | 257 ++++++++++++++++++
16 files changed, 574 insertions(+), 48 deletions(-)
create mode 100644 tools/testing/selftests/cachestat/.gitignore
create mode 100644 tools/testing/selftests/cachestat/Makefile
create mode 100644 tools/testing/selftests/cachestat/test_cachestat.c

base-commit: 1440f576022887004f719883acb094e7e0dd4944
prerequisite-patch-id: 171a43d333e1b267ce14188a5beaea2f313787fb
--
2.39.2

2023-03-08 03:28:05

by Nhat Pham

[permalink] [raw]

Subject: [PATCH v11 1/3] workingset: refactor LRU refault to expose refault recency check

In preparation for computing recently evicted pages in cachestat,
refactor workingset_refault and lru_gen_refault to expose a helper
function that would test if an evicted page is recently evicted.

Signed-off-by: Nhat Pham <[email protected]>
---
include/linux/swap.h | 1 +
mm/workingset.c | 145 +++++++++++++++++++++++++++++--------------
2 files changed, 100 insertions(+), 46 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index a18cf4b7c724..dae6f6f955eb 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -361,6 +361,7 @@ static inline void folio_set_swap_entry(struct folio *folio, swp_entry_t entry)
}

/* linux/mm/workingset.c */
+bool workingset_test_recent(void *shadow, bool file, bool *workingset);
void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg);
void workingset_refault(struct folio *folio, void *shadow);
diff --git a/mm/workingset.c b/mm/workingset.c
index 79585d55c45d..a60729c1b766 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -244,6 +244,29 @@ static void *lru_gen_eviction(struct folio *folio)
return pack_shadow(mem_cgroup_id(memcg), pgdat, token, refs);
}

+/*
+ * Tests if the shadow entry is for a folio that was recently evicted.
+ * Fills in @memcgid, @pglist_data, @token, @workingset with the values
+ * unpacked from shadow.
+ */
+static bool lru_gen_test_recent(void *shadow, bool file, int *memcgid,
+ struct pglist_data **pgdat, unsigned long *token, bool *workingset)
+{
+ struct mem_cgroup *eviction_memcg;
+ struct lruvec *lruvec;
+ struct lru_gen_struct *lrugen;
+ unsigned long min_seq;
+
+ unpack_shadow(shadow, memcgid, pgdat, token, workingset);
+ eviction_memcg = mem_cgroup_from_id(*memcgid);
+
+ lruvec = mem_cgroup_lruvec(eviction_memcg, *pgdat);
+ lrugen = &lruvec->lrugen;
+
+ min_seq = READ_ONCE(lrugen->min_seq[file]);
+ return (*token >> LRU_REFS_WIDTH) == (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH));
+}
+
static void lru_gen_refault(struct folio *folio, void *shadow)
{
int hist, tier, refs;
@@ -258,23 +281,22 @@ static void lru_gen_refault(struct folio *folio, void *shadow)
int type = folio_is_file_lru(folio);
int delta = folio_nr_pages(folio);

- unpack_shadow(shadow, &memcg_id, &pgdat, &token, &workingset);
-
- if (pgdat != folio_pgdat(folio))
- return;
-
rcu_read_lock();

+ if (!lru_gen_test_recent(shadow, type, &memcg_id, &pgdat, &token,
+ &workingset))
+ goto unlock;
+
memcg = folio_memcg_rcu(folio);
if (memcg_id != mem_cgroup_id(memcg))
goto unlock;

+ if (pgdat != folio_pgdat(folio))
+ return;
+
lruvec = mem_cgroup_lruvec(memcg, pgdat);
lrugen = &lruvec->lrugen;
-
min_seq = READ_ONCE(lrugen->min_seq[type]);
- if ((token >> LRU_REFS_WIDTH) != (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH)))
- goto unlock;

hist = lru_hist_from_seq(min_seq);
/* see the comment in folio_lru_refs() */
@@ -306,6 +328,12 @@ static void *lru_gen_eviction(struct folio *folio)
return NULL;
}

+static bool lru_gen_test_recent(void *shadow, bool file, int *memcgid,
+ struct pglist_data **pgdat, unsigned long *token, bool *workingset)
+{
+ return false;
+}
+
static void lru_gen_refault(struct folio *folio, void *shadow)
{
}
@@ -374,39 +402,34 @@ void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg)
}

/**
- * workingset_refault - Evaluate the refault of a previously evicted folio.
- * @folio: The freshly allocated replacement folio.
- * @shadow: Shadow entry of the evicted folio.
- *
- * Calculates and evaluates the refault distance of the previously
- * evicted folio in the context of the node and the memcg whose memory
- * pressure caused the eviction.
+ * workingset_test_recent - tests if the shadow entry is for a folio that was
+ * recently evicted. Also fills in @workingset with the value unpacked from
+ * shadow.
+ * @shadow: the shadow entry to be tested.
+ * @file: whether the corresponding folio is from the file lru.
+ * @workingset: where the workingset value unpacked from shadow should
+ * be stored.
+ *
+ * Return: true if the shadow is for a recently evicted folio; false otherwise.
*/
-void workingset_refault(struct folio *folio, void *shadow)
+bool workingset_test_recent(void *shadow, bool file, bool *workingset)
{
- bool file = folio_is_file_lru(folio);
struct mem_cgroup *eviction_memcg;
struct lruvec *eviction_lruvec;
unsigned long refault_distance;
unsigned long workingset_size;
- struct pglist_data *pgdat;
- struct mem_cgroup *memcg;
- unsigned long eviction;
- struct lruvec *lruvec;
unsigned long refault;
- bool workingset;
int memcgid;
- long nr;
+ struct pglist_data *pgdat;
+ unsigned long eviction;

- if (lru_gen_enabled()) {
- lru_gen_refault(folio, shadow);
- return;
- }
+ if (lru_gen_enabled())
+ return lru_gen_test_recent(shadow, file, &memcgid, &pgdat, &eviction,
+ workingset);

- unpack_shadow(shadow, &memcgid, &pgdat, &eviction, &workingset);
+ unpack_shadow(shadow, &memcgid, &pgdat, &eviction, workingset);
eviction <<= bucket_order;

- rcu_read_lock();
/*
* Look up the memcg associated with the stored ID. It might
* have been deleted since the folio's eviction.
@@ -425,7 +448,8 @@ void workingset_refault(struct folio *folio, void *shadow)
*/
eviction_memcg = mem_cgroup_from_id(memcgid);
if (!mem_cgroup_disabled() && !eviction_memcg)
- goto out;
+ return false;
+
eviction_lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
refault = atomic_long_read(&eviction_lruvec->nonresident_age);

@@ -447,21 +471,6 @@ void workingset_refault(struct folio *folio, void *shadow)
*/
refault_distance = (refault - eviction) & EVICTION_MASK;

- /*
- * The activation decision for this folio is made at the level
- * where the eviction occurred, as that is where the LRU order
- * during folio reclaim is being determined.
- *
- * However, the cgroup that will own the folio is the one that
- * is actually experiencing the refault event.
- */
- nr = folio_nr_pages(folio);
- memcg = folio_memcg(folio);
- pgdat = folio_pgdat(folio);
- lruvec = mem_cgroup_lruvec(memcg, pgdat);
-
- mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
-
mem_cgroup_flush_stats_delayed();
/*
* Compare the distance to the existing workingset size. We
@@ -483,7 +492,51 @@ void workingset_refault(struct folio *folio, void *shadow)
NR_INACTIVE_ANON);
}
}
- if (refault_distance > workingset_size)
+
+ return refault_distance <= workingset_size;
+}
+
+/**
+ * workingset_refault - Evaluate the refault of a previously evicted folio.
+ * @folio: The freshly allocated replacement folio.
+ * @shadow: Shadow entry of the evicted folio.
+ *
+ * Calculates and evaluates the refault distance of the previously
+ * evicted folio in the context of the node and the memcg whose memory
+ * pressure caused the eviction.
+ */
+void workingset_refault(struct folio *folio, void *shadow)
+{
+ bool file = folio_is_file_lru(folio);
+ struct pglist_data *pgdat;
+ struct mem_cgroup *memcg;
+ struct lruvec *lruvec;
+ bool workingset;
+ long nr;
+
+ if (lru_gen_enabled()) {
+ lru_gen_refault(folio, shadow);
+ return;
+ }
+
+ rcu_read_lock();
+
+ /*
+ * The activation decision for this folio is made at the level
+ * where the eviction occurred, as that is where the LRU order
+ * during folio reclaim is being determined.
+ *
+ * However, the cgroup that will own the folio is the one that
+ * is actually experiencing the refault event.
+ */
+ nr = folio_nr_pages(folio);
+ memcg = folio_memcg(folio);
+ pgdat = folio_pgdat(folio);
+ lruvec = mem_cgroup_lruvec(memcg, pgdat);
+
+ mod_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file, nr);
+
+ if (!workingset_test_recent(shadow, file, &workingset))
goto out;

folio_set_active(folio);
--
2.39.2

2023-03-08 03:28:13

by Nhat Pham

[permalink] [raw]

Subject: [PATCH v11 2/3] cachestat: implement cachestat syscall

There is currently no good way to query the page cache state of large
file sets and directory trees. There is mincore(), but it scales poorly:
the kernel writes out a lot of bitmap data that userspace has to
aggregate, when the user really doesn not care about per-page
information in that case. The user also needs to mmap and unmap each
file as it goes along, which can be quite slow as well.

