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Message-ID: <Y8qm69bYnzZk+Vni@bfoster>
Date: Fri, 20 Jan 2023 09:36:27 -0500
From: Brian Foster <bfoster@...hat.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, willy@...radead.org,
linux-api@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v6 2/3] cachestat: implement cachestat syscall
On Tue, Jan 17, 2023 at 11:59:58AM -0800, Nhat Pham 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.
>
> 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.
>
Still not sure about this vs. just requiring the structure size to match
(and maybe padding it), but perhaps linux-api can comment on the best
way to future proof.
I'm not the person to ack a syscall regardless, but since I've had
various previous comments on this patch and no further issues stand out
to me:
Reviewed-by: Brian Foster <bfoster@...hat.com>
> 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 <nphamcs@...il.com>
> ---
> 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/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 ++++++++++++++++++++
> 20 files changed, 198 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/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
>
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