Changelog:
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
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/alpha/kernel/syscalls/syscall.tbl | 1 +
arch/arm/tools/syscall.tbl | 1 +
arch/ia64/kernel/syscalls/syscall.tbl | 1 +
arch/m68k/kernel/syscalls/syscall.tbl | 1 +
arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n32.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n64.tbl | 1 +
arch/mips/kernel/syscalls/syscall_o32.tbl | 1 +
arch/parisc/kernel/syscalls/syscall.tbl | 1 +
arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
arch/s390/kernel/syscalls/syscall.tbl | 1 +
arch/sh/kernel/syscalls/syscall.tbl | 1 +
arch/sparc/kernel/syscalls/syscall.tbl | 1 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
include/linux/fs.h | 3 +
include/linux/swap.h | 1 +
include/linux/syscalls.h | 4 +
include/uapi/asm-generic/unistd.h | 5 +-
include/uapi/linux/mman.h | 9 +
init/Kconfig | 10 +
kernel/sys_ni.c | 1 +
mm/filemap.c | 154 +++++++++++
mm/workingset.c | 142 ++++++----
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/cachestat/.gitignore | 2 +
tools/testing/selftests/cachestat/Makefile | 8 +
.../selftests/cachestat/test_cachestat.c | 260 ++++++++++++++++++
30 files changed, 576 insertions(+), 47 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
--
2.30.2
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 | 142 +++++++++++++++++++++++++++++--------------
2 files changed, 97 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..5f1d20a0a2fd 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -244,6 +244,30 @@ static void *lru_gen_eviction(struct folio *folio)
return pack_shadow(mem_cgroup_id(memcg), pgdat, token, refs);
}
+/*
+ * Test if the folio is recently evicted.
+ *
+ * As a side effect, also populates the references with
+ * values unpacked from the shadow of the evicted folio.
+ */
+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 +282,22 @@ 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 +329,12 @@ 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)
{
}
@@ -373,40 +402,31 @@ void *workingset_eviction(struct folio *folio, struct mem_cgroup *target_memcg)
folio_test_workingset(folio));
}
-/**
- * workingset_refault - Evaluate the refault of a previously evicted folio.
- * @folio: The freshly allocated replacement folio.
- * @shadow: Shadow entry of the evicted folio.
+/*
+ * Test if the folio is recently evicted by checking if
+ * refault distance of shadow exceeds workingset size.
*
- * 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.
+ * As a side effect, populate workingset with the value
+ * unpacked from shadow.
*/
-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 +445,8 @@ 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 +468,6 @@ 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 +489,51 @@ 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);
--
2.30.2
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.
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, off_t off, size_t len,
unsigned int cstat_version, 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.
`cstat_version` is an unsigned integer indicating the specific
version of the cachestat struct. It must be at least 1, and does
not exceed the latest version number (which is currently 1). For
now, user should just pass 1.
The `flags` argument is unused for now, but is included for future
extensibility. User should pass 0 (i.e no flag specified).
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 `cstat_version` or `flags`
EBADF invalid file descriptor.
