Re: [PATCH 1/3] xfs: force writes to delalloc regions to unwritten

[Brian Foster <bfoster@redhat.com>]

On Mon, May 18, 2020 at 05:49:11PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> 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.
> 

We could probably add generic/042 into the auto group once this patch
lands.

> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/libxfs/xfs_bmap.c |   28 +++++++++++++++++-----------
>  1 file changed, 17 insertions(+), 11 deletions(-)
> 
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index fda13cd7add0..825d170e1503 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
...
> @@ -4611,8 +4601,24 @@ 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.
> +	 */
>  	if (whichfork == XFS_COW_FORK)
>  		bma.flags = XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC;
> +	else
> +		bma.flags = XFS_BMAPI_PREALLOC;

The following seems a bit cleaner:

	bma.flags = XFS_BMAPI_PREALLOC;
	if (whichfork == XFS_COW_FORK)
		bma.flags |= XFS_BMAPI_COWFORK;

... but nit aside, LGTM:

Reviewed-by: Brian Foster <bfoster@redhat.com>

>  
>  	if (!xfs_iext_peek_prev_extent(ifp, &bma.icur, &bma.prev))
>  		bma.prev.br_startoff = NULLFILEOFF;
>