Re: [PATCH] smaps: fix BUG_ON in smaps_hugetlb_range

[Michal Hocko <mhocko@suse.com>]

On Mon 21-07-25 11:41:18, David Hildenbrand wrote:
> On 21.07.25 11:35, Michal Hocko wrote:
> > On Mon 21-07-25 11:29:52, David Hildenbrand wrote:
[...]
> > > Heh, I stumbled over that code many times and wondered "why don't we need
> > > the PTL here -- I'm  sure it's fine because otherwise we would be getting
> > > reports.".
> > > 
> > > In pagewalk code we only hold the vma lock -- see walk_hugetlb_range().
> > > 
> > > So I think we should just grab the PTL in all these walkers.
> > 
> > I believe the reason that we try to avoid taking the lock in these paths
> > is that they are userspace accessible and we do not want to expose them
> > to users. I think it would be good to try to rework the code to not
> > require the lock even if we get imprecise numbers. We cannot trigger any
> > oops of course and that is a clear bug here. Can we achieve the fix
> > without taking the lock?
> 
> We grab PTLs whenever we walk page tables, except in hugetlb. So I much
> rather want that changed?

OK, fair enough. If hugetlb is the only one odd here and we are already
living with the ptl already exposed then let's go with a lock here as
well.
-- 
Michal Hocko
SUSE Labs