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

[Wei Yang]

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@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>

---
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@xxxxxxxxx/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@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 | 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) &&
-- 
2.34.1