On Thu, Dec 06, 2018 at 10:17:44PM -0700, Jens Axboe 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: sta...@vger.kernel.org
> Suggested-by: Ming Lei <ming....@redhat.com>
> Reported-by: Bart Van Assche <bvanass...@acm.org>
> Signed-off-by: Jens Axboe <ax...@kernel.dk>
>
> ---
>
> I've thrown the initial hang test reported by Bart at it, works fine.
> My reproducer for the corruption case is also happy, as expected.
>
> I'm running blktests and xfstests on it overnight. If that passes as
> expected, this qualms my initial worries on using ->dispatch as a
> holding place for these types of requests.
>
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 3262d83b9e07..6a7566244de3 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1715,15 +1715,6 @@ 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;
> @@ -1736,18 +1727,6 @@ static blk_status_t __blk_mq_issue_directly(struct
> blk_mq_hw_ctx *hctx,
> return ret;
> }
>
> -/*
> - * Don't allow direct dispatch of anything but regular reads/writes,
> - * as some of the other commands can potentially share request space
> - * with data we need for the IO scheduler. If we attempt a direct dispatch
> - * on those and fail, we can't safely add it to the scheduler afterwards
> - * without potentially overwriting data that the driver has already written.
> - */
> -static bool blk_rq_can_direct_dispatch(struct request *rq)
> -{
> - return req_op(rq) == REQ_OP_READ || req_op(rq) == REQ_OP_WRITE;
> -}
> -
> static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> struct request *rq,
> blk_qc_t *cookie,
> @@ -1769,7 +1748,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct
> blk_mq_hw_ctx *hctx,
> goto insert;
> }
>
> - if (!blk_rq_can_direct_dispatch(rq) || (q->elevator && !bypass_insert))
> + if (q->elevator && !bypass_insert)
> goto insert;
>
> if (!blk_mq_get_dispatch_budget(hctx))
> @@ -1785,7 +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;
> }
>
> @@ -1801,7 +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);
>
> @@ -1831,15 +1810,13 @@ void blk_mq_try_issue_list_directly(struct
> blk_mq_hw_ctx *hctx,
> struct request *rq = list_first_entry(list, struct request,
> queuelist);
>
> - if (!blk_rq_can_direct_dispatch(rq))
> - break;
> -
> list_del_init(&rq->queuelist);
> ret = blk_mq_request_issue_directly(rq);
> if (ret != BLK_STS_OK) {
> if (ret == BLK_STS_RESOURCE ||
> ret == BLK_STS_DEV_RESOURCE) {
> - list_add(&rq->queuelist, list);
> + blk_mq_request_bypass_insert(rq,
> + list_empty(list));
> break;
> }
> blk_mq_end_request(rq, ret);
>
Tested-by: Ming Lei <ming....@redhat.com>
Thanks,
Ming
