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Date:   Tue, 10 Jan 2023 09:22:28 -0500
From:   Brian Foster <bfoster@redhat.com>
To:     Nhat Pham <nphamcs@gmail.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, willy@infradead.org,
        kernel-team@meta.com
Subject: Re: [PATCH v5 2/3] cachestat: implement cachestat syscall
Message-ID: <Y710pFYI8rBX9Eqe@bfoster>
References: <20230104231127.2634648-1-nphamcs@gmail.com>
 <20230104231127.2634648-3-nphamcs@gmail.com>
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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.

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?

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
>