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QUEUE_FLAG_NO_SG_MERGE and non-block-mq

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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@suse.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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QUEUE_FLAG_NO_SG_MERGE and non-block-mq Hannes Reinecke <hare@suse.de> - 2015-11-26 09:20 +0100
  Re: QUEUE_FLAG_NO_SG_MERGE and non-block-mq Ming Lei <ming.lei@canonical.com> - 2015-11-26 10:30 +0100
    Re: QUEUE_FLAG_NO_SG_MERGE and non-block-mq Hannes Reinecke <hare@suse.de> - 2015-11-27 15:40 +0100
      Re: QUEUE_FLAG_NO_SG_MERGE and non-block-mq Jens Axboe <axboe@fb.com> - 2015-11-27 17:20 +0100

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