From: Jens Axboe <axboe@kernel.dk>
Date: Fri, 13 Apr 2018 16:25:57 -0600
Subject: [PATCH] loop: handle short DIO reads
Git-commit: f9de14bc7e7aac77fd44bb2f62206eb9e494f0d2
Patch-mainline: v4.17-rc3
References: bsc#1052766

We ran into an issue with loop and btrfs, where btrfs would complain about
checksum errors. It turns out that is because we don't handle short reads
at all, we just zero fill the remainder. Worse than that, we don't handle
the filling properly, which results in loop trying to advance a single
bio by much more than its size, since it doesn't take chaining into
account.

Handle short reads appropriately, by simply retrying at the new correct
offset. End the remainder of the request with EIO, if we get a 0 read.

Fixes: bc07c10a3603 ("block: loop: support DIO & AIO")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Jan Kara <jack@suse.cz>
---
 drivers/block/loop.c | 34 +++++++++++++++++++++++++++-------
 1 file changed, 27 insertions(+), 7 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 8b2fde2109fc..5d4e31655d96 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -451,16 +451,36 @@ static int lo_req_flush(struct loop_device *lo, struct request *rq)
 static void lo_complete_rq(struct request *rq)
 {
 	struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);
+	blk_status_t ret = BLK_STS_OK;
 
-	if (unlikely(req_op(rq) == REQ_OP_READ && cmd->use_aio &&
-		     cmd->ret >= 0 && cmd->ret < blk_rq_bytes(rq))) {
-		struct bio *bio = rq->bio;
-
-		bio_advance(bio, cmd->ret);
-		zero_fill_bio(bio);
+	if (!cmd->use_aio || cmd->ret < 0 || cmd->ret == blk_rq_bytes(rq) ||
+	    req_op(rq) != REQ_OP_READ) {
+		if (cmd->ret < 0)
+			ret = BLK_STS_IOERR;
+		goto end_io;
 	}
 
-	blk_mq_end_request(rq, cmd->ret < 0 ? BLK_STS_IOERR : BLK_STS_OK);
+	/*
+	 * Short READ - if we got some data, advance our request and
+	 * retry it. If we got no data, end the rest with EIO.
+	 */
+	if (cmd->ret) {
+		blk_update_request(rq, BLK_STS_OK, cmd->ret);
+		cmd->ret = 0;
+		blk_mq_requeue_request(rq, true);
+	} else {
+		if (cmd->use_aio) {
+			struct bio *bio = rq->bio;
+
+			while (bio) {
+				zero_fill_bio(bio);
+				bio = bio->bi_next;
+			}
+		}
+		ret = BLK_STS_IOERR;
+end_io:
+		blk_mq_end_request(rq, ret);
+	}
 }
 
 static void lo_rw_aio_do_completion(struct loop_cmd *cmd)
-- 
2.16.4