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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Fri, 13 Jan 2023 10:35:57 +0100
Message-ID: <CAMuHMdXObtC0_US+fpc9Wq0SEHGcmLfVb19F55SF6x=yMKD4UQ@mail.gmail.com>
Subject: Re: [PATCH v5 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, kernel-team@meta.com
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On Thu, Jan 5, 2023 at 12:13 AM Nhat Pham <nphamcs@gmail.com> 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/m68k/kernel/syscalls/syscall.tbl       |   1 +

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> [m68k]

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