Re: [PATCH] dm: max_segments=1 if merge_bvec_fn is not supported

Andrew Morton
2010-03-18T18:50:07Z

On Mon, 8 Mar 2010 14:14:49 +0100
Lars Ellenberg <lars.ellenberg@linbit.com> wrote:

> On Mon, Mar 08, 2010 at 03:35:37AM -0500, Mikulas Patocka wrote:
> > Hi
> >
> > That patch with limits->max_segments = 1; is wrong. It fixes this bug
> > sometimes and sometimes not.
> >
> > The problem is, if someone attempts to create a bio with two vector
> > entries, the first maps the last sector contained in some page and the
> > second maps the first sector of the next physical page: it has one
> > segment, it has size <= PAGE_SIZE, but it still may cross raid stripe and
> > the raid driver will reject it.
>
> Now that you put it that way ;)
> You are right.
>
> My asumption that "single segment" was
> equalvalent in practice with "single bvec"
> does not hold true in that case.
>
> Then, what about adding seg_boundary_mask restrictions as well?
> max_sectors = PAGE_SIZE >> 9;
> max_segments = 1;
> seg_boundary_mask = PAGE_SIZE -1;
> or some such.
>
> > > > This is not the first time this has been patched, btw.
> > > > See https://bugzilla.redhat.com/show_bug.cgi?id=440093
> > > > and the patch by Mikulas:
> > > > https://bugzilla.redhat.com/attachment.cgi?id=342638&action=diff
> >
> > Look at this patch, it is the proper way how to fix it: create a
> > merge_bvec_fn that reject more than one biovec entry.
>
> If adding seg_boundary_mask is still not sufficient,
> lets merge that patch instead?
> Why has it been dropped, respectively never been merged?
> It became obsolete for dm-linear by 7bc3447b,
> but in general the bug is still there, or am I missing something?
>

This all seemed to die. Does Neil's mysterypatch fix all these issues?

Neil, was that patch tagged for -stable backporting?

Thanks.