md/raid1: don't split discard io for write behind

Currently, discad io is treated the same as normal write io, and for
write behind case, io size is limited to:

BIO_MAX_VECS * (PAGE_SIZE >> 9)

For 0.5KB sector size and 4KB PAGE_SIZE, this is just 1MB. For
consequence, if 'WriteMostly' is set to one of the underlying disks,
then diskcard io will be splited into 1MB and it will take a long time
for the diskcard to finish.

Fix this problem by disable write behind for discard io.

Reported-by: Roman Mamedov <rm@romanrm.net>
Closes: https://lore.kernel.org/all/6a1165f7-c792-c054-b8f0-1ad4f7b8ae01@ultracoder.org/
Reported-and-tested-by: Kirill Kirilenko <kirill@ultracoder.org>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231007112105.407449-1-yukuai1@huaweicloud.com

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 3a78f79ee6d5da075199e17a200d21117be31a15..35d12948e0a963bb0786c19b0c4ad2a5757de876 100644 (file)
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1345,6 +1345,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
        int first_clone;
        int max_sectors;
        bool write_behind = false;
+       bool is_discard = (bio_op(bio) == REQ_OP_DISCARD);
 
        if (mddev_is_clustered(mddev) &&
             md_cluster_ops->area_resyncing(mddev, WRITE,
@@ -1405,7 +1406,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
                 * write-mostly, which means we could allocate write behind
                 * bio later.
                 */
-               if (rdev && test_bit(WriteMostly, &rdev->flags))
+               if (!is_discard && rdev && test_bit(WriteMostly, &rdev->flags))
                        write_behind = true;
 
                if (rdev && unlikely(test_bit(Blocked, &rdev->flags))) {