This patch implements a new syscall that queries cache state of a file
and summarizes the number of cached pages, number of dirty pages, number
of pages marked for writeback, number of (recently) evicted pages, etc.
in a given range. Currently, the syscall is only wired in for x86
architecture.

NAME
cachestat - query the page cache statistics of a file.

SYNOPSIS
#include <sys/mman.h>

struct cachestat {
__u64 nr_cache;
__u64 nr_dirty;
__u64 nr_writeback;
__u64 nr_evicted;
__u64 nr_recently_evicted;
};

int cachestat(unsigned int fd, size_t len, loff_t off,
struct cachestat *cstat, unsigned int flags);

DESCRIPTION
cachestat() queries the number of cached pages, number of dirty
pages, number of pages marked for writeback, number of evicted
pages, number of recently evicted pages, in the bytes range given by
`off` and `len`.

An evicted page is a page that is previously in the page cache but
has been evicted since. A page is recently evicted if its last
eviction was recent enough that its reentry to the cache would
indicate that it is actively being used by the system, and that
there is memory pressure on the system.

These values are returned in a cachestat struct, whose address is
given by the `cstat` argument.

The `off` and `len` arguments must be non-negative integers. If
`len` > 0, the queried range is [`off`, `off` + `len`]. If `len` ==
0, we will query in the range from `off` to the end of the file.

The `flags` argument is unused for now, but is included for future
extensibility. User should pass 0 (i.e no flag specified).

Currently, hugetlbfs is not supported.

Because the status of a page can change after cachestat() checks it
but before it returns to the application, the returned values may
contain stale information.

RETURN VALUE
On success, cachestat returns 0. On error, -1 is returned, and errno
is set to indicate the error.

ERRORS
EFAULT cstat points to an invalid address.

EINVAL invalid offset or flags.

EBADF invalid file descriptor.

EOPNOTSUPP file descriptor is of a hugetlbfs file

Signed-off-by: Nhat Pham <[email protected]>
---
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
include/linux/compat.h | 5 +-
include/linux/syscalls.h | 3 +
include/uapi/asm-generic/unistd.h | 5 +-
include/uapi/linux/mman.h | 9 ++
init/Kconfig | 10 ++
kernel/sys_ni.c | 1 +
mm/filemap.c | 166 +++++++++++++++++++++++++
9 files changed, 199 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 320480a8db4f..01d1cc009250 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -455,3 +455,4 @@
448 i386 process_mrelease sys_process_mrelease
449 i386 futex_waitv sys_futex_waitv
450 i386 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 i386 cachestat sys_cachestat compat_sys_cachestat
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index c84d12608cd2..227538b0ce80 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -372,6 +372,7 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat

#
# Due to a historical design error, certain syscalls are numbered differently
diff --git a/include/linux/compat.h b/include/linux/compat.h
index 594357881b0b..c0f4628c2b4f 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -816,7 +816,10 @@ asmlinkage long compat_sys_sendmsg(int fd, struct compat_msghdr __user *msg,
asmlinkage long compat_sys_recvmsg(int fd, struct compat_msghdr __user *msg,
unsigned int flags);

-/* mm/filemap.c: No generic prototype for readahead */
+/* mm/filemap.c */
+asmlinkage long compat_sys_cachestat(unsigned int fd, size_t len,
+ compat_arg_u64(off), struct cachestat __user *cstat,
+ unsigned int flags);

/* security/keys/keyctl.c */
asmlinkage long compat_sys_keyctl(u32 option,
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index a34b0f9a9972..9f384a6d7e77 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -72,6 +72,7 @@ struct open_how;
struct mount_attr;
struct landlock_ruleset_attr;
enum landlock_rule_type;
+struct cachestat;

#include <linux/types.h>
#include <linux/aio_abi.h>
@@ -1056,6 +1057,8 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
unsigned long home_node,
unsigned long flags);
+asmlinkage long sys_cachestat(unsigned int fd, size_t len, loff_t offset,
+ struct cachestat __user *cstat, unsigned int flags);

/*
* Architecture-specific system calls
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 45fa180cc56a..dc5fcfee7f09 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
#define __NR_set_mempolicy_home_node 450
__SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)

+#define __NR_cachestat 451
+__SC_COMP(__NR_cachestat, sys_cachestat, compat_sys_cachestat)
+
#undef __NR_syscalls
-#define __NR_syscalls 451
+#define __NR_syscalls 452

/*
* 32 bit systems traditionally used different
diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
index f55bc680b5b0..fe03ed0b7587 100644
--- a/include/uapi/linux/mman.h
+++ b/include/uapi/linux/mman.h
@@ -4,6 +4,7 @@

#include <asm/mman.h>
#include <asm-generic/hugetlb_encode.h>
+#include <linux/types.h>

#define MREMAP_MAYMOVE 1
#define MREMAP_FIXED 2
@@ -41,4 +42,12 @@
#define MAP_HUGE_2GB HUGETLB_FLAG_ENCODE_2GB
#define MAP_HUGE_16GB HUGETLB_FLAG_ENCODE_16GB

+struct cachestat {
+ __u64 nr_cache;
+ __u64 nr_dirty;
+ __u64 nr_writeback;
+ __u64 nr_evicted;
+ __u64 nr_recently_evicted;
+};
+
#endif /* _UAPI_LINUX_MMAN_H */
diff --git a/init/Kconfig b/init/Kconfig
index 694f7c160c9c..da96ac29af1d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1798,6 +1798,16 @@ config RSEQ

If unsure, say Y.

+config CACHESTAT_SYSCALL
+ bool "Enable cachestat() system call" if EXPERT
+ default y
+ help
+ Enable the cachestat system call, which queries the page cache
+ statistics of a file (number of cached pages, dirty pages,
+ pages marked for writeback, (recently) evicted pages).
+
+ If unsure say Y here.
+
config DEBUG_RSEQ
default n
bool "Enabled debugging of rseq() system call" if EXPERT
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 860b2dcf3ac4..04bfb1e4d377 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -299,6 +299,7 @@ COND_SYSCALL(set_mempolicy);
COND_SYSCALL(migrate_pages);
COND_SYSCALL(move_pages);
COND_SYSCALL(set_mempolicy_home_node);
+COND_SYSCALL(cachestat);

COND_SYSCALL(perf_event_open);
COND_SYSCALL(accept4);
diff --git a/mm/filemap.c b/mm/filemap.c
index 08341616ae7a..768b95ea551c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -22,6 +22,7 @@
#include <linux/mm.h>
#include <linux/swap.h>
#include <linux/swapops.h>
+#include <linux/syscalls.h>
#include <linux/mman.h>
#include <linux/pagemap.h>
#include <linux/file.h>
@@ -55,6 +56,9 @@
#include <linux/buffer_head.h> /* for try_to_free_buffers */

#include <asm/mman.h>
+#include <linux/mman.h>
+
+#include "swap.h"

