linux-kernel - Re: new patchset to eliminate DM's use of BIOSET_NEED

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Message-ID: <20171122184931.GA28348@redhat.com>
Date:   Wed, 22 Nov 2017 13:49:31 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     NeilBrown <neilb@...e.com>, Jens Axboe <axboe@...nel.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org,
        device-mapper development <dm-devel@...hat.com>,
        Zdenek Kabelac <zkabelac@...hat.com>
Subject: Re: new patchset to eliminate DM's use of BIOSET_NEED_RESCUER

On Wed, Nov 22 2017 at  1:24pm -0500,
Mikulas Patocka <mpatocka@...hat.com> wrote:
 
> Another problem is this:
> 
> struct bio *b = bio_clone_bioset(bio, GFP_NOIO, md->queue->bio_split);
> bio_advance(b, (bio_sectors(b) - ci.sector_count) << 9);
> bio_chain(b, bio);
> 
> What if it blocks because the bioset is exhausted?
> 
> The code basically builds a chain of bios of unlimited length (suppose for 
> example a case when we are splitting on every sector boundary, so there 
> will be one bio for every sector in the original bio), it could exhaust 
> the bioset easily.
> 
> It would be better to use mechanism from md-raid that chains all the 
> sub-bios to the same master bio and doesn't create long chains of bios:
> 
>         if (max_sectors < bio_sectors(bio)) {
>                 struct bio *split = bio_split(bio, max_sectors,
>                                               gfp, conf->bio_split);
>                 bio_chain(split, bio);
>                 generic_make_request(bio);
>                 bio = split;
>                 r1_bio->master_bio = bio;
>                 r1_bio->sectors = max_sectors;
>         }

I'd be happy to take an incremental patch that improves on this commit:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.16&id=b46d6a08f1ae7bf53e4cde28e0ccdf91567d432e

But short of that I'll have to come back to this.

Thanks,
Mike