linux-ext4 - Re: [PATCH V11 14/19] block: handle non-cluster bio out of blk_bio_segment

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Message-ID: <20181122102616.GC27273@ming.t460p>
Date:   Thu, 22 Nov 2018 18:26:17 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Theodore Ts'o <tytso@....edu>, Omar Sandoval <osandov@...com>,
        Sagi Grimberg <sagi@...mberg.me>,
        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, 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>,
        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 V11 14/19] block: handle non-cluster bio out of
 blk_bio_segment_split

On Thu, Nov 22, 2018 at 11:04:28AM +0100, Christoph Hellwig wrote:
> On Thu, Nov 22, 2018 at 05:33:00PM +0800, Ming Lei wrote:
> > However, using virt boundary limit on non-cluster seems over-kill,
> > because the bio will be over-split(each small bvec may be split as one bio)
> > if it includes lots of small segment.
> 
> The combination of the virt boundary of PAGE_SIZE - 1 and a
> max_segment_size of PAGE_SIZE will only split if the to me merged
> segment is in a different page than the previous one, which is exactly
> what we need here.  Multiple small bvec inside the same page (e.g.
> 512 byte buffer_heads) will still be merged.

Suppose one bio includes (pg0, 0, 512) and (pg1, 512, 512):

The split is introduced by the following code in blk_bio_segment_split():

      if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
		  	goto split;

Without this patch, for non-cluster, the two bvecs are just in different
segment, but still handled by one same bio. Now you convert into two bios.

Thanks,
Ming