/*
* Shared mappings implemented 30.11.1994. It's not fully working yet,
@@ -3949,3 +3953,165 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
return try_to_free_buffers(folio);
}
EXPORT_SYMBOL(filemap_release_folio);
+
+/**
+ * filemap_cachestat() - compute the page cache statistics of a mapping
+ * @mapping: The mapping to compute the statistics for.
+ * @first_index: The starting page cache index.
+ * @last_index: The final page index (inclusive).
+ * @cs: the cachestat struct to write the result to.
+ *
+ * This will query the page cache statistics of a mapping in the
+ * page range of [first_index, last_index] (inclusive). The statistics
+ * queried include: number of dirty pages, number of pages marked for
+ * writeback, and the number of (recently) evicted pages.
+ */
+static void filemap_cachestat(struct address_space *mapping, pgoff_t first_index,
+ pgoff_t last_index, struct cachestat *cs)
+{
+ XA_STATE(xas, &mapping->i_pages, first_index);
+ struct folio *folio;
+
+ rcu_read_lock();
+ xas_for_each(&xas, folio, last_index) {
+ unsigned long nr_pages;
+ pgoff_t folio_first_index, folio_last_index;
+
+ if (xas_retry(&xas, folio))
+ continue;
+
+ nr_pages = folio_nr_pages(folio);
+ folio_first_index = folio_pgoff(folio);
+ folio_last_index = folio_first_index + nr_pages - 1;
+
+ /* Folios might straddle the range boundaries, only count covered pages */
+ if (folio_first_index < first_index)
+ nr_pages -= first_index - folio_first_index;
+
+ if (folio_last_index > last_index)
+ nr_pages -= folio_last_index - last_index;
+
+ if (xa_is_value(folio)) {
+ /* page is evicted */
+ void *shadow = (void *)folio;
+ bool workingset; /* not used */
+
+ cs->nr_evicted += nr_pages;
+
+#ifdef CONFIG_SWAP /* implies CONFIG_MMU */
+ if (shmem_mapping(mapping)) {
+ /* shmem file - in swap cache */
+ swp_entry_t swp = radix_to_swp_entry(folio);
+
+ shadow = get_shadow_from_swap_cache(swp);
+ }
+#endif
+ if (workingset_test_recent(shadow, true, &workingset))
+ cs->nr_recently_evicted += nr_pages;
+
+ goto resched;
+ }
+
+ /* page is in cache */
+ cs->nr_cache += nr_pages;
+
+ if (folio_test_dirty(folio))
+ cs->nr_dirty += nr_pages;
+
+ if (folio_test_writeback(folio))
+ cs->nr_writeback += nr_pages;
+
+resched:
+ if (need_resched()) {
+ xas_pause(&xas);
+ cond_resched_rcu();
+ }
+ }
+ rcu_read_unlock();
+}
+
+#ifdef CONFIG_CACHESTAT_SYSCALL
+long ksys_cachestat(unsigned int fd, size_t len, loff_t off,
+ struct cachestat __user *cstat, unsigned int flags)
+{
+ struct fd f = fdget(fd);
+ struct address_space *mapping;
+ struct cachestat cs;
+ pgoff_t first_index = off >> PAGE_SHIFT;
+ pgoff_t last_index =
+ len == 0 ? ULONG_MAX : (off + len - 1) >> PAGE_SHIFT;
+
+ if (!f.file)
+ return -EBADF;
+
+ /* hugetlbfs is not supported */
+ if (is_file_hugepages(f.file)) {
+ fdput(f);
+ return -EOPNOTSUPP;
+ }
+
+ if (off < 0 || flags != 0) {
+ fdput(f);
+ return -EINVAL;
+ }
+
+ memset(&cs, 0, sizeof(struct cachestat));
+ mapping = f.file->f_mapping;
+ filemap_cachestat(mapping, first_index, last_index, &cs);
+ fdput(f);
+
+ if (copy_to_user(cstat, &cs, sizeof(struct cachestat)))
+ return -EFAULT;
+
+ return 0;
+}
+
+/*
+ * The cachestat(5) system call.
+ *
+ * cachestat() returns the page cache statistics of a file in the
+ * bytes range specified by `off` and `len`: number of cached pages,
+ * number of dirty pages, number of pages marked for writeback,
+ * number of evicted pages, and number of recently evicted pages.
+ *
+ * An evicted page is a page that is previously in the page cache
+ * but has been evicted since. A page is recently evicted if its last
+ * eviction was recent enough that its reentry to the cache would
+ * indicate that it is actively being used by the system, and that
+ * there is memory pressure on the system.
+ *
+ * `off` and `len` must be non-negative integers. If `len` > 0,
+ * the queried range is [`off`, `off` + `len`]. If `len` == 0,
+ * we will query in the range from `off` to the end of the file.
+ *
+ * The `flags` argument is unused for now, but is included for future
+ * extensibility. User should pass 0 (i.e no flag specified).
+ *
+ * Currently, hugetlbfs is not supported.
+ *
+ * Because the status of a page can change after cachestat() checks it
+ * but before it returns to the application, the returned values may
+ * contain stale information.
+ *
+ * return values:
+ * zero - success
+ * -EFAULT - cstat points to an illegal address
+ * -EINVAL - invalid arguments
+ * -EBADF - invalid file descriptor
+ * -EOPNOTSUPP - file descriptor is of a hugetlbfs file
+ */
+SYSCALL_DEFINE5(cachestat, unsigned int, fd, size_t, len, loff_t, off,
+ struct cachestat __user *, cstat, unsigned int, flags)
+{
+ return ksys_cachestat(fd, len, off, cstat, flags);
+}
+
+#ifdef CONFIG_COMPAT
+COMPAT_SYSCALL_DEFINE6(cachestat, unsigned int, fd, size_t, len,
+ compat_arg_u64_dual(off), struct cachestat __user *, cstat,
+ unsigned int, flags)
+{
+ return ksys_cachestat(fd, len, compat_arg_u64_glue(off), cstat, flags);
+}
+#endif
+#endif /* CONFIG_CACHESTAT_SYSCALL */
--
2.39.2

2023-03-08 03:28:17

by Nhat Pham

[permalink] [raw]

Subject: [PATCH v11 3/3] selftests: Add selftests for cachestat

Test cachestat on a newly created file, /dev/ files, and /proc/ files.
Also test on a shmem file (which can also be tested with huge pages
since tmpfs supports huge pages).

Signed-off-by: Nhat Pham <[email protected]>
---
MAINTAINERS | 7 +
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/cachestat/.gitignore | 2 +
tools/testing/selftests/cachestat/Makefile | 8 +
.../selftests/cachestat/test_cachestat.c | 257 ++++++++++++++++++
5 files changed, 275 insertions(+)
create mode 100644 tools/testing/selftests/cachestat/.gitignore
create mode 100644 tools/testing/selftests/cachestat/Makefile
create mode 100644 tools/testing/selftests/cachestat/test_cachestat.c

diff --git a/MAINTAINERS b/MAINTAINERS
index a198da986146..792a866353ec 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4552,6 +4552,13 @@ S: Supported
F: Documentation/filesystems/caching/cachefiles.rst
F: fs/cachefiles/

