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

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Date:   Tue, 20 Nov 2018 10:45:03 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Omar Sandoval <osandov@...ndov.com>, 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

Ming Lei <ming.lei@...hat.com> writes:

> 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.

Yes.  CONFIG_THP_SWAP needs to put 512 normal sub-pages into one bio to
write the 512 sub-pages together.  With the help of multipage-bvec, it
needs just bvect to hold 512 normal sub-pages.

Best Regards,
Huang, Ying

> thanks,
> Ming