Re: [PATCH V2] blk-mq: insert passthrough request into hctx->dispatch directly

[Jens Axboe <axboe@kernel.dk>]

On 2/24/20 6:04 PM, Ming Lei wrote:
> For some reason, device may be in one situation which can't handle
> FS request, so STS_RESOURCE is always returned and the FS request
> will be added to hctx->dispatch. However passthrough request may
> be required at that time for fixing the problem. If passthrough
> request is added to scheduler queue, there isn't any chance for
> blk-mq to dispatch it given we prioritize requests in hctx->dispatch.
> Then the FS IO request may never be completed, and IO hang is caused.
> 
> So passthrough request has to be added to hctx->dispatch directly
> for fixing the IO hang.
> 
> Fix this issue by inserting passthrough request into hctx->dispatch
> directly together withing adding FS request to the tail of
> hctx->dispatch in blk_mq_dispatch_rq_list(). Actually we add FS request
> to tail of hctx->dispatch at default, see blk_mq_request_bypass_insert().
> 
> Then it becomes consistent with original legacy IO request
> path, in which passthrough request is always added to q->queue_head.

Applied, thanks Ming.

-- 
Jens Axboe