mm/huge_memory: add device-private THP support to PMD operations

Extend core huge page management functions to handle device-private THP
entries.  This enables proper handling of large device-private folios in
fundamental MM operations.

The following functions have been updated:

- copy_huge_pmd(): Handle device-private entries during fork/clone
- zap_huge_pmd(): Properly free device-private THP during munmap
- change_huge_pmd(): Support protection changes on device-private THP
- __pte_offset_map(): Add device-private entry awareness

Link: https://lkml.kernel.org/r/20251001065707.920170-4-balbirs@nvidia.com
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Balbir Singh <balbirs@nvidia.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Mika Penttilä <mpenttil@redhat.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swapops.h

@@ -594,10 +594,42 @@ static inline int is_pmd_migration_entry(pmd_t pmd)
 }
 #endif  /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
 
+#if defined(CONFIG_ZONE_DEVICE) && defined(CONFIG_ARCH_ENABLE_THP_MIGRATION)
+
+/**
+ * is_pmd_device_private_entry() - Check if PMD contains a device private swap entry
+ * @pmd: The PMD to check
+ *
+ * Returns true if the PMD contains a swap entry that represents a device private
+ * page mapping. This is used for zone device private pages that have been
+ * swapped out but still need special handling during various memory management
+ * operations.
+ *
+ * Return: 1 if PMD contains device private entry, 0 otherwise
+ */
+static inline int is_pmd_device_private_entry(pmd_t pmd)
+{
+	return is_swap_pmd(pmd) && is_device_private_entry(pmd_to_swp_entry(pmd));
+}
+
+#else /* CONFIG_ZONE_DEVICE && CONFIG_ARCH_ENABLE_THP_MIGRATION */
+
+static inline int is_pmd_device_private_entry(pmd_t pmd)
+{
+	return 0;
+}
+
+#endif /* CONFIG_ZONE_DEVICE && CONFIG_ARCH_ENABLE_THP_MIGRATION */
+
 static inline int non_swap_entry(swp_entry_t entry)
 {
 	return swp_type(entry) >= MAX_SWAPFILES;
 }
 
+static inline int is_pmd_non_present_folio_entry(pmd_t pmd)
+{
+	return is_pmd_migration_entry(pmd) || is_pmd_device_private_entry(pmd);
+}
+
 #endif /* CONFIG_MMU */
 #endif /* _LINUX_SWAPOPS_H */

mm/huge_memory.c

@@ -1704,17 +1704,45 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	if (unlikely(is_swap_pmd(pmd))) {
 		swp_entry_t entry = pmd_to_swp_entry(pmd);
 
-		VM_BUG_ON(!is_pmd_migration_entry(pmd));
-		if (!is_readable_migration_entry(entry)) {
-			entry = make_readable_migration_entry(
-							swp_offset(entry));
+		VM_WARN_ON(!is_pmd_non_present_folio_entry(pmd));
+
+		if (is_writable_migration_entry(entry) ||
+		    is_readable_exclusive_migration_entry(entry)) {
+			entry = make_readable_migration_entry(swp_offset(entry));
 			pmd = swp_entry_to_pmd(entry);
 			if (pmd_swp_soft_dirty(*src_pmd))
 				pmd = pmd_swp_mksoft_dirty(pmd);
 			if (pmd_swp_uffd_wp(*src_pmd))
 				pmd = pmd_swp_mkuffd_wp(pmd);
 			set_pmd_at(src_mm, addr, src_pmd, pmd);
+		} else if (is_device_private_entry(entry)) {
+			/*
+			 * For device private entries, since there are no
+			 * read exclusive entries, writable = !readable
+			 */
+			if (is_writable_device_private_entry(entry)) {
+				entry = make_readable_device_private_entry(swp_offset(entry));
+				pmd = swp_entry_to_pmd(entry);
+
+				if (pmd_swp_soft_dirty(*src_pmd))
+					pmd = pmd_swp_mksoft_dirty(pmd);
+				if (pmd_swp_uffd_wp(*src_pmd))
+					pmd = pmd_swp_mkuffd_wp(pmd);
+				set_pmd_at(src_mm, addr, src_pmd, pmd);
+			}
+
+			src_folio = pfn_swap_entry_folio(entry);
+			VM_WARN_ON(!folio_test_large(src_folio));
+
+			folio_get(src_folio);
+			/*
+			 * folio_try_dup_anon_rmap_pmd does not fail for
+			 * device private entries.
+			 */
+			folio_try_dup_anon_rmap_pmd(src_folio, &src_folio->page,
+							dst_vma, src_vma);
 		}
+
 		add_mm_counter(dst_mm, MM_ANONPAGES, HPAGE_PMD_NR);
 		mm_inc_nr_ptes(dst_mm);
 		pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
@@ -2212,15 +2240,16 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 			folio_remove_rmap_pmd(folio, page, vma);
 			WARN_ON_ONCE(folio_mapcount(folio) < 0);
 			VM_BUG_ON_PAGE(!PageHead(page), page);
-		} else if (thp_migration_supported()) {
+		} else if (is_pmd_non_present_folio_entry(orig_pmd)) {
 			swp_entry_t entry;
 
-			VM_BUG_ON(!is_pmd_migration_entry(orig_pmd));
 			entry = pmd_to_swp_entry(orig_pmd);
 			folio = pfn_swap_entry_folio(entry);
 			flush_needed = 0;
-		} else
-			WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
+
+			if (!thp_migration_supported())
+				WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
+		}
 
 		if (folio_test_anon(folio)) {
 			zap_deposited_table(tlb->mm, pmd);
@@ -2240,6 +2269,12 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 				folio_mark_accessed(folio);
 		}
 
+		if (folio_is_device_private(folio)) {
+			folio_remove_rmap_pmd(folio, &folio->page, vma);
+			WARN_ON_ONCE(folio_mapcount(folio) < 0);
+			folio_put(folio);
+		}
+
 		spin_unlock(ptl);
 		if (flush_needed)
 			tlb_remove_page_size(tlb, &folio->page, HPAGE_PMD_SIZE);
@@ -2368,7 +2403,7 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		struct folio *folio = pfn_swap_entry_folio(entry);
 		pmd_t newpmd;
 
-		VM_BUG_ON(!is_pmd_migration_entry(*pmd));
+		VM_WARN_ON(!is_pmd_non_present_folio_entry(*pmd));
 		if (is_writable_migration_entry(entry)) {
 			/*
 			 * A protection check is difficult so
@@ -2381,6 +2416,9 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 			newpmd = swp_entry_to_pmd(entry);
 			if (pmd_swp_soft_dirty(*pmd))
 				newpmd = pmd_swp_mksoft_dirty(newpmd);
+		} else if (is_writable_device_private_entry(entry)) {
+			entry = make_readable_device_private_entry(swp_offset(entry));
+			newpmd = swp_entry_to_pmd(entry);
 		} else {
 			newpmd = *pmd;
 		}

mm/pgtable-generic.c

@@ -292,7 +292,7 @@ pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
 
 	if (pmdvalp)
 		*pmdvalp = pmdval;
-	if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
+	if (unlikely(pmd_none(pmdval) || !pmd_present(pmdval)))
 		goto nomap;
 	if (unlikely(pmd_trans_huge(pmdval)))
 		goto nomap;