[RFC PATCH v1 09/17] dm bufio: avoid blkg association from GFP_NOWAIT bio init

[Yu Kuai]

From: Yu Kuai <yukuai at fygo.io>

dm-bufio allocates a bio with bio_kmalloc(GFP_NOWAIT) and then initializes it
with the target bdev.  That initialization can attach blkcg state and sleep to
create a missing blkg once blkg lookup is protected by q->blkcg_mutex.

Initialize the bio without a bdev, set the bdev fields, and associate blkcg
with nowait=true.  Fall back to dm_io if a missing blkg would need to be
created.

Signed-off-by: Yu Kuai <yukuai at fygo.io>
---
 drivers/md/dm-bufio.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index 26fedf5883ef..2002d9020dd6 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -1347,7 +1347,14 @@ static void use_bio(struct dm_buffer *b, enum req_op op, sector_t sector,
 		use_dmio(b, op, sector, n_sectors, offset, ioprio);
 		return;
 	}
-	bio_init_inline(bio, b->c->bdev, 1, op);
+	bio_init_inline(bio, NULL, 1, op);
+	bio_set_dev_no_blkg(bio, b->c->bdev);
+	if (!bio_associate_blkg(bio, true)) {
+		bio_uninit(bio);
+		kfree(bio);
+		use_dmio(b, op, sector, n_sectors, offset, ioprio);
+		return;
+	}
 	bio->bi_iter.bi_sector = sector;
 	bio->bi_end_io = bio_complete;
 	bio->bi_private = b;
-- 
2.51.0