[PATCH 39/43] libxfs: always initialize internal buffer map

From: Darrick J. Wong <djwong@kernel.org>

The __initbuf function is responsible for initializing the fields of an
xfs_buf.  Buffers are always required to have a mapping, though in the
typical case there's only one mapping, so we can use the internal one.

The single-mapping b_maps init code at the end of the function doesn't
quite get this right though -- if a single-mapping buffer in the cache
was allowed to expire and now is being repurposed, it'll come out with
b_maps == &__b_map, in which case we incorrectly skip initializing the
map.  This has gone unnoticed until now because (AFAICT) the code paths
that use b_maps are the same ones that are called with multi-mapping
buffers, which are initialized correctly.

Anyway, the improperly initialized single-mappings will cause problems
in upcoming patches where we turn b_bn into the cache key and require
the use of b_maps[0].bm_bn for the buffer LBA.  Fix this.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 libxfs/rdwr.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c
index f4e76029..03dc2917 100644
--- a/libxfs/rdwr.c
+++ b/libxfs/rdwr.c

@@ -251,9 +251,11 @@ __initbuf(struct xfs_buf *bp, struct xfs_buftarg *btp, xfs_daddr_t bno,
 	bp->b_ops = NULL;
 	INIT_LIST_HEAD(&bp->b_li_list);
 
-	if (!bp->b_maps) {
-		bp->b_nmaps = 1;
+	if (!bp->b_maps)
 		bp->b_maps = &bp->__b_map;
+
+	if (bp->b_maps == &bp->__b_map) {
+		bp->b_nmaps = 1;
 		bp->b_maps[0].bm_bn = bp->b_bn;
 		bp->b_maps[0].bm_len = bp->b_length;
 	}