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Date: Tue, 20 Dec 2022 09:37:58 -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,
kernel-team@...a.com
Subject: Re: [PATCH v4 3/4] cachestat: implement cachestat syscall
On Fri, Dec 16, 2022 at 11:21:48AM -0800, Nhat Pham wrote:
> Implement 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 status 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,
> size_t cstat_size, 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 (recently)
> evicted pages, in the bytes range given by `off` and `len`.
>
> 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_size` allows users to obtain partial results. The syscall
> will copy the first `csstat_size` bytes to the specified userspace
> memory. `cstat_size` must be a non-negative value that is no larger
> than the current size of the cachestat struct.
>
> 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_size` or `flags`
>
> EBADF invalid file descriptor.
>
> Signed-off-by: Nhat Pham <nphamcs@...il.com>
> ---
Hi Nhat,
Thanks for the tweaks. FWIW, this by and large looks reasonable to me.
Just a couple more random nitty things, if you happen to care about
them..
> 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 | 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 | 137 ++++++++++++++++++++
> 20 files changed, 180 insertions(+), 1 deletion(-)
>
...
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 08341616ae7a..29ffb906caa4 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
...
> @@ -3949,3 +3953,136 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
> return try_to_free_buffers(folio);
> }
> EXPORT_SYMBOL(filemap_release_folio);
> +
> +#ifdef CONFIG_CACHESTAT
> +/**
> + * 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)
> +{
Not sure why the internal helper needs to be wrapped in a config option?
Perhaps it makes more sense to define CONFIG_CACHESTAT_SYSCALL and just
wrap the syscall bits with that..?
I would also think it might make things simpler to split out all the
syscall bits into a separate patch from filemap_cachestat().
> + 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;
> +
> + continue;
> + }
> +
> + /* 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;
I'm not sure this is an issue right now (or if it will ever be for your
use cases), but from a filesystem perspective it is possible to have
large or variable sized folios in cache. At the moment I think the fs
(or XFS+iomap at least) manages dirty/writeback state at folio
granularity, but that may change in the near future if/when iomap
sub-page dirty tracking comes along. I suspect that means it may become
possible to have a large folio of some N number of pages where only a
subset of those pages are actually in dirty/writeback state, and thus
introduces some inaccuracy here because this assumes that folio state
applies to folio_nr_pages() worth of pages. Just something to be aware
of..
Brian
> + }
> + rcu_read_unlock();
> +}
> +EXPORT_SYMBOL(filemap_cachestat);
> +
> +/*
> + * The cachestat(5) system call.
> + *
> + * cachestat() returns the page cache statistics of a file in the
> + * bytes specified by `off` and `len`: number of cached pages,
> + * number of dirty pages, number of pages marked for writeback,
> + * number of (recently) evicted pages.
> + *
> + * `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_size` allows users to obtain partial results. The syscall
> + * will copy the first `csstat_size` bytes to the specified userspace
> + * memory. It also makes the cachestat struct extensible - new fields
> + * can be added in the future without breaking existing usage.
> + * `cstat_size` must be a non-negative value that is no larger than
> + * the current size of the cachestat struct.
> + *
> + * 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,
> + size_t, cstat_size, 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 (off < 0 || cstat_size > sizeof(struct cachestat) || flags != 0)
> + return -EINVAL;
> +
> + if (!f.file)
> + return -EBADF;
> +
> + 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, cstat_size))
> + return -EFAULT;
> +
> + return 0;
> +}
> +#endif /* CONFIG_CACHESTAT */
> --
> 2.30.2
>
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