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

[Lance Yang <lance.yang@linux.dev>]

On Wed, Jun 24, 2026 at 06:53:53AM +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.
>
>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>

Shouldn't we add

Suggested-by: Lorenzo Stoakes <ljs@kernel.org>

as well?

v4 mostly follows Lorenzo's comments, code bits included. Feels only fair.

>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>
>
>---
>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@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 | 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)


Hmm ... looks like there may still be a race here ...

Current code picks the branch from the lockless PMD value:

		pmde = pmdp_get_lockless(pvmw->pmd);

		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
			pmde = *pvmw->pmd;
			if (!pmd_present(pmde)) {
				softleaf_t entry;

				if (!thp_migration_supported() ||
				    !(pvmw->flags & PVMW_MIGRATION))
					return not_found(pvmw);
				entry = softleaf_from_pmd(pmde);

				if (!softleaf_is_migration(entry) ||
				    !check_pmd(softleaf_to_pfn(entry), pvmw))
					return not_found(pvmw);
				return true;
			}
		}

But after taking PTL, the PMD may already be a different non-present PMD
type:

CPU0: pmde = pmdp_get_lockless();   // sees PMD migration entry

CPU1: remove_migration_ptes(src, dst /* device-private */)
        ... via rmap_walk(dst) ...
        page_vma_mapped_walk(&pvmw /* src, PVMW_MIGRATION */)
          returns with PTL held for the PMD migration entry
        remove_migration_pmd(new = dst page)
          installs a device-private PMD
        next page_vma_mapped_walk()
          drops PTL via not_found()

CPU0: takes PTL
      pmde = *pvmw->pmd;            // now device-private PMD

So when PVMW_MIGRATION is not set, current code can return not_found()
before we even decode the locked PMD as a device-private entry.

Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
device-private entries") made the

device-private PMD <-> PMD migration

transition possible.

set_pmd_migration_entry() can replace a device-private PMD with a PMD
migration entry, and remove_migration_pmd() can restore a PMD migration
entry back to a device-private PMD when the new folio is device-private.

Maybe decode the locked softleaf entry first, before the migration-only
checks? Something like this on top:

---8<---
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 17dff8aab9f9..97babd408dba 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -249,10 +249,18 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
 			if (!pmd_present(pmde)) {
 				softleaf_t entry;

+				entry = softleaf_from_pmd(pmde);
+				if (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;
+				}
+
 				if (!thp_migration_supported() ||
 				    !(pvmw->flags & PVMW_MIGRATION))
 					return not_found(pvmw);
-				entry = softleaf_from_pmd(pmde);

 				if (!softleaf_is_migration(entry) ||
 				    !check_pmd(softleaf_to_pfn(entry), pvmw))
@@ -266,7 +274,10 @@ 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 pmd was split under us, or device-private PMD
+			 * changed under us: handle on pte level.
+			 */
 			spin_unlock(pvmw->ptl);
 			pvmw->ptl = NULL;
 		} else if (pmd_is_device_private_entry(pmde)) {
--

Anyway, that stuff is getting kinda messy now. Feels like it really needs
a cleanup on top before it bites us again :)

Cheers, Lance

> 			/* 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) &&
>-- 
>2.34.1
>
>