Re: [PATCH v5 2/3] cachestat: implement cachestat syscall

[Nhat Pham <nphamcs@gmail.com>]

On Tue, Jan 10, 2023 at 6:21 AM Brian Foster <bfoster@redhat.com> wrote:
>
> On Wed, Jan 04, 2023 at 03:11:26PM -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 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,
> >           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 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_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>
> > ---
> >  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                                | 143 ++++++++++++++++++++
> >  20 files changed, 186 insertions(+), 1 deletion(-)
> >
> ...
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 08341616ae7a..d70d47b20700 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> ...
> > @@ -3949,3 +3953,142 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
> >       return try_to_free_buffers(folio);
> >  }
> >  EXPORT_SYMBOL(filemap_release_folio);
> ...
> > +#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_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;
> > +
>
> It looks like we miss an fdput() before returning via the above error
> checks.

Ooops yeah I missed that. I'll fix it.

>
> The only other thing that stands out as a bit odd to me is the
> cstat_size check and associated ability to return a partial cachestat
> struct. Do other syscalls do anything like that? I'd think we'd want to
> ensure we always at least return a fully populated cachestat struct,
> even if it happened to be an old/compat version if the size does ever
> increase. Hm?

Not that I know of, but the idea is that the user might expect a smaller
struct cachestat in their code (and allocate memory accordingly).
With this cstat_size, we can make sure that the expansion of cachestat
struct (with new fields) does not break existing user's code - we only
copy part of the struct.

>
> Brian
>
> > +     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_SYSCALL */
> > --
> > 2.30.2
> >
>