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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Thu, 4 May 2023 19:26:11 +0200
Message-ID: <CAMuHMdWUtb_A-uhXrBg6kC9L2zbC_q3m8oCZoq80ZSJvk6mUAA@mail.gmail.com>
Subject: Re: [PATCH v13 2/3] cachestat: implement cachestat syscall
To:     Nhat Pham <nphamcs@gmail.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, bfoster@redhat.com,
        willy@infradead.org, linux-api@vger.kernel.org,
        kernel-team@meta.com
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Hi Nhat,

On Wed, May 3, 2023 at 3:38 AM Nhat Pham <nphamcs@gmail.com> 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.
>
> Some use cases where this information could come in handy:
>   * Allowing database to decide whether to perform an index scan or
>     direct table queries based on the in-memory cache state of the
>     index.
>   * Visibility into the writeback algorithm, for performance issues
>     diagnostic.
>   * Workload-aware writeback pacing: estimating IO fulfilled by page
>     cache (and IO to be done) within a range of a file, allowing for
>     more frequent syncing when and where there is IO capacity, and
>     batching when there is not.
>   * Computing memory usage of large files/directory trees, analogous to
>     the du tool for disk usage.
>
> More information about these use cases could be found in the following
> thread:
>
> https://lore.kernel.org/lkml/20230315170934.GA97793@cmpxchg.org/
>
> 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. Currently, the syscall is only wired in for x86
> architecture.
>
> NAME
>     cachestat - query the page cache statistics of a file.
>
> SYNOPSIS
>     #include <sys/mman.h>
>
>     struct cachestat_range {
>         __u64 off;
>         __u64 len;
>     };
>
>     struct cachestat {
>         __u64 nr_cache;
>         __u64 nr_dirty;
>         __u64 nr_writeback;
>         __u64 nr_evicted;
>         __u64 nr_recently_evicted;
>     };
>
>     int cachestat(unsigned int fd, struct cachestat_range *cstat_range,
>         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.
>
>     The `flags` argument is unused for now, but is included for future
>     extensibility. User should pass 0 (i.e no flag specified).
>
>     Currently, hugetlbfs is not supported.
>
>     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 VALUE
>     On success, cachestat returns 0. On error, -1 is returned, and errno
>     is set to indicate the error.
>
> ERRORS
>     EFAULT cstat or cstat_args points to an invalid address.
>
>     EINVAL invalid flags.
>
>     EBADF  invalid file descriptor.
>
>     EOPNOTSUPP file descriptor is of a hugetlbfs file
>
> Signed-off-by: Nhat Pham <nphamcs@gmail.com>
> ---
>  arch/x86/entry/syscalls/syscall_32.tbl |   1 +
>  arch/x86/entry/syscalls/syscall_64.tbl |   1 +

This should be wired up on each and every architecture.
Currently we're getting

    <stdin>:1567:2: warning: #warning syscall cachestat not implemented [-Wcpp]

in linux-next for all the missing architectures.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds