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

[Guenter Roeck <linux@roeck-us.net>]

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 merge with this request, then the old tables are no longer valid.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 <axboe@kernel.dk>

Tested-by: Guenter Roeck <linux@roeck-us.net>

... on two systems affected by the problem.

> 
> 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;