linux-kernel - Re: [PATCH] block: make sure big bio is splitted into at most 256 bvecs

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Date:	Tue, 5 Apr 2016 17:10:28 -0800
From:	Kent Overstreet <kent.overstreet@...il.com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jens Axboe <axboe@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
	Eric Wheeler <bcache@...ts.ewheeler.net>,
	Sebastian Roesner <sroesner-kernelorg@...sner-online.de>,
	"4.2+" <stable@...r.kernel.org>, Shaohua Li <shli@...com>
Subject: Re: [PATCH] block: make sure big bio is splitted into at most 256
 bvecs

On Wed, Apr 06, 2016 at 08:59:31AM +0800, Ming Lei wrote:
> On Wed, Apr 6, 2016 at 8:30 AM, Kent Overstreet
> <kent.overstreet@...il.com> wrote:
> > On Wed, Apr 06, 2016 at 01:44:06AM +0800, Ming Lei wrote:
> >> After arbitrary bio size is supported, the incoming bio may
> >> be very big. We have to split the bio into small bios so that
> >> each holds at most BIO_MAX_PAGES bvecs for safety reason, such
> >> as bio_clone().
> >>
> >> This patch fixes the following kernel crash:
> >
> > Ming, let's not do it this way; drivers that don't clone biovecs are the norm -
> > instead, md has its own queue limits that it ought to be setting up correctly.
> 
> Except for md, there are also several usages of bio_clone:
> 
>          - drbd
>          - osdblk
>          - pktcdvd
>          - xen-blkfront
>          - verify code of bcache
> 
> I don't like bio_clone() too, which can cause trouble to multipage bvecs.
> 
> How about fixing the issue by this simple patch first? Then once we limits
> all above queues by max sectors, the global limit can be removed as
> mentioned by the comment.

just do this:

void blk_set_limit_clonable(struct queue_limits *lim)
{
	lim->max_segments = min(lim->max_segments, BIO_MAX_PAGES);
}

and then call that from the appropriate drivers. It should be like 20 minutes of
work.

My issue is that your approach of just enforcing a global limit is a step in the
wrong direction - we want to get _away_ from that and move towards drivers
specifying _directly_ what their limits are: more straightforward, less opaque.

Also, your patch is wrong, as it'll break if there's bvecs that aren't full
pages.