+ mm-huge_memory-separate-out-config_persistent_huge_zero_folio-logic.patch added to mm-new branch

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

The patch titled
     Subject: mm/huge_memory: separate out CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic
has been added to the -mm mm-new branch.  Its filename is
     mm-huge_memory-separate-out-config_persistent_huge_zero_folio-logic.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-separate-out-config_persistent_huge_zero_folio-logic.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
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Subject: mm/huge_memory: separate out CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic
Date: Tue, 28 Jul 2026 13:05:44 +0100

Patch series "mm/huge_memory: fix huge_zero_pfn race".

There is a subtle race in the reference-counted huge_zero_folio
implementation.

The fast path atomic logic fails to account for the fact that the shrinker
(which drops the final huge_zero_refcount pin) can overwrite huge_zero_pfn
with the ~0UL sentinel value in shrink_huge_zero_folio_scan() after a
racing get_huge_zero_folio() installed a valid value there.

This results in huge_zero_folio being correctly set but huge_zero_pfn
being set incorrectly and thus is_huge_zero_pfn() and consequently
is_huge_zero_pmd() will misidentify the huge zero folio as being an
ordinary THP folio.

This can result in the huge zero folio being split and otherwise treated
incorrectly.

The solution to this is very subtle as there is an atomic fast path, and
thus ordering in weakly ordered architectures has to be treated very
carefully.

As a result, this series first reworks the
CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic so it is separated from the
refcounted code in order to make the subsequent fix reasonably
understandable.

The second commit fixes the issue by introducing a spinlock around
huge_zero_[pfn, folio, refcount] write, with careful consideration paid to
load/store ordering in the fast path.


This patch (of 2):

Rather than mixing the refcounted and non-refcounted
CONFIG_PERSISTENT_HUGE_ZERO_FOLIO logic, separate the two out cleanly so
it is clear what happens when this configuration option is set and what
happens when it is not.

Introduce HUGE_ZERO_UNSET_PFN to abstract the ~0UL assignment, only
introduce the refcount and shrinker if !CONFIG_PERSISTENT_HUGE_ZERO_FOLIO,
abstract initialisation and teardown, abstract the huge zero folio
allocation from refcounting.

Also change a BUG_ON() to WARN_ON_ONCE() while we're at it.

Without this change, the subsequent fix for a subtle race is harder to
understand thus this is a dependency of it.

Link: https://lore.kernel.org/20260728-fix-refcounted-huge-zero-v1-1-3f261f5447b4@kernel.org
Fixes: 3b77e8c8cde5 ("mm/thp: make is_huge_zero_pmd() safe and quicker")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Hengbin Zhang <uqbarz@gmail.com>
Cc: Xueyuan Chen <xueyuan.chen@vivo.com>
Cc: <stable@vger.kernel.org> # 6.18.x: dependency of subsequent fix
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/huge_memory.c |  159 ++++++++++++++++++++++++++-------------------
 1 file changed, 94 insertions(+), 65 deletions(-)

--- a/mm/huge_memory.c~mm-huge_memory-separate-out-config_persistent_huge_zero_folio-logic
+++ a/mm/huge_memory.c
@@ -77,9 +77,14 @@ static unsigned long deferred_split_scan
 					 struct shrink_control *sc);
 static bool split_underused_thp = true;
 
-static atomic_t huge_zero_refcount;
+#define HUGE_ZERO_UNSET_PFN (~0UL)
 struct folio *huge_zero_folio __read_mostly;
-unsigned long huge_zero_pfn __read_mostly = ~0UL;
+unsigned long huge_zero_pfn __read_mostly = HUGE_ZERO_UNSET_PFN;
+#ifndef CONFIG_PERSISTENT_HUGE_ZERO_FOLIO
+static atomic_t huge_zero_refcount;
+static struct shrinker *huge_zero_folio_shrinker;
+#endif
+
 unsigned long huge_anon_orders_always __read_mostly;
 unsigned long huge_anon_orders_madvise __read_mostly;
 unsigned long huge_anon_orders_inherit __read_mostly;
@@ -221,22 +226,58 @@ unsigned long __thp_vma_allowable_orders
 	return orders;
 }
 
-static bool get_huge_zero_folio(void)
+static struct folio *alloc_huge_zero_folio(void)
 {
 	struct folio *zero_folio;
-retry:
-	if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
-		return true;
 
 	zero_folio = folio_alloc((GFP_TRANSHUGE | __GFP_ZERO | __GFP_ZEROTAGS) &
 				 ~__GFP_MOVABLE,
 			HPAGE_PMD_ORDER);
 	if (!zero_folio) {
 		count_vm_event(THP_ZERO_PAGE_ALLOC_FAILED);
-		return false;
+		return NULL;
+	}
+	folio_clear_large_rmappable(zero_folio); /* Explicitly not rmappable. */
+	return zero_folio;
+}
+
+#ifdef CONFIG_PERSISTENT_HUGE_ZERO_FOLIO
+static int __init huge_zero_init(void)
+{
+	huge_zero_folio = alloc_huge_zero_folio();
+	if (!huge_zero_folio) {
+		pr_warn("Allocating persistent huge zero folio failed\n");
+	} else {
+		huge_zero_pfn = folio_pfn(huge_zero_folio);
+		count_vm_event(THP_ZERO_PAGE_ALLOC);
 	}
-	/* Ensure zero folio won't have large_rmappable flag set. */
-	folio_clear_large_rmappable(zero_folio);
+	return 0;
+}
+
+static void __init huge_zero_shrinker_exit(void)
+{
+}
+
+struct folio *mm_get_huge_zero_folio(struct mm_struct *mm)
+{
+	return huge_zero_folio;
+}
+
+void mm_put_huge_zero_folio(struct mm_struct *mm)
+{
+}
+#else
+static bool get_huge_zero_folio(void)
+{
+	struct folio *zero_folio;
+retry:
+	if (likely(atomic_inc_not_zero(&huge_zero_refcount)))
+		return true;
+
+	zero_folio = alloc_huge_zero_folio();
+	if (unlikely(!zero_folio))
+		return false;
+
 	preempt_disable();
 	if (cmpxchg(&huge_zero_folio, NULL, zero_folio)) {
 		preempt_enable();
@@ -258,33 +299,7 @@ static void put_huge_zero_folio(void)
 	 * Counter should never go to zero here. Only shrinker can put
 	 * last reference.
 	 */
-	BUG_ON(atomic_dec_and_test(&huge_zero_refcount));
-}
-
-struct folio *mm_get_huge_zero_folio(struct mm_struct *mm)
-{
-	if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO))
-		return huge_zero_folio;
-
-	if (mm_flags_test(MMF_HUGE_ZERO_FOLIO, mm))
-		return READ_ONCE(huge_zero_folio);
-
-	if (!get_huge_zero_folio())
-		return NULL;
-
-	if (mm_flags_test_and_set(MMF_HUGE_ZERO_FOLIO, mm))
-		put_huge_zero_folio();
-
-	return READ_ONCE(huge_zero_folio);
-}
-
-void mm_put_huge_zero_folio(struct mm_struct *mm)
-{
-	if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO))
-		return;
-
-	if (mm_flags_test(MMF_HUGE_ZERO_FOLIO, mm))
-		put_huge_zero_folio();
+	WARN_ON_ONCE(atomic_dec_and_test(&huge_zero_refcount));
 }
 
 static unsigned long shrink_huge_zero_folio_count(struct shrinker *shrink,
@@ -300,7 +315,7 @@ static unsigned long shrink_huge_zero_fo
 	if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
 		struct folio *zero_folio = xchg(&huge_zero_folio, NULL);
 		BUG_ON(zero_folio == NULL);
-		WRITE_ONCE(huge_zero_pfn, ~0UL);
+		WRITE_ONCE(huge_zero_pfn, HUGE_ZERO_UNSET_PFN);
 		folio_put(zero_folio);
 		return HPAGE_PMD_NR;
 	}
@@ -308,7 +323,46 @@ static unsigned long shrink_huge_zero_fo
 	return 0;
 }
 