Signed-off-by: Nhat Pham <[email protected]>
---
arch/alpha/kernel/syscalls/syscall.tbl | 1 +
arch/arm/tools/syscall.tbl | 1 +
arch/ia64/kernel/syscalls/syscall.tbl | 1 +
arch/m68k/kernel/syscalls/syscall.tbl | 1 +
arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n32.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n64.tbl | 1 +
arch/mips/kernel/syscalls/syscall_o32.tbl | 1 +
arch/parisc/kernel/syscalls/syscall.tbl | 1 +
arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
arch/s390/kernel/syscalls/syscall.tbl | 1 +
arch/sh/kernel/syscalls/syscall.tbl | 1 +
arch/sparc/kernel/syscalls/syscall.tbl | 1 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
include/linux/fs.h | 3 +
include/linux/syscalls.h | 4 +
include/uapi/asm-generic/unistd.h | 5 +-
include/uapi/linux/mman.h | 9 ++
init/Kconfig | 10 ++
kernel/sys_ni.c | 1 +
mm/filemap.c | 154 ++++++++++++++++++++
23 files changed, 201 insertions(+), 1 deletion(-)
diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index 8ebacf37a8cf..1f13995d00d7 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -490,3 +490,4 @@
558 common process_mrelease sys_process_mrelease
559 common futex_waitv sys_futex_waitv
560 common set_mempolicy_home_node sys_ni_syscall
+561 common cachestat sys_cachestat
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index ac964612d8b0..8ebed8a13874 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -464,3 +464,4 @@
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
diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
index 72c929d9902b..f8c74ffeeefb 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl
@@ -371,3 +371,4 @@
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
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index b1f3940bc298..4f504783371f 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -450,3 +450,4 @@
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
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index 820145e47350..858d22bf275c 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -456,3 +456,4 @@
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
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index 253ff994ed2e..1976317d4e8b 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -389,3 +389,4 @@
448 n32 process_mrelease sys_process_mrelease
449 n32 futex_waitv sys_futex_waitv
450 n32 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 n32 cachestat sys_cachestat
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 3f1886ad9d80..cfda2511badf 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -365,3 +365,4 @@
448 n64 process_mrelease sys_process_mrelease
449 n64 futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 n64 cachestat sys_cachestat
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 8f243e35a7b2..7692234c3768 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -438,3 +438,4 @@
448 o32 process_mrelease sys_process_mrelease
449 o32 futex_waitv sys_futex_waitv
450 o32 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 o32 cachestat sys_cachestat
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index 8a99c998da9b..7c84a72306d1 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -448,3 +448,4 @@
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
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index 2bca64f96164..937460f0a8ec 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -530,3 +530,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 nospu set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index 799147658dee..7df0329d46cb 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -453,3 +453,4 @@
448 common process_mrelease sys_process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common cachestat sys_cachestat sys_cachestat
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index 2de85c977f54..97377e8c5025 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -453,3 +453,4 @@
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
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index 4398cc6fb68d..faa835f3c54a 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -496,3 +496,4 @@
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
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 320480a8db4f..bc0a3c941b35 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
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/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 52c94ab5c205..2b69c3c035b6 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -421,3 +421,4 @@
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
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e654435f1651..83300f1491e7 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -75,6 +75,7 @@ struct fs_context;
struct fs_parameter_spec;
struct fileattr;
struct iomap_ops;
+struct cachestat;
extern void __init inode_init(void);
extern void __init inode_init_early(void);
@@ -830,6 +831,8 @@ void filemap_invalidate_lock_two(struct address_space *mapping1,
struct address_space *mapping2);
void filemap_invalidate_unlock_two(struct address_space *mapping1,
struct address_space *mapping2);
+void filemap_cachestat(struct address_space *mapping, pgoff_t first_index,
+ pgoff_t last_index, struct cachestat *cs);
/*
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index a34b0f9a9972..d3fe6ba8eb38 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 +1057,9 @@ 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, off_t off, size_t len,
+ unsigned int cstat_version, 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..cd639fae9086 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
+__SYSCALL(__NR_cachestat, 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..0305eaf5b3f5 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 +56,13 @@
#include <linux/buffer_head.h> /* for try_to_free_buffers */
#include <asm/mman.h>
+#include <uapi/linux/mman.h>
+
+#include "swap.h"
+
+#ifdef CONFIG_CACHESTAT_SYSCALL
+#define LATEST_CACHESTAT_VERSION 1
+#endif
/*
* Shared mappings implemented 30.11.1994. It's not fully working yet,
@@ -3949,3 +3957,149 @@ 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.
+ */
+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 subpages */
+ 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();
+}
+EXPORT_SYMBOL(filemap_cachestat);
+
+#ifdef CONFIG_CACHESTAT_SYSCALL
+/*
+ * 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.
+ *
+ * `cstat_version` is an unsigned integer indicating the specific version
+ * of the cachestat struct. It must be at least 1, and does not exceed the
+ * latest version number (which is currently 1). For now, user should
+ * just pass 1.
+ *
+ * The `flags` argument is unused for now, but is included for future
+ * extensibility. User should pass 0 (i.e no flag specified).
