+ mm-huge_memory-implement-device-private-thp-splitting.patch added to mm-new branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: mm/huge_memory: implement device-private THP splitting
has been added to the -mm mm-new branch.  Its filename is
     mm-huge_memory-implement-device-private-thp-splitting.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-huge_memory-implement-device-private-thp-splitting.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
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------------------------------------------------------
From: Balbir Singh <balbirs@nvidia.com>
Subject: mm/huge_memory: implement device-private THP splitting
Date: Mon, 8 Sep 2025 10:04:37 +1000

Add support for splitting device-private THP folios, enabling fallback
to smaller page sizes when large page allocation or migration fails.

Key changes:
- split_huge_pmd(): Handle device-private PMD entries during splitting
- Preserve RMAP_EXCLUSIVE semantics for anonymous exclusive folios
- Skip RMP_USE_SHARED_ZEROPAGE for device-private entries as they
  don't support shared zero page semantics

Link: https://lkml.kernel.org/r/20250908000448.180088-5-balbirs@nvidia.com
Signed-off-by: Balbir Singh <balbirs@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Zi Yan <ziy@nvidia.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: Matthew Brost <matthew.brost@intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/huge_memory.c |  129 +++++++++++++++++++++++++++++++--------------
 1 file changed, 91 insertions(+), 38 deletions(-)

--- a/mm/huge_memory.c~mm-huge_memory-implement-device-private-thp-splitting
+++ a/mm/huge_memory.c
@@ -2880,16 +2880,19 @@ static void __split_huge_pmd_locked(stru
 	struct page *page;
 	pgtable_t pgtable;
 	pmd_t old_pmd, _pmd;
-	bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
-	bool anon_exclusive = false, dirty = false;
+	bool young, write, soft_dirty, uffd_wp = false;
+	bool anon_exclusive = false, dirty = false, present = false;
 	unsigned long addr;
 	pte_t *pte;
 	int i;
+	swp_entry_t swp_entry;
 
 	VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
 	VM_BUG_ON_VMA(vma->vm_start > haddr, vma);
 	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
-	VM_BUG_ON(!is_pmd_migration_entry(*pmd) && !pmd_trans_huge(*pmd));
+
+	VM_WARN_ON(!is_pmd_migration_entry(*pmd) && !pmd_trans_huge(*pmd) &&
+			!is_pmd_device_private_entry(*pmd));
 
 	count_vm_event(THP_SPLIT_PMD);
 
@@ -2937,18 +2940,43 @@ static void __split_huge_pmd_locked(stru
 		return __split_huge_zero_page_pmd(vma, haddr, pmd);
 	}
 
-	pmd_migration = is_pmd_migration_entry(*pmd);
-	if (unlikely(pmd_migration)) {
-		swp_entry_t entry;
 
