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

[David Hildenbrand (Arm)]

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?

-- 
Cheers,

David