linux-kernel - Re: [PATCH 05/16] blk-mq: allow software queue to map to multiple hardware queues

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Date:   Wed, 31 Oct 2018 08:19:10 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Sagi Grimberg <sagi@...mberg.me>, linux-block@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/16] blk-mq: allow software queue to map to multiple
 hardware queues

On 10/30/18 6:49 PM, Sagi Grimberg wrote:
> 
> 
>> @@ -2342,10 +2346,16 @@ static void blk_mq_map_swqueue(struct request_queue *q)
>>   
>>   		ctx = per_cpu_ptr(q->queue_ctx, i);
>>   		hctx = blk_mq_map_queue_type(q, 0, i);
>> -
>> +		hctx->type = 0;
>>   		cpumask_set_cpu(i, hctx->cpumask);
>> -		ctx->index_hw = hctx->nr_ctx;
>> +		ctx->index_hw[hctx->type] = hctx->nr_ctx;
>>   		hctx->ctxs[hctx->nr_ctx++] = ctx;
>> +
>> +		/*
>> +		 * If the nr_ctx type overflows, we have exceeded the
>> +		 * amount of sw queues we can support.
>> +		 */
>> +		BUG_ON(!hctx->nr_ctx);
> 
> Was this added because nr_ctx became short? What was the motivation
> for that? save space? structure alignment?

Right, I made it smaller to not take up more space after this change.
And since it is now smaller, adding a check for wrap seemed like
a good idea.

> Perhaps this should be mentioned in the change log (as it contradicts
> that no functional changes exist in this patch). Or, if this can happen
> even when 32bit nr_ctx, it should be a separate small patch?

Could happen for any type, but more likely now of course. I'll
mention this in the changelog.

-- 
Jens Axboe