linux-kernel - Re: [PATCH 3/6] shmem: move reclaim check early on writepages()

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Date:   Mon, 6 Mar 2023 11:49:05 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     hughd@...gle.com, akpm@...ux-foundation.org, willy@...radead.org,
        brauner@...nel.org, linux-mm@...ck.org, p.raghav@...sung.com,
        da.gomez@...sung.com, a.manzanares@...sung.com, dave@...olabs.net,
        keescook@...omium.org, patches@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] shmem: move reclaim check early on writepages()

On Thu, Mar 2, 2023 at 3:28 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
>
> i915_gem requires huge folios to be split when swapping.
> However we have  check for usage of writepages() to ensure
> it used only for swap purposes later. Avoid the splits if
> we're not being called for reclaim, even if they should in
> theory not happen.
>
> This makes the conditions easier to follow on shem_writepage().
>
> Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>

Nice cleanup.

Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>

> ---
>  mm/shmem.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 2b9ff585a553..a5a6da51087e 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1340,6 +1340,18 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>         swp_entry_t swap;
>         pgoff_t index;
>
> +       /*
> +        * Our capabilities prevent regular writeback or sync from ever calling
> +        * shmem_writepage; but a stacking filesystem might use ->writepage of
> +        * its underlying filesystem, in which case tmpfs should write out to
> +        * swap only in response to memory pressure, and not for the writeback
> +        * threads or sync.
> +        */
> +       if (!wbc->for_reclaim) {
> +               WARN_ON_ONCE(1);        /* Still happens? Tell us about it! */
> +               goto redirty;
> +       }
> +
>         /*
>          * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "always" or
>          * "force", drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
> @@ -1360,18 +1372,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>         if (!total_swap_pages)
>                 goto redirty;
>
> -       /*
> -        * Our capabilities prevent regular writeback or sync from ever calling
> -        * shmem_writepage; but a stacking filesystem might use ->writepage of
> -        * its underlying filesystem, in which case tmpfs should write out to
> -        * swap only in response to memory pressure, and not for the writeback
> -        * threads or sync.
> -        */
> -       if (!wbc->for_reclaim) {
> -               WARN_ON_ONCE(1);        /* Still happens? Tell us about it! */
> -               goto redirty;
> -       }
> -
>         /*
>          * This is somewhat ridiculous, but without plumbing a SWAP_MAP_FALLOC
>          * value into swapfile.c, the only way we can correctly account for a
> --
> 2.39.1
>