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

[Kent Overstreet <kent.overstreet@gmail.com>]

On Wed, Apr 06, 2016 at 09:51:02AM +0800, Ming Lei wrote:
> On Wed, Apr 6, 2016 at 9:28 AM, Kent Overstreet
> <kent.overstreet@gmail.com> wrote:
> > On Wed, Apr 06, 2016 at 09:20:59AM +0800, Ming Lei wrote:
> >> On Wed, Apr 6, 2016 at 9:10 AM, Kent Overstreet
> >> <kent.overstreet@gmail.com> wrote:
> >> > 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@gmail.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);
> >> > }
> >>
> >> As I memtioned it is __not__ correct to use max_segments, and the issue is
> >> related with max sectors, please see the code of bio_clone_bioset():
> >
> > I know how bio_clone_bioset() works but I'm not seeing how that has anything to
> > do with max sectors. The way it copies the biovec is not going to merge
> > segments, if the original bio had non full page segments then so is the clone.
> 
> OK, I see, now it is a totally new limit, and no current queue limit can fit
> the purpose.
> 
> Looks we need to introduce the new limit of io_max_vecs, which can be
> applied into blk_bio_segment_split().
> 
> But a queue flag should be better than queue limit since it is a 'limit' from
> software/driver.

Why is max_segments not appropriate?