[PATCH v3] xfs: ensure st_blocks never goes to zero during COW writes

[PATCH v3] xfs: ensure st_blocks never goes to zero during COW writes

[Christoph Hellwig]

COW writes remove the amount overwritten either directly for delalloc
reservations, or in earlier deferred transactions than adding the new
amount back in the bmap map transaction.  This means st_blocks on an
inode where all data is overwritten using the COW path can temporarily
show a 0 st_blocks.  This can easily be reproduced with the pending
zoned device support where all writes use this path and trips the
check in generic/615, but could also happen on a reflink file without
that.

Fix this by temporarily add the pending blocks to be mapped to
i_delayed_blks while the item is queued.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
---

Changes since v2:
 - even better comments!

Changes since v1:
 - slightly more and slightly improved comments

 fs/xfs/libxfs/xfs_bmap.c |  1 +
 fs/xfs/xfs_bmap_item.c   | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index 88c62e1158ac73..685a3278878ada 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -4895,6 +4895,7 @@ xfs_bmapi_remap(
 	}
 
 	ip->i_nblocks += len;
+	ip->i_delayed_blks -= len; /* see xfs_bmap_defer_add */
 	xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
 
 	if (ifp->if_format == XFS_DINODE_FMT_BTREE)
diff --git a/fs/xfs/xfs_bmap_item.c b/fs/xfs/xfs_bmap_item.c
index 9a7e97a922b6d3..43be8e04cf0ead 100644
--- a/fs/xfs/xfs_bmap_item.c
+++ b/fs/xfs/xfs_bmap_item.c
@@ -357,6 +357,17 @@ xfs_bmap_defer_add(
 	trace_xfs_bmap_defer(bi);
 
 	xfs_bmap_update_get_group(tp->t_mountp, bi);
+
+	/*
+	 * Ensure the deferred mapping is pre-recorded in i_delayed_blks.
+	 *
+	 * Otherwise stat can report zero blocks for an inode that actually has
+	 * data when the entire mapping is in the process of being overwritten
+	 * using the out of place write path. This is undone in xfs_bmapi_remap
+	 * after it has incremented di_nblocks for a successful operation.
+	 */
+	if (bi->bi_type == XFS_BMAP_MAP)
+		bi->bi_owner->i_delayed_blks += bi->bi_bmap.br_blockcount;
 	xfs_defer_add(tp, &bi->bi_list, &xfs_bmap_update_defer_type);
 }
 
@@ -381,6 +392,9 @@ xfs_bmap_update_cancel_item(
 {
 	struct xfs_bmap_intent		*bi = bi_entry(item);
 
+	if (bi->bi_type == XFS_BMAP_MAP)
+		bi->bi_owner->i_delayed_blks -= bi->bi_bmap.br_blockcount;
+
 	xfs_bmap_update_put_group(bi);
 	kmem_cache_free(xfs_bmap_intent_cache, bi);
 }
@@ -478,6 +492,9 @@ xfs_bui_recover_work(
 	bi->bi_owner = *ipp;
 	xfs_bmap_update_get_group(mp, bi);
 
+	/* see xfs_bmap_defer_add for details */
+	if (bi->bi_type == XFS_BMAP_MAP)
+		bi->bi_owner->i_delayed_blks += bi->bi_bmap.br_blockcount;
 	xfs_defer_add_item(dfp, &bi->bi_list);
 	return bi;
 }
-- 
2.43.0

Re: [PATCH v3] xfs: ensure st_blocks never goes to zero during COW writes

[Dave Chinner]

On Tue, Aug 27, 2024 at 07:03:21AM +0200, Christoph Hellwig wrote:
> COW writes remove the amount overwritten either directly for delalloc
> reservations, or in earlier deferred transactions than adding the new
> amount back in the bmap map transaction.  This means st_blocks on an
> inode where all data is overwritten using the COW path can temporarily
> show a 0 st_blocks.  This can easily be reproduced with the pending
> zoned device support where all writes use this path and trips the
> check in generic/615, but could also happen on a reflink file without
> that.
> 
> Fix this by temporarily add the pending blocks to be mapped to
> i_delayed_blks while the item is queued.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>

Looks good to me.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com