+CACHESTAT: PAGE CACHE STATS FOR A FILE
+M: Nhat Pham <[email protected]>
+M: Johannes Weiner <[email protected]>
+L: [email protected]
+S: Maintained
+F: tools/testing/selftests/cachestat/test_cachestat.c
+
CADENCE MIPI-CSI2 BRIDGES
M: Maxime Ripard <[email protected]>
L: [email protected]
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 0464b2c6c1e4..3cad0b38c5c2 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -4,6 +4,7 @@ TARGETS += amd-pstate
TARGETS += arm64
TARGETS += bpf
TARGETS += breakpoints
+TARGETS += cachestat
TARGETS += capabilities
TARGETS += cgroup
TARGETS += clone3
diff --git a/tools/testing/selftests/cachestat/.gitignore b/tools/testing/selftests/cachestat/.gitignore
new file mode 100644
index 000000000000..d6c30b43a4bb
--- /dev/null
+++ b/tools/testing/selftests/cachestat/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+test_cachestat
diff --git a/tools/testing/selftests/cachestat/Makefile b/tools/testing/selftests/cachestat/Makefile
new file mode 100644
index 000000000000..fca73aaa7d14
--- /dev/null
+++ b/tools/testing/selftests/cachestat/Makefile
@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: GPL-2.0
+TEST_GEN_PROGS := test_cachestat
+
+CFLAGS += $(KHDR_INCLUDES)
+CFLAGS += -Wall
+CFLAGS += -lrt
+
+include ../lib.mk
diff --git a/tools/testing/selftests/cachestat/test_cachestat.c b/tools/testing/selftests/cachestat/test_cachestat.c
new file mode 100644
index 000000000000..f62ffb30f8ca
--- /dev/null
+++ b/tools/testing/selftests/cachestat/test_cachestat.c
@@ -0,0 +1,257 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+
+#include <stdio.h>
+#include <stdbool.h>
+#include <linux/kernel.h>
+#include <linux/mman.h>
+#include <sys/mman.h>
+#include <sys/shm.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+#include <string.h>
+#include <fcntl.h>
+#include <errno.h>
+
+#include "../kselftest.h"
+
+static const char * const dev_files[] = {
+ "/dev/zero", "/dev/null", "/dev/urandom",
+ "/proc/version", "/proc"
+};
+static const int cachestat_nr = 451;
+
+void print_cachestat(struct cachestat *cs)
+{
+ ksft_print_msg(
+ "Using cachestat: Cached: %lu, Dirty: %lu, Writeback: %lu, Evicted: %lu, Recently Evicted: %lu\n",
+ cs->nr_cache, cs->nr_dirty, cs->nr_writeback,
+ cs->nr_evicted, cs->nr_recently_evicted);
+}
+
+bool write_exactly(int fd, size_t filesize)
+{
+ char data[filesize];
+ bool ret = true;
+ int random_fd = open("/dev/urandom", O_RDONLY);
+
+ if (random_fd < 0) {
+ ksft_print_msg("Unable to access urandom.\n");
+ ret = false;
+ goto out;
+ } else {
+ int remained = filesize;
+ char *cursor = data;
+
+ while (remained) {
+ ssize_t read_len = read(random_fd, cursor, remained);
+
+ if (read_len <= 0) {
+ ksft_print_msg("Unable to read from urandom.\n");
+ ret = false;
+ goto close_random_fd;
+ }
+
+ remained -= read_len;
+ cursor += read_len;
+ }
+
+ /* write random data to fd */
+ remained = filesize;
+ cursor = data;
+ while (remained) {
+ ssize_t write_len = write(fd, cursor, remained);
+
+ if (write_len <= 0) {
+ ksft_print_msg("Unable write random data to file.\n");
+ ret = false;
+ goto close_random_fd;
+ }
+
+ remained -= write_len;
+ cursor += write_len;
+ }
+ }
+
+close_random_fd:
+ close(random_fd);
+out:
+ return ret;
+}
+
+/*
+ * Open/create the file at filename, (optionally) write random data to it
+ * (exactly num_pages), then test the cachestat syscall on this file.
+ *
+ * If test_fsync == true, fsync the file, then check the number of dirty
+ * pages.
+ */
+bool test_cachestat(const char *filename, bool write_random, bool create,
+ bool test_fsync, unsigned long num_pages, int open_flags,
+ mode_t open_mode)
+{
+ size_t PS = sysconf(_SC_PAGESIZE);
+ int filesize = num_pages * PS;
+ bool ret = true;
+ long syscall_ret;
+ struct cachestat cs;
+
+ int fd = open(filename, open_flags, open_mode);
+
+ if (fd == -1) {
+ ksft_print_msg("Unable to create/open file.\n");
+ ret = false;
+ goto out;
+ } else {
+ ksft_print_msg("Create/open %s\n", filename);
+ }
+
+ if (write_random) {
+ if (!write_exactly(fd, filesize)) {
+ ksft_print_msg("Unable to access urandom.\n");
+ ret = false;
+ goto out1;
+ }
+ }
+
+ syscall_ret = syscall(cachestat_nr, fd, filesize, 0, &cs, 0);
+
+ ksft_print_msg("Cachestat call returned %ld\n", syscall_ret);
+
+ if (syscall_ret) {
+ ksft_print_msg("Cachestat returned non-zero.\n");
+ ret = false;
+ goto out1;
+
+ } else {
+ print_cachestat(&cs);
+
+ if (write_random) {
+ if (cs.nr_cache + cs.nr_evicted != num_pages) {
+ ksft_print_msg(
+ "Total number of cached and evicted pages is off.\n");
+ ret = false;
+ }
+ }
+ }
+
+ if (test_fsync) {
+ if (fsync(fd)) {
+ ksft_print_msg("fsync fails.\n");
+ ret = false;
+ } else {
+ syscall_ret = syscall(cachestat_nr, fd, filesize, 0, &cs, 0);
+
+ ksft_print_msg("Cachestat call (after fsync) returned %ld\n",
+ syscall_ret);
+
+ if (!syscall_ret) {
+ print_cachestat(&cs);
+
+ if (cs.nr_dirty) {
+ ret = false;
+ ksft_print_msg(
+ "Number of dirty should be zero after fsync.\n");
+ }
+ } else {
+ ksft_print_msg("Cachestat (after fsync) returned non-zero.\n");
+ ret = false;
+ goto out1;
+ }
+ }
+ }
+
+out1:
+ close(fd);
+
+ if (create)
+ remove(filename);
+out:
+ return ret;
+}
+
+bool test_cachestat_shmem(void)
+{
+ size_t PS = sysconf(_SC_PAGESIZE);
+ size_t filesize = PS * 512 * 2; /* 2 2MB huge pages */
+ int syscall_ret;
+ off_t off = PS;
+ size_t compute_len = PS * 512;
+ char *filename = "tmpshmcstat";
+ struct cachestat cs;
+ bool ret = true;
+ unsigned long num_pages = compute_len / PS;
+ int fd = shm_open(filename, O_CREAT | O_RDWR, 0600);
+
+ if (fd < 0) {
+ ksft_print_msg("Unable to create shmem file.\n");
+ ret = false;
+ goto out;
+ }
+
+ if (ftruncate(fd, filesize)) {
+ ksft_print_msg("Unable to trucate shmem file.\n");
+ ret = false;
+ goto close_fd;
+ }
+
+ if (!write_exactly(fd, filesize)) {
+ ksft_print_msg("Unable to write to shmem file.\n");
+ ret = false;
+ goto close_fd;
+ }
+
+ syscall_ret = syscall(cachestat_nr, fd, compute_len, off, &cs, 0);
+
+ if (syscall_ret) {
+ ksft_print_msg("Cachestat returned non-zero.\n");
+ ret = false;
+ goto close_fd;
+ } else {
+ print_cachestat(&cs);
+ if (cs.nr_cache + cs.nr_evicted != num_pages) {
+ ksft_print_msg(
+ "Total number of cached and evicted pages is off.\n");
+ ret = false;
+ }
+ }
+
+close_fd:
+ shm_unlink(filename);
+out:
+ return ret;
+}
+
+int main(void)
+{
+ int ret = 0;
+
+ for (int i = 0; i < 5; i++) {
+ const char *dev_filename = dev_files[i];
+
+ if (test_cachestat(dev_filename, false, false, false,
+ 4, O_RDONLY, 0400))
+ ksft_test_result_pass("cachestat works with %s\n", dev_filename);
+ else {
+ ksft_test_result_fail("cachestat fails with %s\n", dev_filename);
+ ret = 1;
+ }
+ }
+
+ if (test_cachestat("tmpfilecachestat", true, true,
+ true, 4, O_CREAT | O_RDWR, 0400 | 0600))
+ ksft_test_result_pass("cachestat works with a normal file\n");
+ else {
+ ksft_test_result_fail("cachestat fails with normal file\n");
+ ret = 1;
+ }
+
+ if (test_cachestat_shmem())
+ ksft_test_result_pass("cachestat works with a shmem file\n");
+ else {
+ ksft_test_result_fail("cachestat fails with a shmem file\n");
+ ret = 1;
+ }
+
+ return ret;
+}
--
2.39.2

2023-03-09 14:11:38

by kernel test robot

[permalink] [raw]

Subject: Re: [PATCH v11 2/3] cachestat: implement cachestat syscall

Hi Nhat,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on 1440f576022887004f719883acb094e7e0dd4944]

url: https://github.com/intel-lab-lkp/linux/commits/Nhat-Pham/workingset-refactor-LRU-refault-to-expose-refault-recency-check/20230308-112932
base: 1440f576022887004f719883acb094e7e0dd4944
patch link: https://lore.kernel.org/r/20230308032748.609510-3-nphamcs%40gmail.com
patch subject: [PATCH v11 2/3] cachestat: implement cachestat syscall
config: hexagon-randconfig-r041-20230308 (https://download.01.org/0day-ci/archive/20230309/[email protected]/config)
compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project 67409911353323ca5edf2049ef0df54132fa1ca7)
reproduce (this is a W=1 build):
wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
chmod +x ~/bin/make.cross
# https://github.com/intel-lab-lkp/linux/commit/ca34a4daf11f6fbb9731205af7acd06fe620e447
git remote add linux-review https://github.com/intel-lab-lkp/linux
git fetch --no-tags linux-review Nhat-Pham/workingset-refactor-LRU-refault-to-expose-refault-recency-check/20230308-112932
git checkout ca34a4daf11f6fbb9731205af7acd06fe620e447
# save the config file
mkdir build_dir && cp config build_dir/.config
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=hexagon olddefconfig
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=hexagon prepare

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <[email protected]>
| Link: https://lore.kernel.org/oe-kbuild-all/[email protected]/

All warnings (new ones prefixed by >>):

