Re: [PATCH v3] blk-mq: punt failed direct issue to dispatch list

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Dec 07 2018 at 12:17am -0500,
Jens Axboe <axboe@kernel.dk> wrote:

> After the direct dispatch corruption fix, we permanently disallow direct
> dispatch of non read/write requests. This works fine off the normal IO
> path, as they will be retried like any other failed direct dispatch
> request. But for the blk_insert_cloned_request() that only DM uses to
> bypass the bottom level scheduler, we always first attempt direct
> dispatch. For some types of requests, that's now a permanent failure,
> and no amount of retrying will make that succeed. This results in a
> livelock.
> 
> Instead of making special cases for what we can direct issue, and now
> having to deal with DM solving the livelock while still retaining a BUSY
> condition feedback loop, always just add a request that has been through
> ->queue_rq() to the hardware queue dispatch list. These are safe to use
> as no merging can take place there. Additionally, if requests do have
> prepped data from drivers, we aren't dependent on them not sharing space
> in the request structure to safely add them to the IO scheduler lists.
> 
> This basically reverts ffe81d45322c and is based on a patch from Ming,
> but with the list insert case covered as well.
> 
> Fixes: ffe81d45322c ("blk-mq: fix corruption with direct issue")
> Cc: stable@vger.kernel.org
> Suggested-by: Ming Lei <ming.lei@redhat.com>
> Reported-by: Bart Van Assche <bvanassche@acm.org>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>

Looks good, thanks!

Acked-by: Mike Snitzer <snitzer@redhat.com>