Re: [bug] block subsystem related crash with latest -git

Jens Axboe
2007-10-18T10:56:31Z

On Thu, Oct 18 2007, Benny Halevy wrote:
> On Oct. 17, 2007, 20:22 +0200, Jens Axboe <jens.axboe@oracle.com> wrote:
> > On Wed, Oct 17 2007, Linus Torvalds wrote:
> >>
> >> On Wed, 17 Oct 2007, Jens Axboe wrote:
> >>>> So avoiding the "sg_next()" on the last entry is pointless.
> >>> Yeah, I didn't quite understand why if sg was valid, why dereferencing
> >>> *(sg + 1)->page would crap out :/
> >> Actually, I take that back. If 'sg' is the last entry in a *non*linked
> >> scatter-gather list (ie we don't use the last entry as a link, we actually
> >> use it as a real SG entry), then "sg_next(sg)" will indeed access past the
> >> end of the whole allocated array, and will access one past the end.
> >>
> >> And with page-alloc debugging, that *will* blow up.
> >>
> >> So I think your change to use "sg_next()" only when you actually need a
> >> next pointer is the correct one after all.
> >
> > Thanks, so I'm not totally crazy :-)
> >
> > Can you just pull:
> >
> > git://git.kernel.dk/data/git/linux-2.6-block.git for-linus
> >
> > then so we get those two pieces correct? Then the remaining issue seems
> > to be a new one that is biting Ingo elsewhere, at least we'll all be on
> > the same page then.
> >
>
> Jens, for_each_sg still calls sg_next on the last entry which will
> dereference a possibly bogus sg->page (for the sg_is_chain(sg)
> condition in sg_next) if the last entry is the last one on the page
> of unchained entry and sg+1 falls over into an uninitialized page.

Things have progressed a lot since, see my recent posting based on
Davem's proposal. Will post another patch soonish, that is also tested.

--
Jens Axboe