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From: Christoph Hellwig <hch@lst.de>
To: axboe@fb.com
Cc: ming.lei@redhat.com, Matias Bjorling <mb@lightnvm.io>,
	linux-block@vger.kernel.org
Subject: fix nr_phys_segments vs iterators accounting
Date: Mon, 13 May 2019 08:37:44 +0200	[thread overview]
Message-ID: <20190513063754.1520-1-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, and that
the bio nr_phys_segments is only needed in the submission path,
so we can remove three members of struct bio.

             reply	other threads:[~2019-05-13  6:38 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-13  6:37 Christoph Hellwig [this message]
2019-05-13  6:37 ` [PATCH 01/10] block: don't decrement nr_phys_segments for physically contigous segments Christoph Hellwig
2019-05-13  9:45   ` Ming Lei
2019-05-13 12:03     ` Christoph Hellwig
2019-05-13 12:37       ` Christoph Hellwig
2019-05-14  4:36       ` Ming Lei
2019-05-14  5:14         ` Christoph Hellwig
2019-05-14  9:05           ` Ming Lei
2019-05-14 13:51             ` Christoph Hellwig
2019-05-14 13:57               ` Hannes Reinecke
2019-05-14 14:27               ` Ming Lei
2019-05-14 14:31                 ` Christoph Hellwig
2019-05-14 14:32                   ` Christoph Hellwig
2019-05-13  6:37 ` [PATCH 02/10] block: force an unlimited segment size on queues with a virt boundary Christoph Hellwig
2019-05-15  8:19   ` Ming Lei
2019-05-13  6:37 ` [PATCH 03/10] block: remove the segment size check in bio_will_gap Christoph Hellwig
2019-05-15  8:34   ` Ming Lei
2019-05-13  6:37 ` [PATCH 04/10] block: remove the bi_seg_{front,back}_size fields in struct bio Christoph Hellwig
2019-05-13  6:37 ` [PATCH 05/10] block: initialize the write priority in blk_rq_bio_prep Christoph Hellwig
2019-05-13 15:04   ` Chaitanya Kulkarni
2019-05-13  6:37 ` [PATCH 06/10] block: remove blk_init_request_from_bio Christoph Hellwig
2019-05-13  6:37 ` [PATCH 07/10] block: remove the bi_phys_segments field in struct bio Christoph Hellwig
2019-05-13  6:37 ` [PATCH 08/10] block: simplify blk_recalc_rq_segments Christoph Hellwig
2019-05-13  6:37 ` [PATCH 09/10] block: untangle the end of blk_bio_segment_split Christoph Hellwig
2019-05-13  6:37 ` [PATCH 10/10] block: mark blk_rq_bio_prep as inline Christoph Hellwig
2019-05-13 14:57   ` Chaitanya Kulkarni
     [not found] ` <CGME20190513063855epcas5p33ef8c4c0a0055bd0b66eadc859796f0f@epcms2p6>
2019-05-13  7:34   ` [PATCH 05/10] block: initialize the write priority in blk_rq_bio_prep Minwoo Im

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