linux-kernel - Re: [PATCH AUTOSEL 5.7 244/274] xfs: force writes to delalloc regions to unwritten

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Date:   Mon, 8 Jun 2020 18:07:27 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Brian Foster <bfoster@...hat.com>, linux-xfs@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 5.7 244/274] xfs: force writes to delalloc
 regions to unwritten

On Mon, Jun 08, 2020 at 07:05:37PM -0400, Sasha Levin wrote:
> From: "Darrick J. Wong" <darrick.wong@...cle.com>
> 
> [ Upstream commit a5949d3faedf492fa7863b914da408047ab46eb0 ]
> 
> When writing to a delalloc region in the data fork, commit the new
> allocations (of the da reservation) as unwritten so that the mappings
> are only marked written once writeback completes successfully.  This
> fixes the problem of stale data exposure if the system goes down during
> targeted writeback of a specific region of a file, as tested by
> generic/042.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Reviewed-by: Brian Foster <bfoster@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>

Err, this doesn't have a Fixes: tag attached to it.  Does it pass
fstests?  Because it doesn't look like you've pulled in "xfs: don't fail
unwritten extent conversion on writeback due to edquot", which is needed
to avoid regressing fstests...

...waitaminute, that whole series lacks Fixes: tags because it wasn't
considered a good enough candidate for automatic backport.

Ummm, does the autosel fstests driver turn on quotas? ;)

--D

> ---
>  fs/xfs/libxfs/xfs_bmap.c | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index fda13cd7add0..f8fe83c9348d 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -4193,17 +4193,7 @@ xfs_bmapi_allocate(
>  	bma->got.br_blockcount = bma->length;
>  	bma->got.br_state = XFS_EXT_NORM;
>  
> -	/*
> -	 * In the data fork, a wasdelay extent has been initialized, so
> -	 * shouldn't be flagged as unwritten.
> -	 *
> -	 * For the cow fork, however, we convert delalloc reservations
> -	 * (extents allocated for speculative preallocation) to
> -	 * allocated unwritten extents, and only convert the unwritten
> -	 * extents to real extents when we're about to write the data.
> -	 */
> -	if ((!bma->wasdel || (bma->flags & XFS_BMAPI_COWFORK)) &&
> -	    (bma->flags & XFS_BMAPI_PREALLOC))
> +	if (bma->flags & XFS_BMAPI_PREALLOC)
>  		bma->got.br_state = XFS_EXT_UNWRITTEN;
>  
>  	if (bma->wasdel)
> @@ -4611,8 +4601,23 @@ xfs_bmapi_convert_delalloc(
>  	bma.offset = bma.got.br_startoff;
>  	bma.length = max_t(xfs_filblks_t, bma.got.br_blockcount, MAXEXTLEN);
>  	bma.minleft = xfs_bmapi_minleft(tp, ip, whichfork);
> +
> +	/*
> +	 * When we're converting the delalloc reservations backing dirty pages
> +	 * in the page cache, we must be careful about how we create the new
> +	 * extents:
> +	 *
> +	 * New CoW fork extents are created unwritten, turned into real extents
> +	 * when we're about to write the data to disk, and mapped into the data
> +	 * fork after the write finishes.  End of story.
> +	 *
> +	 * New data fork extents must be mapped in as unwritten and converted
> +	 * to real extents after the write succeeds to avoid exposing stale
> +	 * disk contents if we crash.
> +	 */
> +	bma.flags = XFS_BMAPI_PREALLOC;
>  	if (whichfork == XFS_COW_FORK)
> -		bma.flags = XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC;
> +		bma.flags |= XFS_BMAPI_COWFORK;
>  
>  	if (!xfs_iext_peek_prev_extent(ifp, &bma.icur, &bma.prev))
>  		bma.prev.br_startoff = NULLFILEOFF;
> -- 
> 2.25.1
>