On Wed, Apr 06, 2016 at 08:59:31AM +0800, Ming Lei wrote:
> On Wed, Apr 6, 2016 at 8:30 AM, Kent Overstreet
> <[email protected]> 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.
