linux-kernel - Re: [PATCH 4/6] shmem: skip page split if we're not reclaiming

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Date:   Mon, 6 Mar 2023 11:49:57 -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 4/6] shmem: skip page split if we're not reclaiming

On Thu, Mar 2, 2023 at 3:28 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
>
> In theory when info->flags & VM_LOCKED we should not be getting
> shem_writepage() called so we should be verifying this with a
> WARN_ON_ONCE(). Since we should not be swapping then best to ensure
> we also don't do the folio split earlier too. So just move the check
> early to avoid folio splits in case its a dubious call.
>
> We also have a similar early bail when !total_swap_pages so just move
> that earlier to avoid the possible folio split in the same situation.
>
> Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>

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

> ---
>  mm/shmem.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index a5a6da51087e..6006dbb7dbcb 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1352,6 +1352,12 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>                 goto redirty;
>         }
>
> +       if (WARN_ON_ONCE(info->flags & VM_LOCKED))
> +               goto redirty;
> +
> +       if (!total_swap_pages)
> +               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,
> @@ -1367,10 +1373,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>         }
>
>         index = folio->index;
> -       if (info->flags & VM_LOCKED)
> -               goto redirty;
> -       if (!total_swap_pages)
> -               goto redirty;
>
>         /*
>          * This is somewhat ridiculous, but without plumbing a SWAP_MAP_FALLOC
> --
> 2.39.1
>