In file included from arch/hexagon/kernel/asm-offsets.c:12:
>> include/linux/compat.h:821:31: warning: declaration of 'struct cachestat' will not be visible outside of this function [-Wvisibility]
compat_arg_u64(off), struct cachestat __user *cstat,
^
In file included from arch/hexagon/kernel/asm-offsets.c:15:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:547:31: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __raw_readb(PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:560:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le16_to_cpu((__le16 __force)__raw_readw(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:37:51: note: expanded from macro '__le16_to_cpu'
#define __le16_to_cpu(x) ((__force __u16)(__le16)(x))
^
In file included from arch/hexagon/kernel/asm-offsets.c:15:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:573:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le32_to_cpu((__le32 __force)__raw_readl(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:35:51: note: expanded from macro '__le32_to_cpu'
#define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
^
In file included from arch/hexagon/kernel/asm-offsets.c:15:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:584:33: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writeb(value, PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:594:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writew((u16 __force)cpu_to_le16(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:604:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writel((u32 __force)cpu_to_le32(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
7 warnings generated.
--
In file included from mm/filemap.c:20:
In file included from include/linux/kernel_stat.h:9:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:547:31: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __raw_readb(PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:560:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le16_to_cpu((__le16 __force)__raw_readw(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:37:51: note: expanded from macro '__le16_to_cpu'
#define __le16_to_cpu(x) ((__force __u16)(__le16)(x))
^
In file included from mm/filemap.c:20:
In file included from include/linux/kernel_stat.h:9:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:573:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le32_to_cpu((__le32 __force)__raw_readl(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:35:51: note: expanded from macro '__le32_to_cpu'
#define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
^
In file included from mm/filemap.c:20:
In file included from include/linux/kernel_stat.h:9:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:584:33: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writeb(value, PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:594:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writew((u16 __force)cpu_to_le16(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:604:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writel((u32 __force)cpu_to_le32(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
>> mm/filemap.c:4034:6: warning: no previous prototype for function 'ksys_cachestat' [-Wmissing-prototypes]
long ksys_cachestat(unsigned int fd, size_t len, loff_t off,
^
mm/filemap.c:4034:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
long ksys_cachestat(unsigned int fd, size_t len, loff_t off,
^
static
7 warnings generated.
--
In file included from security/keys/gc.c:11:
In file included from security/keys/internal.h:19:
>> include/linux/compat.h:821:31: warning: declaration of 'struct cachestat' will not be visible outside of this function [-Wvisibility]
compat_arg_u64(off), struct cachestat __user *cstat,
^
1 warning generated.
--
In file included from security/keys/key.c:16:
In file included from include/linux/ima.h:13:
In file included from include/linux/kexec.h:19:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:547:31: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __raw_readb(PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:560:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le16_to_cpu((__le16 __force)__raw_readw(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:37:51: note: expanded from macro '__le16_to_cpu'
#define __le16_to_cpu(x) ((__force __u16)(__le16)(x))
^
In file included from security/keys/key.c:16:
In file included from include/linux/ima.h:13:
In file included from include/linux/kexec.h:19:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:573:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le32_to_cpu((__le32 __force)__raw_readl(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:35:51: note: expanded from macro '__le32_to_cpu'
#define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
^
In file included from security/keys/key.c:16:
In file included from include/linux/ima.h:13:
In file included from include/linux/kexec.h:19:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:584:33: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writeb(value, PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:594:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writew((u16 __force)cpu_to_le16(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:604:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writel((u32 __force)cpu_to_le32(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
In file included from security/keys/key.c:18:
In file included from security/keys/internal.h:19:
>> include/linux/compat.h:821:31: warning: declaration of 'struct cachestat' will not be visible outside of this function [-Wvisibility]
compat_arg_u64(off), struct cachestat __user *cstat,
^
7 warnings generated.
--
In file included from arch/hexagon/kernel/asm-offsets.c:12:
>> include/linux/compat.h:821:31: warning: declaration of 'struct cachestat' will not be visible outside of this function [-Wvisibility]
compat_arg_u64(off), struct cachestat __user *cstat,
^
In file included from arch/hexagon/kernel/asm-offsets.c:15:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:547:31: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __raw_readb(PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:560:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le16_to_cpu((__le16 __force)__raw_readw(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:37:51: note: expanded from macro '__le16_to_cpu'
#define __le16_to_cpu(x) ((__force __u16)(__le16)(x))
^
In file included from arch/hexagon/kernel/asm-offsets.c:15:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:573:61: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
val = __le32_to_cpu((__le32 __force)__raw_readl(PCI_IOBASE + addr));
~~~~~~~~~~ ^
include/uapi/linux/byteorder/little_endian.h:35:51: note: expanded from macro '__le32_to_cpu'
#define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
^
In file included from arch/hexagon/kernel/asm-offsets.c:15:
In file included from include/linux/interrupt.h:11:
In file included from include/linux/hardirq.h:11:
In file included from ./arch/hexagon/include/generated/asm/hardirq.h:1:
In file included from include/asm-generic/hardirq.h:17:
In file included from include/linux/irq.h:20:
In file included from include/linux/io.h:13:
In file included from arch/hexagon/include/asm/io.h:334:
include/asm-generic/io.h:584:33: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writeb(value, PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:594:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writew((u16 __force)cpu_to_le16(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
include/asm-generic/io.h:604:59: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
__raw_writel((u32 __force)cpu_to_le32(value), PCI_IOBASE + addr);
~~~~~~~~~~ ^
7 warnings generated.
<stdin>:1517:2: warning: syscall clone3 not implemented [-W#warnings]
#warning syscall clone3 not implemented
^
1 warning generated.

vim +821 include/linux/compat.h

809
810 /* net/socket.c */
811 asmlinkage long compat_sys_recvfrom(int fd, void __user *buf, compat_size_t len,
812 unsigned flags, struct sockaddr __user *addr,
813 int __user *addrlen);
814 asmlinkage long compat_sys_sendmsg(int fd, struct compat_msghdr __user *msg,
815 unsigned flags);
816 asmlinkage long compat_sys_recvmsg(int fd, struct compat_msghdr __user *msg,
817 unsigned int flags);
818
819 /* mm/filemap.c */
820 asmlinkage long compat_sys_cachestat(unsigned int fd, size_t len,
> 821 compat_arg_u64(off), struct cachestat __user *cstat,
822 unsigned int flags);
823

--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests

2023-03-09 14:11:45

by kernel test robot

[permalink] [raw]

Subject: Re: [PATCH v11 2/3] cachestat: implement cachestat syscall

2023-03-09 17:07:05

by Nhat Pham

[permalink] [raw]

Subject: [PATCH v11 2/3] cachestat: implement cachestat syscall

There is currently no good way to query the page cache state of large
file sets and directory trees. There is mincore(), but it scales poorly:
the kernel writes out a lot of bitmap data that userspace has to
aggregate, when the user really doesn not care about per-page
information in that case. The user also needs to mmap and unmap each
file as it goes along, which can be quite slow as well.

This patch implements a new syscall that queries cache state of a file
and summarizes the number of cached pages, number of dirty pages, number
of pages marked for writeback, number of (recently) evicted pages, etc.
in a given range. Currently, the syscall is only wired in for x86
architecture.

NAME
cachestat - query the page cache statistics of a file.

SYNOPSIS
#include <sys/mman.h>

struct cachestat {
__u64 nr_cache;
__u64 nr_dirty;
__u64 nr_writeback;
__u64 nr_evicted;
__u64 nr_recently_evicted;
};

int cachestat(unsigned int fd, size_t len, loff_t off,
struct cachestat *cstat, unsigned int flags);

DESCRIPTION
cachestat() queries the number of cached pages, number of dirty
pages, number of pages marked for writeback, number of evicted
pages, number of recently evicted pages, in the bytes range given by
`off` and `len`.

An evicted page is a page that is previously in the page cache but
has been evicted since. A page is recently evicted if its last
eviction was recent enough that its reentry to the cache would
indicate that it is actively being used by the system, and that
there is memory pressure on the system.

These values are returned in a cachestat struct, whose address is
given by the `cstat` argument.

The `off` and `len` arguments must be non-negative integers. If
`len` > 0, the queried range is [`off`, `off` + `len`]. If `len` ==
0, we will query in the range from `off` to the end of the file.

The `flags` argument is unused for now, but is included for future
extensibility. User should pass 0 (i.e no flag specified).

Currently, hugetlbfs is not supported.

Because the status of a page can change after cachestat() checks it
but before it returns to the application, the returned values may
contain stale information.

RETURN VALUE
On success, cachestat returns 0. On error, -1 is returned, and errno
is set to indicate the error.

ERRORS
EFAULT cstat points to an invalid address.

EINVAL invalid offset or flags.

EBADF invalid file descriptor.

