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

[Balbir Singh]

On Tue, Jul 07, 2026 at 03:01:56PM +0100, Lorenzo Stoakes wrote:
> +cc Balbir
> 
> Thanks for the series!
> 
> I think Balbir should be cc'd no?
>

Thanks, my linux-mm monitoring has been largely skimming through the
list. Explict cc's are always appreciated.

> 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?
> 

Let me run through this and audit them. My usage of device private PMD
did not expose them, but I can see a bunch of reports in the link above



> >
> > 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
> >
>

Thanks Lorenzo!

Balbir