linux-ext4 - Re: [PATCH 5/8] nowait aio: return on congested block device

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Date:   Wed, 8 Mar 2017 09:03:08 +0200
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>, jack@...e.com
Cc:     hch@...radead.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 5/8] nowait aio: return on congested block device


> -		if (likely(blk_queue_enter(q, false) == 0)) {
> +		if (likely(blk_queue_enter(q, bio_flagged(bio, BIO_NOWAIT)) == 0)) {
>  			ret = q->make_request_fn(q, bio);

I think that for ->make_request to not block we'd need to set
BLK_MQ_REQ_NOWAIT in blk_mq_alloc_data to avoid blocking on a tag
allocation.

Something like the untested addition below:
--
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 09af8ff18719..40e78b57fb44 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -119,6 +119,9 @@ struct request *blk_mq_sched_get_request(struct 
request_queue *q,
         if (likely(!data->hctx))
                 data->hctx = blk_mq_map_queue(q, data->ctx->cpu);

+       if (likely(bio) && bio_flagged(bio, BIO_NOWAIT))
+               data->flags |= BLK_MQ_REQ_NOWAIT;
+
         if (e) {
                 data->flags |= BLK_MQ_REQ_INTERNAL;
--