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

[Wei Yang <richard.weiyang@gmail.com>]

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>
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)
 			/* 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