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Date:   Wed, 28 Nov 2018 06:44:00 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     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>,
        Christoph Hellwig <hch@....de>, 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 V12 00/20] block: support multi-page bvec

On 11/25/18 7:17 PM, Ming Lei wrote:
> Hi,
> 
> This patchset brings multi-page bvec into block layer:
> 
> 1) what is multi-page bvec?
> 
> Multipage bvecs means that one 'struct bio_bvec' can hold multiple pages
> which are physically contiguous instead of one single page used in linux
> kernel for long time.
> 
> 2) why is multi-page bvec introduced?
> 
> Kent proposed the idea[1] first. 
> 
> As system's RAM becomes much bigger than before, and huge page, transparent
> huge page and memory compaction are widely used, it is a bit easy now
> to see physically contiguous pages from fs in I/O. On the other hand, from
> block layer's view, it isn't necessary to store intermediate pages into bvec,
> and it is enough to just store the physicallly contiguous 'segment' in each
> io vector.
> 
> Also huge pages are being brought to filesystem and swap [2][6], we can
> do IO on a hugepage each time[3], which requires that one bio can transfer
> at least one huge page one time. Turns out it isn't flexiable to change
> BIO_MAX_PAGES simply[3][5]. Multipage bvec can fit in this case very well.
> As we saw, if CONFIG_THP_SWAP is enabled, BIO_MAX_PAGES can be configured
> as much bigger, such as 512, which requires at least two 4K pages for holding
> the bvec table.

I'm pretty happy with this patchset at this point, looks like it just
needs a respin to address the last comments. My only concern is whether
it's a good idea to target this for 4.21, or if we should wait until
4.22. 4.21 has a fairly substantial amount of changes in terms of block
already, it's not the best timing for something of this magnitude too.

I'm going back and forth on those one a bit. Any concerns with
pushing this to 4.22?

-- 
Jens Axboe

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