linux-ext4 - Re: [PATCH V12 00/20] block: support multi-page bvec

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Date:   Thu, 29 Nov 2018 09:30:00 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>, Mike Snitzer <snitzer@...hat.com>,
        "Ewan D. Milne" <emilne@...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 Wed, Nov 28, 2018 at 06:44:00AM -0700, Jens Axboe wrote:
> 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

I will address the last comment from Omar on patch of '[PATCH V12 01/20] btrfs:
remove various bio_offset arguments', we may use the approach in V11 simply.

> 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.

Yeah, I understand.

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

My only one concern is about the warning of "blk_cloned_rq_check_limits:
over max segments limit" on dm multipath, and seems Ewan and Mike is waiting
for this fix.

thanks,
Ming