From: Ming Lei <ming.lei@redhat.com>
To: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Nikolay Borisov <nborisov@suse.com>, Jens Axboe <axboe@kernel.dk>,
Omar Sandoval <osandov@osandov.com>,
linux-block@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: Possible bio merging breakage in mp bio rework
Date: Mon, 8 Apr 2019 18:19:04 +0800 [thread overview]
Message-ID: <20190408101903.GC19589@ming.t460p> (raw)
In-Reply-To: <eb586b8f-f8d4-f455-6203-4ceae2420274@suse.de>
On Mon, Apr 08, 2019 at 11:52:59AM +0200, Johannes Thumshirn wrote:
> 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?
bio split is one important IO model in block layer, which simplifies
stacked driver(dm, md, bcache, ...) a lot.
It is very reasonable to apply the queue limits in its. make_request_fn().
Otherwise, it will cause huge mess in stacking driver if queue limits
are applied in bio_add_page(), see previous .merge_bvec_fn's implementation
in these stacking drivers.
Not only bio_add_page(), there is also bio clone involved.
>
> Can't we look at the limits in e.g. bio_add_page() and decide if we need
> to split there?
bio_add_page() is absolutely the fast path, and it is much more efficient
to apply the limit just once in the queue's .make_request_fn.
Thanks,
Ming
next prev parent reply other threads:[~2019-04-08 10:19 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-05 16:04 Possible bio merging breakage in mp bio rework Nikolay Borisov
2019-04-06 0:16 ` Ming Lei
2019-04-06 6:09 ` Nikolay Borisov
2019-04-06 8:00 ` Qu Wenruo
2019-04-06 12:30 ` Ming Lei
2019-04-08 9:52 ` Johannes Thumshirn
2019-04-08 10:19 ` Ming Lei [this message]
2019-04-08 10:22 ` Johannes Thumshirn
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