xfs: ensure st_blocks never goes to zero during COW writes

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 <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Reviewed-by: Dave Chinner <[email protected]>
Signed-off-by: Chandan Babu R <[email protected]>

fs/xfs/libxfs/xfs_bmap.c

@@ -4856,6 +4856,7 @@ 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)

fs/xfs/xfs_bmap_item.c

@@ -346,6 +346,17 @@ 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);
 }
 
@@ -367,6 +378,9 @@ 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);
 }
@@ -464,6 +478,9 @@ 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;
 }