EOPNOTSUPP file descriptor is of a hugetlbfs file

Signed-off-by: Nhat Pham <[email protected]>
---
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
include/linux/compat.h | 4 +
include/linux/syscalls.h | 3 +
include/uapi/asm-generic/unistd.h | 5 +-
include/uapi/linux/mman.h | 9 ++
init/Kconfig | 10 ++
kernel/sys_ni.c | 1 +
mm/filemap.c | 166 +++++++++++++++++++++++++
9 files changed, 199 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 320480a8db4f..01d1cc009250 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -455,3 +455,4 @@
448 i386 process_mrelease sys_process_mrelease
449 i386 futex_waitv sys_futex_waitv
450 i386 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 i386 cachestat sys_cachestat compat_sys_cachestat
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index c84d12608cd2..227538b0ce80 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -372,6 +372,7 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat

#
# Due to a historical design error, certain syscalls are numbered differently
diff --git a/include/linux/compat.h b/include/linux/compat.h
index 594357881b0b..6e2b7b8bdb1e 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -426,6 +426,7 @@ struct compat_sysctl_args;
struct compat_kexec_segment;
struct compat_mq_attr;
struct compat_msgbuf;
+struct cachestat;

void copy_siginfo_to_external32(struct compat_siginfo *to,
const struct kernel_siginfo *from);
@@ -817,6 +818,9 @@ asmlinkage long compat_sys_recvmsg(int fd, struct compat_msghdr __user *msg,
unsigned int flags);

/* mm/filemap.c: No generic prototype for readahead */
+asmlinkage long compat_sys_cachestat(unsigned int fd, size_t len,
+ compat_arg_u64(off), struct cachestat __user *cstat,
+ unsigned int flags);

/* security/keys/keyctl.c */
asmlinkage long compat_sys_keyctl(u32 option,
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index a34b0f9a9972..9f384a6d7e77 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -72,6 +72,7 @@ struct open_how;
struct mount_attr;
struct landlock_ruleset_attr;
enum landlock_rule_type;
+struct cachestat;

#include <linux/types.h>
#include <linux/aio_abi.h>
@@ -1056,6 +1057,8 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
unsigned long home_node,
unsigned long flags);
+asmlinkage long sys_cachestat(unsigned int fd, size_t len, loff_t offset,
+ struct cachestat __user *cstat, unsigned int flags);

/*
* Architecture-specific system calls
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 45fa180cc56a..dc5fcfee7f09 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
#define __NR_set_mempolicy_home_node 450
__SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)

+#define __NR_cachestat 451
+__SC_COMP(__NR_cachestat, sys_cachestat, compat_sys_cachestat)
+
#undef __NR_syscalls
-#define __NR_syscalls 451
+#define __NR_syscalls 452

/*
* 32 bit systems traditionally used different
diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
index f55bc680b5b0..fe03ed0b7587 100644
--- a/include/uapi/linux/mman.h
+++ b/include/uapi/linux/mman.h
@@ -4,6 +4,7 @@

#include <asm/mman.h>
#include <asm-generic/hugetlb_encode.h>
+#include <linux/types.h>

#define MREMAP_MAYMOVE 1
#define MREMAP_FIXED 2
@@ -41,4 +42,12 @@
#define MAP_HUGE_2GB HUGETLB_FLAG_ENCODE_2GB
#define MAP_HUGE_16GB HUGETLB_FLAG_ENCODE_16GB

+struct cachestat {
+ __u64 nr_cache;
+ __u64 nr_dirty;
+ __u64 nr_writeback;
+ __u64 nr_evicted;
+ __u64 nr_recently_evicted;
+};
+
#endif /* _UAPI_LINUX_MMAN_H */
diff --git a/init/Kconfig b/init/Kconfig
index 694f7c160c9c..da96ac29af1d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1798,6 +1798,16 @@ config RSEQ

If unsure, say Y.

+config CACHESTAT_SYSCALL
+ bool "Enable cachestat() system call" if EXPERT
+ default y
+ help
+ Enable the cachestat system call, which queries the page cache
+ statistics of a file (number of cached pages, dirty pages,
+ pages marked for writeback, (recently) evicted pages).
+
+ If unsure say Y here.
+
config DEBUG_RSEQ
default n
bool "Enabled debugging of rseq() system call" if EXPERT
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 860b2dcf3ac4..04bfb1e4d377 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -299,6 +299,7 @@ COND_SYSCALL(set_mempolicy);
COND_SYSCALL(migrate_pages);
COND_SYSCALL(move_pages);
COND_SYSCALL(set_mempolicy_home_node);
+COND_SYSCALL(cachestat);

COND_SYSCALL(perf_event_open);
COND_SYSCALL(accept4);
diff --git a/mm/filemap.c b/mm/filemap.c
index 08341616ae7a..749c4f59c625 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -22,6 +22,7 @@
#include <linux/mm.h>
#include <linux/swap.h>
#include <linux/swapops.h>
+#include <linux/syscalls.h>
#include <linux/mman.h>
#include <linux/pagemap.h>
#include <linux/file.h>
@@ -55,6 +56,9 @@
#include <linux/buffer_head.h> /* for try_to_free_buffers */

#include <asm/mman.h>
+#include <linux/mman.h>
+
+#include "swap.h"

/*
* Shared mappings implemented 30.11.1994. It's not fully working yet,
@@ -3949,3 +3953,165 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
return try_to_free_buffers(folio);
}
EXPORT_SYMBOL(filemap_release_folio);
+
+/**
+ * filemap_cachestat() - compute the page cache statistics of a mapping
+ * @mapping: The mapping to compute the statistics for.
+ * @first_index: The starting page cache index.
+ * @last_index: The final page index (inclusive).
+ * @cs: the cachestat struct to write the result to.
+ *
+ * This will query the page cache statistics of a mapping in the
+ * page range of [first_index, last_index] (inclusive). The statistics
+ * queried include: number of dirty pages, number of pages marked for
+ * writeback, and the number of (recently) evicted pages.
+ */
+static void filemap_cachestat(struct address_space *mapping, pgoff_t first_index,
+ pgoff_t last_index, struct cachestat *cs)
+{
+ XA_STATE(xas, &mapping->i_pages, first_index);
+ struct folio *folio;
+
+ rcu_read_lock();
+ xas_for_each(&xas, folio, last_index) {
+ unsigned long nr_pages;
+ pgoff_t folio_first_index, folio_last_index;
+
+ if (xas_retry(&xas, folio))
+ continue;
+
+ nr_pages = folio_nr_pages(folio);
+ folio_first_index = folio_pgoff(folio);
+ folio_last_index = folio_first_index + nr_pages - 1;
+
+ /* Folios might straddle the range boundaries, only count covered pages */
+ if (folio_first_index < first_index)
+ nr_pages -= first_index - folio_first_index;
+
+ if (folio_last_index > last_index)
+ nr_pages -= folio_last_index - last_index;
+
+ if (xa_is_value(folio)) {
+ /* page is evicted */
+ void *shadow = (void *)folio;
+ bool workingset; /* not used */
+
+ cs->nr_evicted += nr_pages;
+
+#ifdef CONFIG_SWAP /* implies CONFIG_MMU */
+ if (shmem_mapping(mapping)) {
+ /* shmem file - in swap cache */
+ swp_entry_t swp = radix_to_swp_entry(folio);
+
+ shadow = get_shadow_from_swap_cache(swp);
+ }
+#endif
+ if (workingset_test_recent(shadow, true, &workingset))
+ cs->nr_recently_evicted += nr_pages;
+
+ goto resched;
+ }
+
+ /* page is in cache */
+ cs->nr_cache += nr_pages;
+
+ if (folio_test_dirty(folio))
+ cs->nr_dirty += nr_pages;
+
+ if (folio_test_writeback(folio))
+ cs->nr_writeback += nr_pages;
+
+resched:
+ if (need_resched()) {
+ xas_pause(&xas);
+ cond_resched_rcu();
+ }
+ }
+ rcu_read_unlock();
+}
+
+#ifdef CONFIG_CACHESTAT_SYSCALL
+static long ksys_cachestat(unsigned int fd, size_t len, loff_t off,
+ struct cachestat __user *cstat, unsigned int flags)
+{
+ struct fd f = fdget(fd);
+ struct address_space *mapping;
+ struct cachestat cs;
+ pgoff_t first_index = off >> PAGE_SHIFT;
+ pgoff_t last_index =
+ len == 0 ? ULONG_MAX : (off + len - 1) >> PAGE_SHIFT;
+
+ if (!f.file)
+ return -EBADF;
+
+ /* hugetlbfs is not supported */
+ if (is_file_hugepages(f.file)) {
+ fdput(f);
+ return -EOPNOTSUPP;
+ }
+
+ if (off < 0 || flags != 0) {
+ fdput(f);
+ return -EINVAL;
+ }
+
+ memset(&cs, 0, sizeof(struct cachestat));
+ mapping = f.file->f_mapping;
+ filemap_cachestat(mapping, first_index, last_index, &cs);
+ fdput(f);
+
+ if (copy_to_user(cstat, &cs, sizeof(struct cachestat)))
+ return -EFAULT;
+
+ return 0;
+}
+
+/*
+ * The cachestat(5) system call.
+ *
+ * cachestat() returns the page cache statistics of a file in the
+ * bytes range specified by `off` and `len`: number of cached pages,
+ * number of dirty pages, number of pages marked for writeback,
+ * number of evicted pages, and number of recently evicted pages.
+ *
+ * An evicted page is a page that is previously in the page cache
+ * but has been evicted since. A page is recently evicted if its last
+ * eviction was recent enough that its reentry to the cache would
+ * indicate that it is actively being used by the system, and that
+ * there is memory pressure on the system.
+ *
+ * `off` and `len` must be non-negative integers. If `len` > 0,
+ * the queried range is [`off`, `off` + `len`]. If `len` == 0,
+ * we will query in the range from `off` to the end of the file.
+ *
+ * The `flags` argument is unused for now, but is included for future
+ * extensibility. User should pass 0 (i.e no flag specified).
+ *
+ * Currently, hugetlbfs is not supported.
+ *
+ * Because the status of a page can change after cachestat() checks it
+ * but before it returns to the application, the returned values may
+ * contain stale information.
+ *
+ * return values:
+ * zero - success
+ * -EFAULT - cstat points to an illegal address
+ * -EINVAL - invalid arguments
+ * -EBADF - invalid file descriptor
+ * -EOPNOTSUPP - file descriptor is of a hugetlbfs file
+ */
+SYSCALL_DEFINE5(cachestat, unsigned int, fd, size_t, len, loff_t, off,
+ struct cachestat __user *, cstat, unsigned int, flags)
+{
+ return ksys_cachestat(fd, len, off, cstat, flags);
+}
+
+#ifdef CONFIG_COMPAT
+COMPAT_SYSCALL_DEFINE6(cachestat, unsigned int, fd, size_t, len,
+ compat_arg_u64_dual(off), struct cachestat __user *, cstat,
+ unsigned int, flags)
+{
+ return ksys_cachestat(fd, len, compat_arg_u64_glue(off), cstat, flags);
+}
+#endif
+#endif /* CONFIG_CACHESTAT_SYSCALL */
--
2.39.2

2023-03-14 23:00:52

by Andrew Morton

[permalink] [raw]

Subject: Re: [PATCH v11 0/3] cachestat: a new syscall for page cache state of files

On Tue, 7 Mar 2023 19:27:45 -0800 Nhat Pham <[email protected]> wrote:

> There is currently no good way to query the page cache state of large
> file sets and directory trees. There is mincore(), but it scales poorly:
> the kernel writes out a lot of bitmap data that userspace has to
> aggregate, when the user really doesn not care about per-page information
> in that case. The user also needs to mmap and unmap each file as it goes
> along, which can be quite slow as well.

A while ago I asked about the security implications - could cachestat()
be used to figure out what parts of a file another user is reading.
This also applies to mincore(), but cachestat() newly permits user A to
work out which parts of a file user B has *written* to.

I don't recall seeing a response to this, and there is no discussion in
the changelogs.

Secondly, I'm not seeing description of any use cases. OK, it's faster
and better than mincore(), but who cares? In other words, what
end-user value compels us to add this feature to Linux?

> struct cachestat {
> __u64 nr_cache;
> __u64 nr_dirty;
> __u64 nr_writeback;
> __u64 nr_evicted;
> __u64 nr_recently_evicted;
> };

And these fields are really getting into the weedy details of internal
kernel implementation. Bear in mind that we must support this API for
ever.

Particularly the "evicted" things. The workingset code was implemented
eight years ago, which is actually relatively recent. It could be that
eight years from now it will have been removed and possibly replaced
workingset with something else. Then what do we do?

For these reasons, and because of the lack of enthusiasm I have seen
from others, I don't think a case has yet been made for the addition of
this new syscall.

2023-03-15 17:09:44

by Johannes Weiner

[permalink] [raw]

Subject: Re: [PATCH v11 0/3] cachestat: a new syscall for page cache state of files

Hi Andrew,

On Tue, Mar 14, 2023 at 04:00:41PM -0700, Andrew Morton wrote:
> On Tue, 7 Mar 2023 19:27:45 -0800 Nhat Pham <[email protected]> wrote:
>
> > There is currently no good way to query the page cache state of large
> > file sets and directory trees. There is mincore(), but it scales poorly:
> > the kernel writes out a lot of bitmap data that userspace has to
> > aggregate, when the user really doesn not care about per-page information
> > in that case. The user also needs to mmap and unmap each file as it goes
> > along, which can be quite slow as well.
>
> A while ago I asked about the security implications - could cachestat()
> be used to figure out what parts of a file another user is reading.
> This also applies to mincore(), but cachestat() newly permits user A to
> work out which parts of a file user B has *written* to.

The caller of cachestat() must have the file open for reading. If they
can read the contents that B has written, is the fact that they can
see dirty state really a concern?

Nhat and I were discussing this offlist at the time, but weren't
creative enough to come up with an abuse scenario.

> I don't recall seeing a response to this, and there is no discussion in
> the changelogs.

It might have drowned in the noise, but he did reply:

https://lore.kernel.org/lkml/CAKEwX=Ppf=WbOuV2Rh3+V8ohOYXo=CnfSu9qqSh-DpVvfy2nhA@mail.gmail.com/

> Secondly, I'm not seeing description of any use cases. OK, it's faster
> and better than mincore(), but who cares? In other words, what
> end-user value compels us to add this feature to Linux?

Years ago there was a thread about adding dirty bits to mincore(), I
don't know if you remember this:

https://lkml.org/lkml/2013/2/10/162

In that thread, Rusty described a usecase of maintaining a journaling
file alongside a main file. The idea for testing the dirty state isn't
to call sync but to see whether the journal needs to be updated.

The efficiency of mincore() was touched on too. Andres Freund (CC'd,
hopefully I got the email address right) mentioned that Postgres has a
usecase for deciding whether to do an index scan or query tables
directly, based on whether the index is cached. Postgres works with
files rather than memory regions, and Andres mentioned that the index
could be quite large. The consensus was that having to go through
mmap(), and getting a bytemap representing each page when all you need
is a summary for the queried range, was too painful in practice.

Most recently, the database team at Meta reached out to us and asked
about the ability to query dirty state again. The motivation for this
was twofold. One was simply visibility into the writeback algorithm,
i.e. trying to figure out what it's doing when investigating
performance problems.

The second usecase they brought up was to advise writeback from
userspace to manage the tradeoff between integrity and IO utilization:
if IO capacity is available, sync more frequently; if not, let the
work batch up. Blindly syncing through the file in chunks doesn't work
because you don't know in advance how much IO they'll end up doing (or
how much they've done, afterwards.) So it's difficult to build an
algorithm that will reasonably pace through sparsely dirtied regions
without the risk of overwhelming the IO device on dense ones. And it's
not straight-forward to do this from the kernel, since it doesn't know
the IO headroom the application needs for reading (which is dynamic).

The page cache is often the biggest memory consumer, and so the kernel
heuristics that manage it have a big impact on performance. We have a
rich interface to augment those heuristics with fadvise and the sync
family, but it's not a stretch to say that it's difficult to use them
if you cannot get good insights into what the other hand is doing.

Another query we get almost monthly is service owners trying to
understand where their memory is going and what's causing unexpected
pressure on a host. They see the cache in vmstat, but between a
complex application, shared libraries or a runtime (jvm, hhvm etc.)
and a myriad of host management agents, there is so much going on on
the machine that it's hard to find out who is touching which
files. When it comes to disk usage, the kernel provides the ability to
quickly stat entire filesystem subtrees and drill down with tools like
du. It sure would be useful to have the same for memory usage.

Our current cache interface is seriously lacking in that regard.

It would be great to have a stable, canonical and versatile interface
to inspect what the cache is doing. One that blends in with the
broader VFS and buffered IO interface: an easy to discover, easy to
use syscall (not an obscure tracepoint or fcntl or a drgn script); an
fd instead of a vma; a VFS-based permission model; efficient handling
of the wide range of file sizes that exist in the real world.

cachestat() fits that bill.

> > struct cachestat {
> > __u64 nr_cache;
> > __u64 nr_dirty;
> > __u64 nr_writeback;
> > __u64 nr_evicted;
> > __u64 nr_recently_evicted;
> > };
>
> And these fields are really getting into the weedy details of internal
> kernel implementation. Bear in mind that we must support this API for
> ever.
>
> Particularly the "evicted" things. The workingset code was implemented
> eight years ago, which is actually relatively recent. It could be that
> eight years from now it will have been removed and possibly replaced
> workingset with something else. Then what do we do?

;) I'm definitely biased here, but I don't think it's realistic that
we'd ever go back to a cache that doesn't maintain *some* form of
non-residency information.

We now have two reclaim implementations that rely on it at its
core. And psi is designed around the concept of initial faults vs
refaults; that's an ABI we have to maintain indefinitely anyway, and
is widely used for OOM killing and load shedding in datacenters, on
Android, by all systemd-based installations etc.

It seems unlikely that this is a fluke. But even if I'm completely
wrong about that, I think we have options that wouldn't spell the end
of the world. We could report 0 for those fields and be perfectly
backward compatible. There is a flags field that allows versioning of
struct cachestat, too.

2023-03-15 19:15:15

by Andres Freund

[permalink] [raw]

Subject: Re: [PATCH v11 0/3] cachestat: a new syscall for page cache state of files

Hi,

On 2023-03-15 13:09:34 -0400, Johannes Weiner wrote:
> On Tue, Mar 14, 2023 at 04:00:41PM -0700, Andrew Morton wrote:
> > A while ago I asked about the security implications - could cachestat()
> > be used to figure out what parts of a file another user is reading.
> > This also applies to mincore(), but cachestat() newly permits user A to
> > work out which parts of a file user B has *written* to.
>
> The caller of cachestat() must have the file open for reading. If they
> can read the contents that B has written, is the fact that they can
> see dirty state really a concern?

Random idea: Only fill ->dirty/writeback if the fd is open for writing.

> > Secondly, I'm not seeing description of any use cases. OK, it's faster
> > and better than mincore(), but who cares? In other words, what
> > end-user value compels us to add this feature to Linux?
>
> Years ago there was a thread about adding dirty bits to mincore(), I
> don't know if you remember this:
>
> https://lkml.org/lkml/2013/2/10/162
>
> In that thread, Rusty described a usecase of maintaining a journaling
> file alongside a main file. The idea for testing the dirty state isn't
> to call sync but to see whether the journal needs to be updated.
>
> The efficiency of mincore() was touched on too. Andres Freund (CC'd,
> hopefully I got the email address right) mentioned that Postgres has a
> usecase for deciding whether to do an index scan or query tables
> directly, based on whether the index is cached. Postgres works with
> files rather than memory regions, and Andres mentioned that the index
> could be quite large.

This is still relevant, FWIW. And not just for deciding on the optimal query
plan, but also for reporting purposes. We can show the user what part of the
query has done how much IO, but that can end up being quite confusing because
we're not aware of how much IO was fullfilled by the page cache.

> Most recently, the database team at Meta reached out to us and asked
> about the ability to query dirty state again. The motivation for this
> was twofold. One was simply visibility into the writeback algorithm,
> i.e. trying to figure out what it's doing when investigating
> performance problems.
>
> The second usecase they brought up was to advise writeback from
> userspace to manage the tradeoff between integrity and IO utilization:
> if IO capacity is available, sync more frequently; if not, let the
> work batch up. Blindly syncing through the file in chunks doesn't work
> because you don't know in advance how much IO they'll end up doing (or
> how much they've done, afterwards.) So it's difficult to build an
> algorithm that will reasonably pace through sparsely dirtied regions
> without the risk of overwhelming the IO device on dense ones. And it's
> not straight-forward to do this from the kernel, since it doesn't know
> the IO headroom the application needs for reading (which is dynamic).

We ended up building something very roughly like that in userspace - each
backend tracks the last N writes, and once the numbers reaches a certain
limit, we sort and collapse the outstanding ranges and issue
sync_file_range(SYNC_FILE_RANGE_WRITE) for them. Different types of tasks have
different limits. Without that latency in write heavy workloads is ... not
good (to this day, but to a lesser degree than 5-10 years ago).

> Another query we get almost monthly is service owners trying to
> understand where their memory is going and what's causing unexpected
> pressure on a host. They see the cache in vmstat, but between a
> complex application, shared libraries or a runtime (jvm, hhvm etc.)
> and a myriad of host management agents, there is so much going on on
> the machine that it's hard to find out who is touching which
> files. When it comes to disk usage, the kernel provides the ability to
> quickly stat entire filesystem subtrees and drill down with tools like
> du. It sure would be useful to have the same for memory usage.

+1

Greetings,

Andres Freund

2023-03-24 22:00:59

by Nhat Pham

[permalink] [raw]

Subject: Re: [PATCH v11 0/3] cachestat: a new syscall for page cache state of files

On Wed, Mar 15, 2023 at 12:15 PM Andres Freund <[email protected]> wrote:
>
> Hi,
>
> On 2023-03-15 13:09:34 -0400, Johannes Weiner wrote:
> > On Tue, Mar 14, 2023 at 04:00:41PM -0700, Andrew Morton wrote:
> > > A while ago I asked about the security implications - could cachestat()
> > > be used to figure out what parts of a file another user is reading.
> > > This also applies to mincore(), but cachestat() newly permits user A to
> > > work out which parts of a file user B has *written* to.
> >
> > The caller of cachestat() must have the file open for reading. If they
> > can read the contents that B has written, is the fact that they can
> > see dirty state really a concern?
>
> Random idea: Only fill ->dirty/writeback if the fd is open for writing.
>
>
> > > Secondly, I'm not seeing description of any use cases. OK, it's faster
> > > and better than mincore(), but who cares? In other words, what
> > > end-user value compels us to add this feature to Linux?
> >
> > Years ago there was a thread about adding dirty bits to mincore(), I
> > don't know if you remember this:
> >
> > https://lkml.org/lkml/2013/2/10/162
> >
> > In that thread, Rusty described a usecase of maintaining a journaling
> > file alongside a main file. The idea for testing the dirty state isn't
> > to call sync but to see whether the journal needs to be updated.
> >
> > The efficiency of mincore() was touched on too. Andres Freund (CC'd,
> > hopefully I got the email address right) mentioned that Postgres has a
> > usecase for deciding whether to do an index scan or query tables
> > directly, based on whether the index is cached. Postgres works with
> > files rather than memory regions, and Andres mentioned that the index
> > could be quite large.
>
> This is still relevant, FWIW. And not just for deciding on the optimal query
> plan, but also for reporting purposes. We can show the user what part of the
> query has done how much IO, but that can end up being quite confusing because
> we're not aware of how much IO was fullfilled by the page cache.
>
>
> > Most recently, the database team at Meta reached out to us and asked
> > about the ability to query dirty state again. The motivation for this
> > was twofold. One was simply visibility into the writeback algorithm,
> > i.e. trying to figure out what it's doing when investigating
> > performance problems.
> >
> > The second usecase they brought up was to advise writeback from
> > userspace to manage the tradeoff between integrity and IO utilization:
> > if IO capacity is available, sync more frequently; if not, let the
> > work batch up. Blindly syncing through the file in chunks doesn't work
> > because you don't know in advance how much IO they'll end up doing (or
> > how much they've done, afterwards.) So it's difficult to build an
> > algorithm that will reasonably pace through sparsely dirtied regions
> > without the risk of overwhelming the IO device on dense ones. And it's
> > not straight-forward to do this from the kernel, since it doesn't know
> > the IO headroom the application needs for reading (which is dynamic).
>
> We ended up building something very roughly like that in userspace - each
> backend tracks the last N writes, and once the numbers reaches a certain
> limit, we sort and collapse the outstanding ranges and issue
> sync_file_range(SYNC_FILE_RANGE_WRITE) for them. Different types of tasks have
> different limits. Without that latency in write heavy workloads is ... not
> good (to this day, but to a lesser degree than 5-10 years ago).
>
>
> > Another query we get almost monthly is service owners trying to
> > understand where their memory is going and what's causing unexpected
> > pressure on a host. They see the cache in vmstat, but between a
> > complex application, shared libraries or a runtime (jvm, hhvm etc.)
> > and a myriad of host management agents, there is so much going on on
> > the machine that it's hard to find out who is touching which
> > files. When it comes to disk usage, the kernel provides the ability to
> > quickly stat entire filesystem subtrees and drill down with tools like
> > du. It sure would be useful to have the same for memory usage.
>
> +1
>
> Greetings,
>
> Andres Freund

Thanks for the suggestion/discussion regarding cachestat's use cases,
Johannes and Andres! I'll put a summary of these points (along with a link to
the original discussion thread) in the cover letter and commit message
of the new version of the patch set.

In the meantime, feel free to let me know if there is something else cachestat
could help with (along with any improvements that could facilitate such
use cases)

Best,
Nhat