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

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Message-ID: <2a3386e7-9b09-f22e-ee39-a37a046d54ec@redhat.com>
Date:   Mon, 6 Mar 2023 15:01:52 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Luis Chamberlain <mcgrof@...nel.org>, hughd@...gle.com,
        akpm@...ux-foundation.org, willy@...radead.org, brauner@...nel.org
Cc:     linux-mm@...ck.org, p.raghav@...sung.com, da.gomez@...sung.com,
        a.manzanares@...sung.com, dave@...olabs.net, yosryahmed@...gle.com,
        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 03.03.23 00:27, Luis Chamberlain 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>
> ---
>   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) {

if (WARN_ON_ONCE(!wbc->for_reclaim))

> +		WARN_ON_ONCE(1);	/* Still happens? Tell us about it! */

And drop the comment :) That's what WARN_ON_ONCE is all about.

> +		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

LGTM

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb