Re: [PATCH v2 2/2] selftests/mm: add stale walk->action race test

[Hyunwoo Kim]

On Mon, Aug 17, 2026 at 06:32:21PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Tue, Aug 11, 2026 at 08:49:46PM +0200, David Hildenbrand (Arm) wrote:
> > On 8/11/26 20:23, Lorenzo Stoakes (ARM) wrote:
> > > On Tue, Aug 11, 2026 at 07:11:59PM +0200, David Hildenbrand (Arm) wrote:
> > >> On 8/11/26 18:18, Hyunwoo Kim wrote:
> > >>> The added pagewalk_race_test maps two PMDs and faults in 2MB of the first
> > >>> one.  A second thread then faults in the second PMD and drops it again with
> > >>> MADV_DONTNEED in a loop, while the main thread reads Rss for the mapping
> > >>> from /proc/self/smaps.
> > >>>
> > >>> Clearing the second PMD while smaps_pte_range() runs leaves walk->action
> > >>> erroneously set to ACTION_AGAIN, which causes the PUD walk to be retried,
> > >>> so the first PMD is counted twice and Rss comes out twice as large as what
> > >>> was faulted in.
> > >>>
> > >>> mincore() is the caller named in the fix, but the second walk writes past
> > >>> the length mincore() copies back, so it cannot be seen from userspace
> > >>> there.  smaps reports what the callbacks counted, so the duplicate shows up
> > >>> in Rss.
> > >>>
> > >>> A failure can only come from the kernel counting the same page twice, so
> > >>> missing the race is harmless.  On an unfixed kernel the test fails after a
> > >>> few hundred reads at most and takes about half a second.
> > >>>
> > >>> Assisted-by: Claude:claude-opus-5
> > >>> Signed-off-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
> > >>> ---
> > >>
> > >> I'm sorry, but I don't consider such a specialized reproducer for a problem we
> > >> hit once a good fit for a selftest.
> > >>
> > >> Or am I missing something, that this here is more generic?
> > >
> > > To be fair I did ask him for this. :)
> > >
> > > I thought it would be useful to have a regression test because there's certainly
> > > no harm in it? The code is very sensitive and catching a similar mistake in
> > > future could be useful.
> > >
> > > If you're sure this isn't valuable then he could just drop it, but it's my fault
> > > sorry Hyunwoo for giving you extra work in this case! My bad.
> >
> > pagewalk_race_test is just rather odd. If this would be part of a bigger test
> > that covers more things than just one odd race, it might be better.
> >
> > But we certainly don't want one new test file for each kernel bug we ever
> > trigger, right? :)
> 
> Right yeah :) sure we can drop this then. Sorry Hyunwoo!

That's okay :)


Best regards,
Hyunwoo Kim