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SubjectRe: [PATCH v4 3/4] cachestat: implement cachestat syscall
On Thu, Dec 22, 2022 at 01:50:19PM -0800, Nhat Pham wrote:
> On Tue, Dec 20, 2022 at 6:37 AM Brian Foster <bfoster@redhat.com> wrote:
> >
> > 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@gmail.com>
> > > ---
> >
> > Hi Nhat,
> >
> > Thanks for the tweaks. FWIW, this by and large looks reasonable to me.
>
> Thanks for suggesting the tweaks! I tried refactoring it out and it
> did look cleaner to me (and one fewer file that I have to keep
> track of...)
>
> > 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..?
>
> No particularly strong reasons - I was just bundling the two together
> because I was not entirely sure if it's interesting or has any use
> case outside of the syscall itself.
>
> Do you have something in mind?
>

Not immediately, though it looks like a straightforward and potentially
useful helper. It wouldn't surprise me if it grew more in-kernel users
eventually. :)

That said, I only suggested this for cleanup/consistency reasons. I.e.,
it seems there is plenty of precedent for CONFIG_*_SYSCALL config
options, and the one or two I happened to peek at looked like they only
wrapped the syscall bits as opposed to the entire implementation (which
also seems like a clean and elegant approach to me). Somebody could
easily come along and make that change later if/when they might want to
use the helper (though it might be annoying to change the name of the
config option), so I have no strong opinion on either suggestion.

Brian

> >
> > I would also think it might make things simpler to split out all the
> > syscall bits into a separate patch from filemap_cachestat().
>
> Same as above.
>
> >
> > > + 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
>
> Oof, I coded this with the mental framework of folio-as-a-unit, and
> assumed that the dirty/writeback state is managed at the granularity of folio.
> Thanks for bringing this up Brian! I'll have to watch out for this as iomap
> evolves (and the subpage tracking becomes a thing).
>
> >
> > > + }
> > > + 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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