Re: [PATCH] blk-mq: fix corruption with direct issue

[Ming Lei]

On Tue, Dec 04, 2018 at 03:47:46PM -0700, Jens Axboe wrote:
> If we attempt a direct issue to a SCSI device, and it returns BUSY, then
> we queue the request up normally. However, the SCSI layer may have
> already setup SG tables etc for this particular command. If we later
> merge with this request, then the old tables are no longer valid. Once
> we issue the IO, we only read/write the original part of the request,
> not the new state of it.
> 
> This causes data corruption, and is most often noticed with the file
> system complaining about the just read data being invalid:
> 
> [  235.934465] EXT4-fs error (device sda1): ext4_iget:4831: inode #7142: comm 
> dpkg-query: bad extra_isize 24937 (inode size 256)
> 
> because most of it is garbage...
> 
> This doesn't happen from the normal issue path, as we will simply defer
> the request to the hardware queue dispatch list if we fail. Once it's on
> the dispatch list, we never merge with it.
> 
> Fix this from the direct issue path by flagging the request as
> REQ_NOMERGE so we don't change the size of it before issue.
> 
> See also:
>   https://bugzilla.kernel.org/show_bug.cgi?id=201685
> 
> Fixes: 6ce3dd6eec1 ("blk-mq: issue directly if hw queue isn't busy in case of 
> 'none'")
> Signed-off-by: Jens Axboe <ax...@kernel.dk>
> 
> ---
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 3f91c6e5b17a..d8f518c6ea38 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1715,6 +1715,15 @@ static blk_status_t __blk_mq_issue_directly(struct 
> blk_mq_hw_ctx *hctx,
>               break;
>       case BLK_STS_RESOURCE:
>       case BLK_STS_DEV_RESOURCE:
> +             /*
> +              * If direct dispatch fails, we cannot allow any merging on
> +              * this IO. Drivers (like SCSI) may have set up permanent state
> +              * for this request, like SG tables and mappings, and if we
> +              * merge to it later on then we'll still only do IO to the
> +              * original part.
> +              */
> +             rq->cmd_flags |= REQ_NOMERGE;
> +
>               blk_mq_update_dispatch_busy(hctx, true);
>               __blk_mq_requeue_request(rq);
>               break;
> 

Not sure it is enough to just mark it as NOMERGE, for example, driver
may have setup the .special_vec for discard, and NOMERGE may not prevent
request from entering elevator queue completely. Cause 'rq.rb_node' and
'rq.special_vec' share same space.

So how about inserting this request via blk_mq_request_bypass_insert()
in case that direct issue returns BUSY? Then it is invariant that
any request queued via .queue_rq() won't enter scheduler queue.

--

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3f91c6e5b17a..4b2db0b2909e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1764,7 +1764,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct 
blk_mq_hw_ctx *hctx,
        if (bypass_insert)
                return BLK_STS_RESOURCE;
 
-       blk_mq_sched_insert_request(rq, false, run_queue, false);
+       blk_mq_request_bypass_insert(rq, run_queue);
        return BLK_STS_OK;
 }
 
@@ -1780,7 +1780,7 @@ static void blk_mq_try_issue_directly(struct 
blk_mq_hw_ctx *hctx,
 
        ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false);
        if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE)
-               blk_mq_sched_insert_request(rq, false, true, false);
+               blk_mq_request_bypass_insert(rq, true);
        else if (ret != BLK_STS_OK)
                blk_mq_end_request(rq, ret);
 


Thanks,
Ming