-static struct shrinker *huge_zero_folio_shrinker;
+static int __init huge_zero_init(void)
+{
+	huge_zero_folio_shrinker = shrinker_alloc(0, "thp-zero");
+	if (!huge_zero_folio_shrinker) {
+		shrinker_free(deferred_split_shrinker);
+		list_lru_destroy(&deferred_split_lru);
+		return -ENOMEM;
+	}
+
+	huge_zero_folio_shrinker->count_objects = shrink_huge_zero_folio_count;
+	huge_zero_folio_shrinker->scan_objects = shrink_huge_zero_folio_scan;
+	shrinker_register(huge_zero_folio_shrinker);
+	return 0;
+}
+
+static void __init huge_zero_shrinker_exit(void)
+{
+	shrinker_free(huge_zero_folio_shrinker);
+}
+
+struct folio *mm_get_huge_zero_folio(struct mm_struct *mm)
+{
+	if (mm_flags_test(MMF_HUGE_ZERO_FOLIO, mm))
+		return READ_ONCE(huge_zero_folio);
+
+	if (!get_huge_zero_folio())
+		return NULL;
+
+	if (mm_flags_test_and_set(MMF_HUGE_ZERO_FOLIO, mm))
+		put_huge_zero_folio();
+
+	return READ_ONCE(huge_zero_folio);
+}
+
+void mm_put_huge_zero_folio(struct mm_struct *mm)
+{
+	if (mm_flags_test(MMF_HUGE_ZERO_FOLIO, mm))
+		put_huge_zero_folio();
+}
+#endif /* CONFIG_PERSISTENT_HUGE_ZERO_FOLIO */
 
 #ifdef CONFIG_SYSFS
 static ssize_t enabled_show(struct kobject *kobj,
@@ -970,39 +1024,14 @@ static int __init thp_shrinker_init(void
 	deferred_split_shrinker->scan_objects = deferred_split_scan;
 	shrinker_register(deferred_split_shrinker);
 
-	if (IS_ENABLED(CONFIG_PERSISTENT_HUGE_ZERO_FOLIO)) {
-		/*
-		 * Bump the reference of the huge_zero_folio and do not
-		 * initialize the shrinker.
-		 *
-		 * huge_zero_folio will always be NULL on failure. We assume
-		 * that get_huge_zero_folio() will most likely not fail as
-		 * thp_shrinker_init() is invoked early on during boot.
-		 */
-		if (!get_huge_zero_folio())
-			pr_warn("Allocating persistent huge zero folio failed\n");
-		return 0;
-	}
-
-	huge_zero_folio_shrinker = shrinker_alloc(0, "thp-zero");
-	if (!huge_zero_folio_shrinker) {
-		shrinker_free(deferred_split_shrinker);
-		list_lru_destroy(&deferred_split_lru);
-		return -ENOMEM;
-	}
-
-	huge_zero_folio_shrinker->count_objects = shrink_huge_zero_folio_count;
-	huge_zero_folio_shrinker->scan_objects = shrink_huge_zero_folio_scan;
-	shrinker_register(huge_zero_folio_shrinker);
-
-	return 0;
+	return huge_zero_init();
 }
 
 static void __init thp_shrinker_exit(void)
 {
-	shrinker_free(huge_zero_folio_shrinker);
 	shrinker_free(deferred_split_shrinker);
 	list_lru_destroy(&deferred_split_lru);
+	huge_zero_shrinker_exit();
 }
 
