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Date:   Mon, 19 Nov 2018 17:06:46 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Omar Sandoval <osandov@...ndov.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Dave Chinner <dchinner@...hat.com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Shaohua Li <shli@...nel.org>,
        linux-raid@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, Gao Xiang <gaoxiang25@...wei.com>,
        Christoph Hellwig <hch@....de>, Theodore Ts'o <tytso@....edu>,
        linux-ext4@...r.kernel.org, Coly Li <colyli@...e.de>,
        linux-bcache@...r.kernel.org, Boaz Harrosh <ooo@...ctrozaur.com>,
        Bob Peterson <rpeterso@...hat.com>, cluster-devel@...hat.com
Subject: Re: [PATCH V10 17/19] block: don't use bio->bi_vcnt to figure out
 segment number

On Thu, Nov 15, 2018 at 06:11:40PM -0800, Omar Sandoval wrote:
> On Thu, Nov 15, 2018 at 04:53:04PM +0800, Ming Lei wrote:
> > It is wrong to use bio->bi_vcnt to figure out how many segments
> > there are in the bio even though CLONED flag isn't set on this bio,
> > because this bio may be splitted or advanced.
> > 
> > So always use bio_segments() in blk_recount_segments(), and it shouldn't
> > cause any performance loss now because the physical segment number is figured
> > out in blk_queue_split() and BIO_SEG_VALID is set meantime since
> > bdced438acd83ad83a6c ("block: setup bi_phys_segments after splitting").
> > 
> > Cc: Dave Chinner <dchinner@...hat.com>
> > Cc: Kent Overstreet <kent.overstreet@...il.com>
> > Fixes: 7f60dcaaf91 ("block: blk-merge: fix blk_recount_segments()")
> 
> From what I can tell, the problem was originally introduced by
> 76d8137a3113 ("blk-merge: recaculate segment if it isn't less than max segments")
> 
> Is that right?

Indeed, will update it in next version.

Thanks,
Ming

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