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

[Lorenzo Stoakes]

+cc Balbir

Thanks for the series!

I think Balbir should be cc'd no?

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.

>
> [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