Re: [PATCH 2/2] loop: handle short DIO reads

[Ming Lei]

On Fri, Apr 13, 2018 at 04:30:36PM -0600, Jens Axboe wrote:
> 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.
> 
> Signed-off-by: Jens Axboe 
> ---
>  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)
> -- 
> 2.7.4
> 

Looks fine, short read will be guaranteed to complete when zero read
is triggered.

Reviewed-by: Ming Lei 

-- 
Ming

[PATCH 2/2] loop: handle short DIO reads

[Jens Axboe]

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.

Signed-off-by: Jens Axboe 
---
 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)
-- 
2.7.4