[PATCH 5.15 071/917] md/raid1: only allocate write behind bio for WriteMostly device

From: Guoqing Jiang <redacted>

commit fd3b6975e9c11c4fa00965f82a0bfbb3b7b44101 upstream.

Commit 6607cd319b6b91bff94e90f798a61c031650b514 ("raid1: ensure write
behind bio has less than BIO_MAX_VECS sectors") tried to guarantee the
size of behind bio is not bigger than BIO_MAX_VECS sectors.

Unfortunately the same calltrace still could happen since an array could
enable write-behind without write mostly device.

To match the manpage of mdadm (which says "write-behind is only attempted
on drives marked as write-mostly"), we need to check WriteMostly flag to
avoid such unexpected behavior.

[1]. https://bugzilla.kernel.org/show_bug.cgi?id=213181#c25

Cc: stable@vger.kernel.org # v5.12+
Cc: Jens Stutte <redacted>
Reported-by: Jens Stutte <redacted>
Signed-off-by: Guoqing Jiang <redacted>
Signed-off-by: Song Liu <redacted>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/md/raid1.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c

@@ -1496,7 +1496,7 @@ static void raid1_write_request(struct m
 		if (!r1_bio->bios[i])
 			continue;
 
-		if (first_clone) {
+		if (first_clone && test_bit(WriteMostly, &rdev->flags)) {
 			/* do behind I/O ?
 			 * Not if there are too many, or cannot
 			 * allocate memory, or a reader on WriteMostly