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From:   Nhat Pham <nphamcs@gmail.com>
To:     akpm@linux-foundation.org
Cc:     hannes@cmpxchg.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, bfoster@redhat.com,
        willy@infradead.org, linux-api@vger.kernel.org,
        kernel-team@meta.com
Subject: [PATCH v12 2/3] cachestat: implement cachestat syscall
Date:   Fri, 21 Apr 2023 16:14:20 -0700
Message-Id: <20230421231421.2401346-3-nphamcs@gmail.com>
In-Reply-To: <20230421231421.2401346-1-nphamcs@gmail.com>
References: <20230421231421.2401346-1-nphamcs@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
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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 couple of use cases where this information could come in handy:
  * Deciding whether to perform an index scan or direct table queries
    based on the in-memory cache state of the index.
  * Visibility into the writeback algorithm, for performance issues
    diagnostic.
  * Userspace writeback advise - balancing between integrity and IO
    utilization.
  * Computing memory usage of large files/directory trees, analogous to
    the du tool for disk usage.

More information about these use cases could be found in the following
thread:

https://lore.kernel.org/lkml/20230315170934.GA97793@cmpxchg.org/

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 <nphamcs@gmail.com>
---
 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                           | 179 +++++++++++++++++++++++++
 9 files changed, 212 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 44b1736c95b5..ed7227603c7e 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -424,6 +424,7 @@ 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);
@@ -815,6 +816,9 @@ 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 33a0ee3bcb2e..73c174fda520 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>
@@ -1058,6 +1059,8 @@ 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 1fb5f313d18f..10de68cbd6b6 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1837,6 +1837,16 @@ 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 a34abfe8c654..9819722569a5 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>
@@ -57,6 +58,9 @@
 #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,
@@ -4119,3 +4123,178 @@ 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;
+
+		if (xa_is_value(folio)) {
+			/* page is evicted */
+			void *shadow = (void *)folio;
+			bool workingset; /* not used */
+			int order = xa_get_order(xas.xa, xas.xa_index);
+
+			nr_pages = 1 << order;
+			/* rounds down to the nearest multiple of 2^order */
+			folio_first_index = xas.xa_index >> order << order;
+			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;
+
+			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;
+		}
+
+		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;
+
+		/* 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 */
-- 
2.34.1