Re: [Patch mm-hotfixes v4] mm/page_vma_mapped: fix device-private PMD handling

[David Hildenbrand (Arm)]

On 6/24/26 08:53, 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.
> 
> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Balbir Singh <balbirs@xxxxxxxxxx>
> Cc: SeongJae Park <sj@xxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> 
> ---
> v4:
>   * 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@xxxxxxxxx/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@xxxxxxxxx/
> ---
>  mm/page_vma_mapped.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 2ccbabfb2cc1..17dff8aab9f9 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -269,14 +269,24 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>  			/* THP 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);
> +		} else if (pmd_is_device_private_entry(pmde)) {
> +			softleaf_t entry;
> +
> +			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> +			pmde = *pvmw->pmd;
> +			entry = softleaf_from_pmd(pmde);
>  
> -			if (softleaf_is_device_private(entry)) {
> -				pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> +			if (likely(softleaf_is_device_private(entry))) {
> +				if (pvmw->flags & PVMW_MIGRATION)
> +					return not_found(pvmw);
> +				if (!check_pmd(softleaf_to_pfn(entry), pvmw))
> +					return not_found(pvmw);
>  				return true;
>  			}
> -
> +			/* device-private pmd was split under us: handle on pte level */
> +			spin_unlock(pvmw->ptl);
> +			pvmw->ptl = NULL;
> +		} else if (!pmd_present(pmde)) {
>  			if ((pvmw->flags & PVMW_SYNC) &&
>  			    thp_vma_suitable_order(vma, pvmw->address,
>  						   PMD_ORDER) &&

This is extremely hard to review given the existing crap handling here. I'm
really sorry, but it makes my head hurt (I'm not kidding :) ).

It's completely unclear why we only have to check for a subset of the cases
after taking the lock.

Could we simply extend the existing migration pmd handling and leave the
!pmd_present() case for pmd_none()?

That leaves no question to "which transitions are actually allowed", including
"could we accidentally assume something is a page table when really it isn't".


So what about something like the following?

The "thp_migration_supported()" is not required when checking for
pmd_is_migration_entry(), as that defaults to "false" when not compiled in.

Untested: