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

From: John Hubbard <jhubbard@nvidia.com>
To: Jan Kara <jack@suse.cz>, linux-mm@kvack.org
Cc: linux-fsdevel@vger.kernel.org,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@infradead.org>,
	David Hildenbrand <david@redhat.com>
Subject: Re: [PATCH] mm: Do not reclaim private data from pinned page
Date: Mon, 1 May 2023 11:12:31 -0700	[thread overview]
Message-ID: <b8ace783-a102-f6fe-3744-91deb3993184@nvidia.com> (raw)
In-Reply-To: <20230428124140.30166-1-jack@suse.cz>

On 4/28/23 05:41, Jan Kara 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).
> 
> Link: https://lore.kernel.org/linux-mm/20180103100430.GE4911@quack2.suse.cz
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  mm/vmscan.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> This was the non-controversial part of my series [1] dealing with pinned pages
> in filesystems. It is already a win as it avoids crashes in the filesystem and
> we can drop workarounds for this in ext4. Can we merge it please?
> 
> [1] https://lore.kernel.org/all/20230209121046.25360-1-jack@suse.cz/
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bf3eedf0209c..401a379ea99a 100644
> --- 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;
> +

This is huge! At long last. In fact, with this in the queue, I'm going to close
out our internal bug report from 2018 that launched this whole maybe-dma-pinned 
odyssey. :)

Reviewed-by: John Hubbard <jhubbard@nvidia.com>

thanks,
-- 
John Hubbard
NVIDIA

>  		mapping = folio_mapping(folio);
>  		if (folio_test_dirty(folio)) {
>  			/*