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

[Joshua Hahn]

On Tue,  7 Jul 2026 06:45:06 -0700 Usama Arif <usama.arif@xxxxxxxxx> 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 :)

Hello Usama,

Thank you for the fixes! I think all of them look good and they are fixes
so please feel free to add my

Reviewed-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>

One thing I wanted to maybe consider is for 1/3, whether it could be
good to gate by !pmd_present() like the sites in 2/3 and 3/3, maybe
that will be more robust in the future if there are other cases we
should protect against (and the same as queue_folios_pte_range, which
uses the !pte_present() check too).

For 2/3 and 3/3 we already have the !pmd_present() check, would it be
worth considering just changing the VM_BUG_ON condition to include
!pmd_is_device_private_entry()?

Just wanted to toss my 2c. Thanks again for the fixes Usama!
Joshua

> [1] https://sashiko.dev/#/patchset/20260703173903.3789516-1-usama.arif%40linux.dev?part=6
>  
> 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