On 1/28/22 4:03 PM, Darrick J. Wong wrote:
On Fri, Jan 28, 2022 at 02:31:11PM -0600, Eric Sandeen wrote:
On 1/19/22 6:21 PM, Darrick J. Wong wrote:
From: Darrick J. Wong <djwong@xxxxxxxxxx>
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.
In this case b_nmaps must already be 1, right. And it's the bn and
length in b_maps[0] that fail to be initialized?
I wonder, then, if it's any more clear to reorganize it just a little bit,
like:
if (!bp->b_maps) {
bp->b_maps = &bp->__b_map;
bp->b_nmaps = 1;
}
if (bp->b_maps == &bp->__b_map) {
bp->b_maps[0].bm_bn = bp->b_bn;
bp->b_maps[0].bm_len = bp->b_length;
}
because AFAICT b_nmaps only needs to be reset to 1 if we didn't already
get here with b_maps == &__b_map?
That would also work, though it's less obvious (to me anyway) that
b_nmaps is always 1 when bp->b_maps == &bp->__b_map.
Ok. I'll leave it alone.
-Eric
--D
If this is just navel-gazing I can leave it as is. If you think it's
any clearer, I'll make the change. (or if I've gotten it completely wrong,
sorry!)
Thanks,
-Eric
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@xxxxxxxxxx>
---
libxfs/rdwr.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/libxfs/rdwr.c b/libxfs/rdwr.c
index 5086bdbc..a55e3a79 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;
}
