Re: [PATCH v2 3/3] mm/huge_memory: skip device-private PMDs in madvise_free_huge_pmd

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/8/26 14:20, Usama Arif wrote:
> madvise_free_pte_range() checks pmd_trans_huge(*pmd) unlocked, then
> madvise_free_huge_pmd() takes pmd_trans_huge_lock(). pmd_is_huge()
> returns true for a device-private PMD, so orig_pmd can be device-private
> and enter the !pmd_present() branch.
> 
> Allow device-private PMDs in that non-present assertion and continue to
> out before calling pmd_folio(). This keeps the assertion for unexpected
> PMD softleafs while skipping device-private PMDs like other non-present
> PMDs in this path.
> 
> Potential trigger: an HMM-based GPU driver races with madvise(MADV_FREE):
> migrate_vma_pages() flips the PMD to a device-private entry between the
> caller's pmd_trans_huge() check and the callee's pmd_trans_huge_lock().
> 
> Fixes: 368076f52ebe ("mm/huge_memory: add device-private THP support to PMD operations")
> Cc: <stable@vger.kernel.org>
> Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
> Reviewed-by: Balbir Singh <balbirs@nvidia.com>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  mm/huge_memory.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index c0892cc533a9..ddbdc83b4cae 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2298,7 +2298,8 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  
>  	if (unlikely(!pmd_present(orig_pmd))) {
>  		VM_BUG_ON(thp_migration_supported() &&
> -				  !pmd_is_migration_entry(orig_pmd));
> +				  !pmd_is_migration_entry(orig_pmd) &&
> +				  !pmd_is_device_private_entry(orig_pmd));

Same comment as to #2.

-- 
Cheers,

David