linux-kernel - Re: [PATCH V10 15/19] block: always define BIO_MAX

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Date:   Mon, 19 Nov 2018 17:04:16 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Omar Sandoval <osandov@...ndov.com>,
        Huang Ying <ying.huang@...el.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 15/19] block: always define BIO_MAX_PAGES as 256

On Thu, Nov 15, 2018 at 05:59:36PM -0800, Omar Sandoval wrote:
> On Thu, Nov 15, 2018 at 04:53:02PM +0800, Ming Lei wrote:
> > Now multi-page bvec can cover CONFIG_THP_SWAP, so we don't need to
> > increase BIO_MAX_PAGES for it.
> 
> You mentioned to it in the cover letter, but this needs more explanation
> in the commit message. Why did CONFIG_THP_SWAP require > 256? Why does
> multipage bvecs remove that requirement?

CONFIG_THP_SWAP needs to split one TH page into normal pages and adds
them all to one bio. With multipage-bvec, it just takes one bvec to
hold them all.

thanks,
Ming