Re: [PATCH 1/6] md/raid1: only allocate write behind bio for WriteMostly device

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>
---
  drivers/md/raid1.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 19598bd38939..6ba12f0f0f03 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c

@@ -1496,7 +1496,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
  		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