Re: [PATCH] mm: Do not reclaim private data from pinned page

From: Andrew Morton <akpm@linux-foundation.org>
To: Jan Kara <jack@suse.cz>
Cc: <linux-mm@kvack.org>, <linux-fsdevel@vger.kernel.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm: Do not reclaim private data from pinned page
Date: Tue, 2 May 2023 13:20:20 -0700	[thread overview]
Message-ID: <20230502132020.5a720158307c11d5b8efe1d9@linux-foundation.org> (raw)
In-Reply-To: <20230428124140.30166-1-jack@suse.cz>

On Fri, 28 Apr 2023 14:41:40 +0200 Jan Kara <jack@suse.cz> wrote:

> If the page is pinned, there's no point in trying to reclaim it.
> Furthermore if the page is from the page cache we don't want to reclaim
> fs-private data from the page because the pinning process may be writing
> to the page at any time and reclaiming fs private info on a dirty page
> can upset the filesystem (see link below).

Obviously I'll add a cc:stable here.  I'm suspecting it's so old that
there's no real Fixes: target that makes sense?

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1901,6 +1901,16 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  			}
>  		}
>  
> +		/*
> +		 * Folio is unmapped now so it cannot be newly pinned anymore.
> +		 * No point in trying to reclaim folio if it is pinned.
> +		 * Furthermore we don't want to reclaim underlying fs metadata
> +		 * if the folio is pinned and thus potentially modified by the
> +		 * pinning process as that may upset the filesystem.
> +		 */
> +		if (folio_maybe_dma_pinned(folio))
> +			goto activate_locked;
> +

So I expect the -stable maintainers will be looking for a pre-folios
version of this when the time comes.