Re: [PATCH 0/3] mm: handle device-private PMDs in walk callbacks

[Usama Arif]

On 07/07/2026 15:01, Lorenzo Stoakes wrote:
> +cc Balbir
> 
> Thanks for the series!
> 
> I think Balbir should be cc'd no?
> 

Ah yes, sorry about that! I just sent the patches to the list I got from
get_maintainers.pl.

I think maybe we should modify get_maintainers.pl so that if there is
a "Fixes" in commit message, than the author of that commit is also
in the output from that script?

> On Tue, Jul 07, 2026 at 06:45:06AM -0700, Usama Arif wrote:
>> Since commit 368076f52ebe ("mm/huge_memory: add device-private THP support
>> to PMD operations") a PMD may hold a device-private swap entry whenever
>> an HMM-based GPU driver migrates an anonymous THP folio to device memory
>> via migrate_vma_pages().
>>
>> pmd_trans_huge_lock() succeeds for such PMDs (pmd_is_huge() returns true
>> for any non-present, non-none huge PMD), so several MM walk callbacks
>> that used to assume present THP or migration entry are now reachable with
>> a device-private PMD. The results range from a VM_BUG_ON() firing on debug
>> kernels, to an oops on a bogus vmemmap dereference, to silently isolating
>> an unrelated live folio from LRU in the aliasing case.
>>
>> The first 2 fixes were reported as pre-existing issues by sashiko in my
>> PMD swap entry series [1]. Hopefully sashiko won't point these out
>> in the next PMD swap entry series :)
> 
> Yeah I don't love these 'existing problem' reports (distracting from other work,
> add workload, really it feels like that should be a passive reporting mode for
> sashiko not arbitrarily added to other series), but sending it as a separate
> series is indeed the right way :)
> 
> These of course should never block an unrelated series.
> 

Yes agreed!

>>
>> [1] https://sashiko.dev/#/patchset/20260703173903.3789516-1-usama.arif%40linux.dev?part=6
> 
> Hmm we've had a whole host of issues with device-private PMDs recently.
> 
> Balbir - would it be possible to audit all of the code paths and proactively see
> if there's anything else that could hit problems here?
> 
>>
>> Usama Arif (3):
>>   mm/mempolicy: skip device-private PMDs when queueing folios
>>   mm/madvise: skip device-private PMDs in cold and pageout walks
>>   mm/huge_memory: skip device-private PMDs in madvise_free_huge_pmd
>>
>>  mm/huge_memory.c | 3 +++
>>  mm/madvise.c     | 3 +++
>>  mm/mempolicy.c   | 2 ++
>>  3 files changed, 8 insertions(+)
>>
>> --
>> 2.53.0-Meta
>>
> 
> Cheers, Lorenzo