linux-kernel - QUEUE_FLAG_NO_SG_MERGE and non-block-mq

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Date:	Thu, 26 Nov 2015 09:13:25 +0100
From:	Hannes Reinecke <hare@...e.de>
To:	Jens Axboe <axboe@...com>
CC:	Ming Lei <ming.lei@...onical.com>, Christoph Hellwig <hch@....de>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	"linux-scsi@...r.kernel.org" <Linux-scsi@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: QUEUE_FLAG_NO_SG_MERGE and non-block-mq

Hi all,

while investigating the crash in scsi_lib.c I found a rather curious
behaviour for QUEUE_FLAG_NO_SG_MERGE.

While the flag is evaluated in blk_recalc_rq_segments and
blk_recount_segments (resulting in nr_phys_segments being
computed based on that flag) it is completely ignored
during blk_rq_map_sg() or the actual merging itself.

This typically shouldn't be an issue, seeing that with
QUEUE_FLAG_NO_SG_MERGE nr_phys_segments will always be
larger than the actual segment count.

However, it still makes me wonder:
What is the point of having a QUEUE_FLAG_NO_SG_MERGE
which doesn't work as advertised?
Or, to be precise, which only works for blk-mq?
Should we make it work for non-block-mq, too?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
hare@...e.de			               +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton
HRB 21284 (AG Nürnberg)
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