Re: [Patch mm-hotfixes v5] mm/page_vma_mapped: fix device-private PMD handling

[Balbir Singh <balbirs@nvidia.com>]

On Tue, Jun 30, 2026 at 02:15:40AM +0000, Wei Yang wrote:
> Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
> device-private entries") introduced the concept of device-private
> PMD entries, but did not correctly update the rmap walk code to
> account for them.
> 
> As a result, when page_vma_mapped_walk() encounters device-private
> PMD entries, it takes no action other than to acquire the PMD lock
> and exit.
> 
> However this is highly problematic for two reasons - firstly,
> device private entries possess a PFN so check_pmd() needs to be
> called to ensure an overlapping PFN range.
> 
> Secondly, and more importantly, if PVMW_MIGRATION is set the
> caller assumes the returned entry is a migration entry, resulting
> in memory corruption when the caller tries to interpret the device
> private entry as such.
> 
> In addition, commit 146287290023 ("mm/huge_memory: implement
> device-private THP splitting") allowed device private PMDs to be
> split like THP mappings, but again did not update this code path.
> 
> As a result, we might race a PMD split prior to acquiring the PMD
> lock.
> 
> This patch addresses all of these issues by invoking check_pmd(),
> ensuring PMVW_MIGRATION is not set and checks whether a split raced
> us we do for PMD THP and migration entries.
> 
> Instead of checking for a subset of the cases after taking the
> pmd_lock(), put device-private along with pmd_trans_huge() and
> pmd_is_migration_entry(). Also remove thp_migration_supported() as
> it is already guarded by pmd_is_migration_entry().
> 
> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Suggested-by: David Hildenbrand <david@kernel.org>
> Cc: David Hildenbrand <david@kernel.org>
> Cc: Balbir Singh <balbirs@nvidia.com>
> Cc: SeongJae Park <sj@kernel.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Lorenzo Stoakes <ljs@kernel.org>
> Cc: Lance Yang <lance.yang@linux.dev>
> 
> ---
> v5:
>   * put device-private pmd handling along with the other two cases
>   * remove thp_migration_supported()
> v4: https://lore.kernel.org/all/20260624065353.1622-1-richard.weiyang@gmail.com/T/#u
>   * refine subject and commit log based on Lorenzo's suggestion
>   * put pmd device-private entry handling in its own if branch,
>     suggested by Lorenzo
> 
> v3:
>   * remove cleanup part, only fix the issue for device-private entry
>   * refine user effect description based on Lorenzo's suggestion
> 
> v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@gmail.com/T/#u
>   * specify the possible error case of current code and user visible effect
>   * besides fix, cleanup the pmd entry handling based on David's suggestion
> 
> v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@gmail.com/
> ---
>  mm/page_vma_mapped.c | 30 ++++++++++++++++--------------
>  1 file changed, 16 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 2ccbabfb2cc1..2d6c58488e3a 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -243,21 +243,30 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  		 */
>  		pmde = pmdp_get_lockless(pvmw->pmd);
>  
> -		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
> +		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
> +		    pmd_is_device_private_entry(pmde)) {
>  			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
>  			pmde = *pvmw->pmd;
> -			if (!pmd_present(pmde)) {
> +			if (pmd_is_migration_entry(pmde)) {
>  				softleaf_t entry;
>  
> -				if (!thp_migration_supported() ||
> -				    !(pvmw->flags & PVMW_MIGRATION))
> +				if (!(pvmw->flags & PVMW_MIGRATION))
>  					return not_found(pvmw);
>  				entry = softleaf_from_pmd(pmde);
> +				if (!check_pmd(softleaf_to_pfn(entry), pvmw))
> +					return not_found(pvmw);
> +				return true;
> +			} else if (pmd_is_device_private_entry(pmde)) {
> +				softleaf_t entry;
>  
> -				if (!softleaf_is_migration(entry) ||
> -				    !check_pmd(softleaf_to_pfn(entry), pvmw))
> +				if (pvmw->flags & PVMW_MIGRATION)
> +					return not_found(pvmw);
> +				entry = softleaf_from_pmd(pmde);
> +				if (!check_pmd(softleaf_to_pfn(entry), pvmw))
>  					return not_found(pvmw);
>  				return true;
> +			} else if (!pmd_present(pmde)) {
> +				return not_found(pvmw);
>  			}
>  			if (likely(pmd_trans_huge(pmde))) {
>  				if (pvmw->flags & PVMW_MIGRATION)
> @@ -266,17 +275,10 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  					return not_found(pvmw);
>  				return true;
>  			}
> -			/* THP pmd was split under us: handle on pte level */
> +			/* THP/device-private pmd was split under us: handle on pte level */
>  			spin_unlock(pvmw->ptl);
>  			pvmw->ptl = NULL;
>  		} else if (!pmd_present(pmde)) {
> -			const softleaf_t entry = softleaf_from_pmd(pmde);
> -
> -			if (softleaf_is_device_private(entry)) {
> -				pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> -				return true;
> -			}
> -
>  			if ((pvmw->flags & PVMW_SYNC) &&
>  			    thp_vma_suitable_order(vma, pvmw->address,
>  						   PMD_ORDER) &&
> --

Thanks!

Acked-by: Balbir Singh <balbirs@nvidia.com>