+	present = pmd_present(*pmd);
+	if (unlikely(!present)) {
+		swp_entry = pmd_to_swp_entry(*pmd);
 		old_pmd = *pmd;
-		entry = pmd_to_swp_entry(old_pmd);
-		page = pfn_swap_entry_to_page(entry);
-		write = is_writable_migration_entry(entry);
-		if (PageAnon(page))
-			anon_exclusive = is_readable_exclusive_migration_entry(entry);
-		young = is_migration_entry_young(entry);
-		dirty = is_migration_entry_dirty(entry);
+
+		folio = pfn_swap_entry_folio(swp_entry);
+		VM_WARN_ON(!is_migration_entry(swp_entry) &&
+				!is_device_private_entry(swp_entry));
+		page = pfn_swap_entry_to_page(swp_entry);
+
+		if (is_pmd_migration_entry(old_pmd)) {
+			write = is_writable_migration_entry(swp_entry);
+			if (PageAnon(page))
+				anon_exclusive =
+					is_readable_exclusive_migration_entry(
+								swp_entry);
+			young = is_migration_entry_young(swp_entry);
+			dirty = is_migration_entry_dirty(swp_entry);
+		} else if (is_pmd_device_private_entry(old_pmd)) {
+			write = is_writable_device_private_entry(swp_entry);
+			anon_exclusive = PageAnonExclusive(page);
+			if (freeze && anon_exclusive &&
+			    folio_try_share_anon_rmap_pmd(folio, page))
+				freeze = false;
+			if (!freeze) {
+				rmap_t rmap_flags = RMAP_NONE;
+
+				folio_ref_add(folio, HPAGE_PMD_NR - 1);
+				if (anon_exclusive)
+					rmap_flags |= RMAP_EXCLUSIVE;
+
+				folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR,
+						 vma, haddr, rmap_flags);
+			}
+		}
+
 		soft_dirty = pmd_swp_soft_dirty(old_pmd);
 		uffd_wp = pmd_swp_uffd_wp(old_pmd);
 	} else {
@@ -3034,30 +3062,49 @@ static void __split_huge_pmd_locked(stru
 	 * Note that NUMA hinting access restrictions are not transferred to
 	 * avoid any possibility of altering permissions across VMAs.
 	 */
-	if (freeze || pmd_migration) {
+	if (freeze || !present) {
 		for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
 			pte_t entry;
-			swp_entry_t swp_entry;
-
-			if (write)
-				swp_entry = make_writable_migration_entry(
-							page_to_pfn(page + i));
-			else if (anon_exclusive)
-				swp_entry = make_readable_exclusive_migration_entry(
-							page_to_pfn(page + i));
-			else
-				swp_entry = make_readable_migration_entry(
-							page_to_pfn(page + i));
-			if (young)
-				swp_entry = make_migration_entry_young(swp_entry);
-			if (dirty)
-				swp_entry = make_migration_entry_dirty(swp_entry);
-			entry = swp_entry_to_pte(swp_entry);
-			if (soft_dirty)
-				entry = pte_swp_mksoft_dirty(entry);
-			if (uffd_wp)
-				entry = pte_swp_mkuffd_wp(entry);
-
+			if (freeze || is_migration_entry(swp_entry)) {
+				if (write)
+					swp_entry = make_writable_migration_entry(
+								page_to_pfn(page + i));
+				else if (anon_exclusive)
+					swp_entry = make_readable_exclusive_migration_entry(
+								page_to_pfn(page + i));
+				else
+					swp_entry = make_readable_migration_entry(
+								page_to_pfn(page + i));
+				if (young)
+					swp_entry = make_migration_entry_young(swp_entry);
+				if (dirty)
+					swp_entry = make_migration_entry_dirty(swp_entry);
+				entry = swp_entry_to_pte(swp_entry);
+				if (soft_dirty)
+					entry = pte_swp_mksoft_dirty(entry);
+				if (uffd_wp)
+					entry = pte_swp_mkuffd_wp(entry);
+			} else {
+				/*
+				 * anon_exclusive was already propagated to the relevant
+				 * pages corresponding to the pte entries when freeze
+				 * is false.
+				 */
+				if (write)
+					swp_entry = make_writable_device_private_entry(
+								page_to_pfn(page + i));
+				else
+					swp_entry = make_readable_device_private_entry(
+								page_to_pfn(page + i));
+				/*
+				 * Young and dirty bits are not progated via swp_entry
+				 */
+				entry = swp_entry_to_pte(swp_entry);
+				if (soft_dirty)
+					entry = pte_swp_mksoft_dirty(entry);
+				if (uffd_wp)
+					entry = pte_swp_mkuffd_wp(entry);
+			}
 			VM_WARN_ON(!pte_none(ptep_get(pte + i)));
 			set_pte_at(mm, addr, pte + i, entry);
 		}
@@ -3084,7 +3131,7 @@ static void __split_huge_pmd_locked(stru
 	}
 	pte_unmap(pte);
 
-	if (!pmd_migration)
+	if (!is_pmd_migration_entry(*pmd))
 		folio_remove_rmap_pmd(folio, page, vma);
 	if (freeze)
 		put_page(page);
@@ -3096,8 +3143,10 @@ static void __split_huge_pmd_locked(stru
 void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
 			   pmd_t *pmd, bool freeze)
 {
+
 	VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
-	if (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd))
+	if (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd) ||
+		is_pmd_device_private_entry(*pmd))
 		__split_huge_pmd_locked(vma, pmd, address, freeze);
 }
 
@@ -3276,6 +3325,9 @@ static void lru_add_split_folio(struct f
 	VM_BUG_ON_FOLIO(folio_test_lru(new_folio), folio);
 	lockdep_assert_held(&lruvec->lru_lock);
 
+	if (folio_is_device_private(folio))
+		return;
+
 	if (list) {
 		/* page reclaim is reclaiming a huge page */
 		VM_WARN_ON(folio_test_lru(folio));
@@ -3894,8 +3946,9 @@ fail:
 	if (nr_shmem_dropped)
 		shmem_uncharge(mapping->host, nr_shmem_dropped);
 
-	if (!ret && is_anon)
+	if (!ret && is_anon && !folio_is_device_private(folio))
 		remap_flags = RMP_USE_SHARED_ZEROPAGE;
+
 	remap_page(folio, 1 << order, remap_flags);
 
 	/*
_

Patches currently in -mm which might be from balbirs@nvidia.com are

mm-zone_device-support-large-zone-device-private-folios.patch
mm-huge_memory-add-device-private-thp-support-to-pmd-operations.patch
mm-rmap-extend-rmap-and-migration-support-device-private-entries.patch
mm-huge_memory-implement-device-private-thp-splitting.patch
mm-migrate_device-handle-partially-mapped-folios-during-collection.patch
mm-migrate_device-implement-thp-migration-of-zone-device-pages.patch
mm-memory-fault-add-thp-fault-handling-for-zone-device-private-pages.patch
lib-test_hmm-add-zone-device-private-thp-test-infrastructure.patch
mm-memremap-add-driver-callback-support-for-folio-splitting.patch
mm-migrate_device-add-thp-splitting-during-migration.patch
lib-test_hmm-add-large-page-allocation-failure-testing.patch
selftests-mm-hmm-tests-new-tests-for-zone-device-thp-migration.patch
selftests-mm-hmm-tests-new-throughput-tests-including-thp.patch
gpu-drm-nouveau-enable-thp-support-for-gpu-memory-migration.patch