linux-kernel - Re: Possible bio merging breakage in mp bio rework

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Message-ID: <eb586b8f-f8d4-f455-6203-4ceae2420274@suse.de>
Date:   Mon, 8 Apr 2019 11:52:59 +0200
From:   Johannes Thumshirn <jthumshirn@...e.de>
To:     Ming Lei <ming.lei@...hat.com>, Nikolay Borisov <nborisov@...e.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Omar Sandoval <osandov@...ndov.com>,
        linux-block@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>
Subject: Re: Possible bio merging breakage in mp bio rework

On 06/04/2019 02:16, Ming Lei wrote:
> Hi Nikolay,
> 
> On Fri, Apr 05, 2019 at 07:04:18PM +0300, Nikolay Borisov wrote:
>> Hello Ming, 
>>
>> Following the mp biovec rework what is the maximum 
>> data that a bio could contain? Should it be PAGE_SIZE * bio_vec 
> 
> There isn't any maximum data limit on the bio submitted from fs,
> and block layer will make the final bio sent to driver correct
> by applying all kinds of queue limit, such as max segment size,
> max segment number, max sectors, ...

Naive question, why are we creating possibly huge bios just to split
them according the the LLDD's limits afterwards?

Can't we look at the limits in e.g. bio_add_page() and decide if we need
to split there?

This is just me thinking about it, I haven't though if there are any
resulting performance penalties from it, yet.

Byte,
	Johannes
-- 
Johannes Thumshirn                            SUSE Labs Filesystems
jthumshirn@...e.de                                +49 911 74053 689
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