 static int __init hugepage_init(void)
_

Patches currently in -mm which might be from ljs@kernel.org are

mm-vmalloc-acquire-init_mm-lock-on-huge-vmap-to-avoid-ptdump-uaf.patch
mm-ptdump-always-stabilise-against-page-table-freeing-using-init_mm.patch
arm64-remove-redundant-concurrent-ptdump-uaf-mitigation.patch
x86-mm-pat-acquire-init_mm-write-lock-on-collapse-to-avoid-uaf.patch
x86-mm-pat-acquire-init_mm-read-lock-on-attribute-change-to-avoid-uaf.patch
x86-mm-pat-allocate-split-page-tables-as-kernel-page-tables.patch
mm-move-vma_start_pgoff-into-mmh-and-clean-up.patch
mm-add-kdoc-comments-for-vma_start-last_pgoff.patch
tools-testing-vma-use-vma_start_pgoff-in-merge-tests.patch
mm-introduce-and-use-vma_end_pgoff.patch
mm-rmap-update-mm-interval_treec-comments.patch
mm-rmap-parameterise-vma_interval_tree_-by-address_space.patch
mm-rmap-elide-unnecessary-static-inlines-in-interval_treec.patch
mm-rmap-rename-vma_interval_tree_-to-mapping_rmap_tree_.patch
mm-rmap-parameterise-anon_vma_interval_tree_-by-anon_vma.patch
mm-rmap-rename-anon_vma_interval_tree_-params-and-use-pgoff_t.patch
mm-rmap-rename-anon_vma_interval_tree_-to-anon_rmap_tree_.patch
maintainers-move-mm-interval_treec-to-rmap-section.patch
mm-vma-introduce-and-use-vmg_pages-vmg__pgoff.patch
mm-vma-clean-up-anon_vma_compatible.patch
mm-vma-refactor-vmg_adjust_set_range-for-clarity.patch
mm-vma-minor-cleanup-of-expand_.patch
mm-introduce-and-use-linear_page_delta.patch
mm-vma-use-vma_start_pgoff-linear_page_index-in-mm-code.patch
mm-prefer-vma__pgoff-to-vma-vm_pgoff-in-kernel.patch
mm-vma-remove-duplicative-vma_pgoff_offset-helper.patch
mm-use-linear_page_-consistently.patch
mm-vma-introduce-vma_assert_can_modify.patch
mm-vma-add-and-use-vma__pgoff.patch
mm-vma-move-__install_special_mapping-to-vmac.patch
mm-vma-make-vma_set_range-static-drop-insert_vm_struct-decl.patch
mm-vma-update-vma_shrink-to-not-pass-start-pgoff-parameters.patch
mm-vma-update-vmg_adjust_set_range-to-offset-pgoff-instead.patch
mm-vma-slightly-rework-the-anonymous-check-in-__mmap_new_vma.patch
mm-vma-introduce-and-use-vma_set_pgoff.patch
mm-vma-correct-incorrect-vmah-inclusion.patch
mm-vma-use-guard-clauses-in-can_vma_merge_.patch
tools-testing-vma-default-vma-mm-flag-bits-to-64-bit.patch
tools-testing-vma-output-compared-expression-on-assert_.patch
mm-introduce-vma_flags_can_grow-and-vma_can_grow.patch
mm-vma-update-do_mmap-to-use-vma_flags_t.patch
mm-convert-__get_unmapped_area-to-use-vma_flags_t.patch
mm-update-generic_get_unmapped_area-to-use-vma_flags_t.patch
mm-prefer-mm-def_vma_flags-in-mm-logic.patch
mm-vma-convert-vm_pgprot_modify-to-use-vma_flags_t-and-rename.patch
mm-vma-rename-vma_get_page_prot-to-vma_flags_to_page_prot.patch
mm-introduce-vma_get_page_prot-and-use-it.patch
mm-vma-update-create_init_stack_vma-to-use-vma_flags_t.patch
mm-vma-convert-miscellaneous-uses-of-vma-flags-in-core-mm.patch
mm-mlock-convert-mlock-code-to-use-vma_flags_t.patch
mm-mprotect-convert-mprotect-code-to-use-vma_flags_t.patch
mm-mremap-convert-mremap-code-to-use-vma_flags_t.patch
mm-mseal-remove-superfluous-comments-fix-confusion-around-mm.patch
mm-mseal-limit-scope-of-mseal-address-zero-to-address-zero.patch
mm-mseal-remove-further-superfluous-comments-do_mseal.patch
mm-vma-introduce-vma-virtual-page-offset-field-and-add-helpers.patch
mm-introduce-linear_virt_page_index.patch
mm-abstract-vma_address-and-introduce-vma_anon_address.patch
mm-update-print_bad_page_map-to-show-virtual-page-index.patch
mm-introduce-and-use-vma_filebacked_address.patch
mm-propagate-vma-virtual-page-offset-on-map-remap-split-merge.patch
mm-rmap-track-whether-the-page-vma-mapped-walk-is-anonymous.patch
mm-introduce-and-use-linear_folio_page_index.patch
mm-rmap-use-virt-pgoff-for-map_private-file-backed-anon-folios.patch
tools-testing-vma-expand-vma-merge-tests-to-assert-virt-pgoff.patch
tools-testing-selftests-mm-test-virtual-page-offset-merge-behaviour.patch
mm-vma-only-permit-map_private-dev-zero-to-be-mapped-anonymous.patch
mm-vma-make-map_private-mapped-dev-zero-mappings-truly-anonymous.patch
tools-testing-vma-add-test-to-assert-map_private-dev-zero-is-anon.patch
tools-testing-selftests-mm-add-map_private-dev-zero-merge-tests.patch
mm-huge_memory-separate-out-config_persistent_huge_zero_folio-logic.patch
mm-huge_memory-fix-huge_zero_pfn-race.patch