[PATCH v2 3/6] perf/core: Fix arch_perf_get_page_size()

[Matthew Wilcox]

On Thu, Nov 26, 2020 at 02:06:19PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 26, 2020 at 12:56:06PM +0000, Matthew Wilcox wrote:
> > On Thu, Nov 26, 2020 at 01:42:07PM +0100, Peter Zijlstra wrote:
> > > +	pgdp = pgd_offset(mm, addr);
> > > +	pgd = READ_ONCE(*pgdp);
> > 
> > I forget how x86-32-PAE maps to Linux's PGD/P4D/PUD/PMD scheme, but
> > according to volume 3, section 4.4.2, PAE paging uses a 64-bit PDE, so
> > whether a PDE is a PGD or a PMD, we're only reading it with READ_ONCE
> > rather than the lockless-retry method used by ptep_get_lockless().
> > So it's potentially racy?  Do we need a pmdp_get_lockless() or
> > pgdp_get_lockless()?
> 
> Oh gawd... this isn't new here though, right? Current gup_fast also gets
> that wrong, if it is in deed wrong.
> 
> I suppose it's a race far more likely today, with THP and all, than it
> ever was back then.

Right, it's not new.  I wouldn't block this patchset for that fix.
Just want to get the problem on your radar ;-)  I just never reviewed
the gup fast codepath before, and this jumped out at me.