+ *
+ * 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
+ */
+SYSCALL_DEFINE6(cachestat, unsigned int, fd, off_t, off, size_t, len,
+ unsigned int, cstat_version, 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;
+
+ if (off < 0 || flags != 0 || cstat_version < 1 ||
+ cstat_version > LATEST_CACHESTAT_VERSION) {
+ 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;
+}
+#endif /* CONFIG_CACHESTAT_SYSCALL */
--
2.30.2
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 | 260 ++++++++++++++++++
5 files changed, 278 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..dc2894028eee
--- /dev/null
+++ b/tools/testing/selftests/cachestat/test_cachestat.c
@@ -0,0 +1,260 @@
+// 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;
+static const int cstat_version = 1; /* first version */
+
+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");
+ 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, 0, filesize,
+ cstat_version, &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, 0, filesize,
+ cstat_version, &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, off, compute_len,
+ cstat_version, &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;
+}
--
2.30.2
On Thu, Jan 26, 2023, at 18:53, Nhat Pham wrote:
>
> 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, off_t off, size_t len,
> unsigned int cstat_version, struct cachestat *cstat,
> unsigned int flags);
Is this "off_t off" argument intentionally limited to the old
32-bit type on 32-bit architectures? Unfortunately I fear
there are no good options to pass an offset here:
- if you make it a 32-bit type, this breaks calling it from
normal userspace that defines off_t as a 64-bit type
- if you change it to a 64-bit loff_t, there are three
separate calling conventions for 64-bit, 32-bit with
aligned register pairs and other 32-bit, plus you
exceed the usual limit of six system call arguments
A separate problem may be the cstat_version argument, usually
we don't use interface versions but instead use a new
system call number if something changes in an incompatible
way.
Arnd
On Fri, Jan 27, 2023 at 04:46:38PM +0100, Arnd Bergmann wrote:
> On Thu, Jan 26, 2023, at 18:53, Nhat Pham wrote:
> >
> > 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, off_t off, size_t len,
> > unsigned int cstat_version, struct cachestat *cstat,
> > unsigned int flags);
>
> Is this "off_t off" argument intentionally limited to the old
> 32-bit type on 32-bit architectures? Unfortunately I fear
> there are no good options to pass an offset here:
>
> - if you make it a 32-bit type, this breaks calling it from
> normal userspace that defines off_t as a 64-bit type
>
> - if you change it to a 64-bit loff_t, there are three
> separate calling conventions for 64-bit, 32-bit with
> aligned register pairs and other 32-bit, plus you
> exceed the usual limit of six system call arguments
That's a good point, thanks for raising it, Arnd.
> A separate problem may be the cstat_version argument, usually
> we don't use interface versions but instead use a new
> system call number if something changes in an incompatible
> way.
I suppose from a userspace POV, a version argument vs calling a
separate syscall doesn't make much of a difference. So going with
loff_t and dropping cstat_version seems like a sensible way forward.
As an added bonus, versioning the syscall itself means the signature
can change in v2. This allows dropping the unused flags arg for now.
That would leave it at:
int cachestat(unsigned int, loff_t, size_t len, struct cachestat *cstat);
and should we ever require extensions - new fields, flags - they would
come through a new cachestat2().
Would anybody have objections to that?
On Fri, Jan 27, 2023, at 20:46, Johannes Weiner wrote:
> On Fri, Jan 27, 2023 at 04:46:38PM +0100, Arnd Bergmann wrote:
>>
>> - if you make it a 32-bit type, this breaks calling it from
>> normal userspace that defines off_t as a 64-bit type
>>
>> - if you change it to a 64-bit loff_t, there are three
>> separate calling conventions for 64-bit, 32-bit with
>> aligned register pairs and other 32-bit, plus you
>> exceed the usual limit of six system call arguments
>
> That's a good point, thanks for raising it, Arnd.
>
>> A separate problem may be the cstat_version argument, usually
>> we don't use interface versions but instead use a new
>> system call number if something changes in an incompatible
>> way.
>
> I suppose from a userspace POV, a version argument vs calling a
> separate syscall doesn't make much of a difference. So going with
> loff_t and dropping cstat_version seems like a sensible way forward.
