+ mm-memory-fix-pmd-pud-checks-in-follow_pfnmap_start.patch added to mm-hotfixes-unstable branch

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

The patch titled
     Subject: mm/memory: fix PMD/PUD checks in follow_pfnmap_start()
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-memory-fix-pmd-pud-checks-in-follow_pfnmap_start.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memory-fix-pmd-pud-checks-in-follow_pfnmap_start.patch

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

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------------------------------------------------------
From: "David Hildenbrand (Arm)" <david@kernel.org>
Subject: mm/memory: fix PMD/PUD checks in follow_pfnmap_start()
Date: Mon, 23 Mar 2026 21:20:18 +0100

follow_pfnmap_start() suffers from two problems:

(1) We are not re-fetching the pmd/pud after taking the PTL

Therefore, we are not properly stabilizing what the lock lock actually
protects.  If there is concurrent zapping, we would indicate to the caller
that we found an entry, however, that entry might already have been
invalidated, or contain a different PFN after taking the lock.

Properly use pmdp_get() / pudp_get() after taking the lock.

(2) pmd_leaf() / pud_leaf() are not well defined on non-present entries

pmd_leaf()/pud_leaf() could wrongly trigger on non-present entries.

There is no real guarantee that pmd_leaf()/pud_leaf() returns something
reasonable on non-present entries.  Most architectures indeed either
perform a present check or make it work by smart use of flags.

However, for example loongarch checks the _PAGE_HUGE flag in pmd_leaf(),
and always sets the _PAGE_HUGE flag in __swp_entry_to_pmd().  Whereby
pmd_trans_huge() explicitly checks pmd_present(), pmd_leaf() does not do
that.

Let's check pmd_present()/pud_present() before assuming "the is a present
PMD leaf" when spotting pmd_leaf()/pud_leaf(), like other page table
handling code that traverses user page tables does.

Given that non-present PMD entries are likely rare in VM_IO|VM_PFNMAP, (1)
is likely more relevant than (2).  It is questionable how often (1) would
actually trigger, but let's CC stable to be sure.

This was found by code inspection.

Link: https://lkml.kernel.org/r/20260323-follow_pfnmap_fix-v1-1-5b0ec10872b3@kernel.org
Fixes: 6da8e9634bb7 ("mm: new follow_pfnmap API")
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memory.c |   18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

--- a/mm/memory.c~mm-memory-fix-pmd-pud-checks-in-follow_pfnmap_start
+++ a/mm/memory.c
@@ -6815,11 +6815,16 @@ retry:
 
 	pudp = pud_offset(p4dp, address);
 	pud = pudp_get(pudp);
-	if (pud_none(pud))
+	if (!pud_present(pud))
 		goto out;
 	if (pud_leaf(pud)) {
 		lock = pud_lock(mm, pudp);
-		if (!unlikely(pud_leaf(pud))) {
+		pud = pudp_get(pudp);
+
+		if (unlikely(!pud_present(pud))) {
+			spin_unlock(lock);
+			goto out;
+		} else if (unlikely(!pud_leaf(pud))) {
 			spin_unlock(lock);
 			goto retry;
 		}
@@ -6831,9 +6836,16 @@ retry:
 
 	pmdp = pmd_offset(pudp, address);
 	pmd = pmdp_get_lockless(pmdp);
+	if (!pmd_present(pmd))
+		goto out;
 	if (pmd_leaf(pmd)) {
 		lock = pmd_lock(mm, pmdp);
-		if (!unlikely(pmd_leaf(pmd))) {
+		pmd = pmdp_get(pmdp);
+
+		if (unlikely(!pmd_present(pmd))) {
+			spin_unlock(lock);
+			goto out;
+		} else if (unlikely(!pmd_leaf(pmd))) {
 			spin_unlock(lock);
 			goto retry;
 		}
_

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

mm-memory-fix-pmd-pud-checks-in-follow_pfnmap_start.patch
mm-centralizefix-comments-about-compound_mapcount-in-new-sync_with_folio_pmd_zap.patch
mm-pagewalk-drop-fw_migration.patch
mm-madvise-drop-range-checks-in-madvise_free_single_vma.patch
mm-memory-remove-zap_details-parameter-from-zap_page_range_single.patch
mm-memory-remove-zap_details-parameter-from-zap_page_range_single-fix.patch
mm-memory-inline-unmap_mapping_range_vma-into-unmap_mapping_range_tree.patch
mm-memory-simplify-calculation-in-unmap_mapping_range_tree.patch
mm-oom_kill-use-mmu_notify_clear-in-__oom_reap_task_mm.patch
mm-oom_kill-factor-out-zapping-of-vma-into-zap_vma_for_reaping.patch
mm-memory-rename-unmap_single_vma-to-__zap_vma_range.patch
mm-memory-move-adjusting-of-address-range-to-unmap_vmas.patch
mm-memory-convert-details-even_cows-into-details-skip_cows.patch
mm-memory-use-__zap_vma_range-in-zap_vma_for_reaping.patch
mm-memory-inline-unmap_page_range-into-__zap_vma_range.patch
mm-memory-inline-unmap_page_range-into-__zap_vma_range-fix.patch
mm-rename-zap_vma_pages-to-zap_vma.patch
mm-rename-zap_page_range_single_batched-to-zap_vma_range_batched.patch
mm-rename-zap_page_range_single-to-zap_vma_range.patch
mm-rename-zap_vma_ptes-to-zap_special_vma_range.patch
mm-memory-support-vm_mixedmap-in-zap_special_vma_range.patch
kasan-docs-slub-is-the-only-remaining-slab-implementation.patch
mm-move-vma_kernel_pagesize-from-hugetlb-to-mmh.patch
mm-move-vma_mmu_pagesize-from-hugetlb-to-vmac.patch
kvm-remove-hugetlbh-inclusion.patch
kvm-ppc-remove-hugetlbh-inclusion.patch
mm-memory_hotplug-remove-for_each_valid_pfn-usage.patch
mm-sparse-remove-warn_ons-from-onlineoffline_mem_sections.patch
mm-kconfig-make-config_memory_hotplug-depend-on-config_sparsemem_vmemmap.patch
mm-memory_hotplug-simplify-check_pfn_span.patch
mm-sparse-remove-config_sparsemem_vmemmap-leftovers-for-config_memory_hotplug.patch
mm-bootmem_info-remove-handling-for-config_sparsemem_vmemmap.patch
mm-bootmem_info-avoid-using-sparse_decode_mem_map.patch
mm-sparse-remove-sparse_decode_mem_map.patch
mm-sparse-remove-config_memory_hotplug-specific-usemap-allocation-handling.patch
mm-prepare-to-move-subsection_map_init-to-mm-sparse-vmemmapc.patch
mm-sparse-drop-set_section_nid-from-sparse_add_section.patch
mm-sparse-move-sparse_init_one_section-to-internalh.patch
mm-sparse-move-sparse_init_one_section-to-internalh-fix.patch
mm-sparse-move-__section_mark_present-to-internalh.patch
mm-sparse-move-memory-hotplug-bits-to-sparse-vmemmapc.patch
mm-remove-config_arch_enable_memory_hotremove.patch
mm-introduce-config_numa_migration-and-simplify-config_migration.patch