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From: Christoph Hellwig <hch@lst.de>
To: axboe@fb.com
Cc: ming.lei@redhat.com, linux-block@vger.kernel.org
Subject: fix nr_phys_segments vs iterators accounting v2
Date: Thu, 16 May 2019 10:40:54 +0200
Message-ID: <20190516084058.20678-1-hch@lst.de> (raw)

Hi all,

we have had a problem for a while where the number of segments that
the bvec iterators will iterate over don't match the value in
req->nr_phys_segments, causing problems for anyone looking at
nr_phys_segments and iterating over bvec directly instead of using
blk_rq_map_sg.  The first patch in this series fixes this by
making sure nr_phys_segments matches the actual number of segments.
Drivers using blk_rq_map_sg will still get the lower number returned
from function eventually, but the fact that we don't reduce the
value earlier will not allow some merges that we might otherwise
allow.

With that in place I also noticed that we do not properly account
segements sizes on devices with a virt_boundary, but it turns out that
segment sizes fundamentally don't make sense for such devices, as their
"segment" is a fixed size "device page", and not a variable sized
scatter/gather elements as in the block layer, so we make that fact
formal.

Once all that is sorted out it is pretty clear that there is no
good reason to have the front/back segement accounting to start
with.

Changes since v1:
  - update a commit log
  - add fixes tags
  - drop the follow on patches not suitable for 5.2

             reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16  8:40 Christoph Hellwig [this message]
2019-05-16  8:40 ` [PATCH 1/4] block: don't decrement nr_phys_segments for physically contigous segments Christoph Hellwig
2019-05-16  8:48   ` Hannes Reinecke
2019-05-16 13:17   ` Ming Lei
2019-05-17 23:02     ` Ming Lei
2019-05-20 11:11     ` Christoph Hellwig
2019-05-21  1:04       ` Ming Lei
2019-05-16  8:40 ` [PATCH 2/4] block: force an unlimited segment size on queues with a virt boundary Christoph Hellwig
2019-05-16  8:49   ` Hannes Reinecke
2019-05-16  8:40 ` [PATCH 3/4] block: remove the segment size check in bio_will_gap Christoph Hellwig
2019-05-16  8:49   ` Hannes Reinecke
2019-05-16  8:40 ` [PATCH 4/4] block: remove the bi_seg_{front,back}_size fields in struct bio Christoph Hellwig
2019-05-16  8:50   ` Hannes Reinecke
2019-05-20 11:17 ` fix nr_phys_segments vs iterators accounting v2 Christoph Hellwig
2019-05-21  1:09   ` Jens Axboe
2019-05-21  1:17     ` Ming Lei
2019-05-21  1:20       ` Jens Axboe
2019-05-21  1:29         ` Ming Lei
2019-05-21  5:11           ` Christoph Hellwig

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