>
> As an added bonus, versioning the syscall itself means the signature
> can change in v2. This allows dropping the unused flags arg for now.
>
> That would leave it at:
>
> int cachestat(unsigned int, loff_t, size_t len, struct cachestat *cstat);
There is still a problem of incompatible calling conventions:
on architectures that require even/odd register pairs, this would
end up like
int cachestat(unsigned int, long unused, u32 off_low, u32 off_high,
size_t len, struct cachestat *cstat);
A more portable way to do this would be to pass the offset by
reference, but that makes it a bit awkward in userspace.
Or the arguments could be rearranged to put the low/high argument
pair first/second, third/fourth or fifth/sixth argument, at least
on the kernel ABI to avoid having another situation like
sys_arm_fadvise64_64.
> and should we ever require extensions - new fields, flags - they would
> come through a new cachestat2().
>
> Would anybody have objections to that?
If there is room for another argument, I would keep the 'flags'
as a way for extending in a compatible way, that is pretty standard
now, just not flags plus version.
Arnd
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/20230128-171134
base: 1440f576022887004f719883acb094e7e0dd4944
patch link: https://lore.kernel.org/r/20230126175356.1582123-3-nphamcs%40gmail.com
patch subject: [PATCH v8 2/3] cachestat: implement cachestat syscall
config: s390-randconfig-s051-20230129 (https://download.01.org/0day-ci/archive/20230129/[email protected]/config)
compiler: s390-linux-gcc (GCC) 12.1.0
reproduce:
wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
chmod +x ~/bin/make.cross
# apt-get install sparse
# sparse version: v0.6.4-39-gce1a6720-dirty
# https://github.com/intel-lab-lkp/linux/commit/a05ffdcecfe9ac147066fb8472e6fb75491d0eed
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/20230128-171134
git checkout a05ffdcecfe9ac147066fb8472e6fb75491d0eed
# save the config file
mkdir build_dir && cp config build_dir/.config
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=s390 olddefconfig
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=s390 SHELL=/bin/bash
If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <[email protected]>
sparse warnings: (new ones prefixed by >>)
>> mm/filemap.c:4075:1: sparse: sparse: Using plain integer as NULL pointer
>> mm/filemap.c:4075:1: sparse: sparse: Using plain integer as NULL pointer
mm/filemap.c:1416:17: sparse: sparse: context imbalance in 'migration_entry_wait_on_locked' - unexpected unlock
vim +4075 mm/filemap.c
4037
4038 #ifdef CONFIG_CACHESTAT_SYSCALL
4039 /*
4040 * The cachestat(5) system call.
4041 *
4042 * cachestat() returns the page cache statistics of a file in the
4043 * bytes range specified by `off` and `len`: number of cached pages,
4044 * number of dirty pages, number of pages marked for writeback,
4045 * number of evicted pages, and number of recently evicted pages.
4046 *
4047 * An evicted page is a page that is previously in the page cache
4048 * but has been evicted since. A page is recently evicted if its last
4049 * eviction was recent enough that its reentry to the cache would
4050 * indicate that it is actively being used by the system, and that
4051 * there is memory pressure on the system.
4052 *
4053 * `off` and `len` must be non-negative integers. If `len` > 0,
4054 * the queried range is [`off`, `off` + `len`]. If `len` == 0,
4055 * we will query in the range from `off` to the end of the file.
4056 *
4057 * `cstat_version` is an unsigned integer indicating the specific version
4058 * of the cachestat struct. It must be at least 1, and does not exceed the
4059 * latest version number (which is currently 1). For now, user should
4060 * just pass 1.
4061 *
4062 * The `flags` argument is unused for now, but is included for future
4063 * extensibility. User should pass 0 (i.e no flag specified).
4064 *
4065 * Because the status of a page can change after cachestat() checks it
4066 * but before it returns to the application, the returned values may
4067 * contain stale information.
4068 *
4069 * return values:
4070 * zero - success
4071 * -EFAULT - cstat points to an illegal address
4072 * -EINVAL - invalid arguments
4073 * -EBADF - invalid file descriptor
4074 */
> 4075 SYSCALL_DEFINE6(cachestat, unsigned int, fd, off_t, off, size_t, len,
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests