mm/page_vma_mapped: fix device-private PMD handling

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().

[akpm@linux-foundation.org: fix Raspberry Pi 1 build, per David]
Link: https://lore.kernel.org/20260630021540.17297-1-richard.weiyang@gmail.com
Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Suggested-by: David Hildenbrand <david@kernel.org>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Tested-by: Klara Modin <klarasmodin@gmail.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: <stable@vger.kernel.org>q
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index 2ccbabfb2cc17a6ab9c14f2df0753cbbbfbae199..bac2eb5de63d6149fb5e073ba1ad24f07736a1ac 100644 (file)
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -243,21 +243,31 @@ restart:
                 */
                pmde = pmdp_get_lockless(pvmw->pmd);
 
-               if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
+               if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+                   (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 +276,10 @@ restart:
                                        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) &&