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	Mon, 18 Aug 2025 15:21:25 +0000
Date: Mon, 18 Aug 2025 16:21:25 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Trond Myklebust <trondmy@kernel.org>
Cc: Anna Schumaker <anna@kernel.org>, linux-nfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, Mike Snitzer <snitzer@kernel.org>
Subject: Re: [PATCH v3 2/2] NFS: Enable the RWF_DONTCACHE flag for the NFS
 client
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References: <cover.1755527537.git.trond.myklebust@hammerspace.com>
 <001e5575d7ddbcdb925626151a7dcc7353445543.1755527537.git.trond.myklebust@hammerspace.com>
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In-Reply-To: <001e5575d7ddbcdb925626151a7dcc7353445543.1755527537.git.trond.myklebust@hammerspace.com>

On Mon, Aug 18, 2025 at 07:39:50AM -0700, Trond Myklebust wrote:
> @@ -349,8 +349,12 @@ static void nfs_folio_end_writeback(struct folio *folio)
>  static void nfs_page_end_writeback(struct nfs_page *req)
>  {
>  	if (nfs_page_group_sync_on_bit(req, PG_WB_END)) {
> +		struct folio *folio = nfs_page_to_folio(req);
> +
> +		if (folio_test_clear_dropbehind(folio))
> +			set_bit(PG_DROPBEHIND, &req->wb_head->wb_flags);
>  		nfs_unlock_request(req);
> @@ -787,8 +791,15 @@ static void nfs_inode_remove_request(struct nfs_page *req)
>  			clear_bit(PG_MAPPED, &req->wb_head->wb_flags);
>  		}
>  		spin_unlock(&mapping->i_private_lock);
> -	}
> -	nfs_page_group_unlock(req);
> +		nfs_page_group_unlock(req);
> +
> +		if (test_and_clear_bit(PG_DROPBEHIND,
> +				       &req->wb_head->wb_flags)) {
> +			folio_set_dropbehind(folio);
> +			folio_end_dropbehind(folio);
> +		}

I don't think this technique is "safe".  By clearing the flag early,
the page cache can't see that a folio that was created by dropbehind
has now been reused and should have its dropbehind flag cleared.  So we
might see pages dropped from the cache that really should not be.