+ mm-introduce-copy-on-fork-vmas-and-make-vm_maybe_guard-one.patch added to mm-new branch

+ mm-introduce-copy-on-fork-vmas-and-make-vm_maybe_guard-one.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: mm: introduce copy-on-fork VMAs and make VM_MAYBE_GUARD one
has been added to the -mm mm-new branch.  Its filename is
     mm-introduce-copy-on-fork-vmas-and-make-vm_maybe_guard-one.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-introduce-copy-on-fork-vmas-and-make-vm_maybe_guard-one.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
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fixup patches in mm-new.

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------------------------------------------------------
From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Subject: mm: introduce copy-on-fork VMAs and make VM_MAYBE_GUARD one
Date: Fri, 7 Nov 2025 16:11:49 +0000

Gather all the VMA flags whose presence implies that page tables must be
copied on fork into a single bitmap - VM_COPY_ON_FORK - and use this
rather than specifying individual flags in vma_needs_copy().

We also add VM_MAYBE_GUARD to this list, as it being set on a VMA implies
that there may be metadata contained in the page tables (that is - guard
markers) which would will not and cannot be propagated upon fork.

This was already being done manually previously in vma_needs_copy(), but
this makes it very explicit, alongside VM_PFNMAP, VM_MIXEDMAP and
VM_UFFD_WP all of which imply the same.

Note that VM_STICKY flags ought generally to be marked VM_COPY_ON_FORK too
- because equally a flag being VM_STICKY indicates that the VMA contains
metadat that is not propagated by being faulted in - i.e.  that the VMA
metadata does not fully describe the VMA alone, and thus we must propagate
whatever metadata there is on a fork.

However, for maximum flexibility, we do not make this necessarily the case
here.

Link: https://lkml.kernel.org/r/ca74d10646662f53627a42d49fda3f39932e20d2.1762531708.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm.h               |   26 ++++++++++++++++++++++++++
 mm/memory.c                      |   18 ++++--------------
 tools/testing/vma/vma_internal.h |   26 ++++++++++++++++++++++++++
 3 files changed, 56 insertions(+), 14 deletions(-)

--- a/include/linux/mm.h~mm-introduce-copy-on-fork-vmas-and-make-vm_maybe_guard-one
+++ a/include/linux/mm.h
@@ -557,6 +557,32 @@ extern unsigned int kobjsize(const void
 #define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
 
 /*
+ * Flags which should result in page tables being copied on fork. These are
+ * flags which indicate that the VMA maps page tables which cannot be
+ * reconsistuted upon page fault, so necessitate page table copying upon
+ *
+ * VM_PFNMAP / VM_MIXEDMAP - These contain kernel-mapped data which cannot be
+ *                           reasonably reconstructed on page fault.
+ *
+ *              VM_UFFD_WP - Encodes metadata about an installed uffd
+ *                           write protect handler, which cannot be
+ *                           reconstructed on page fault.
+ *
+ *                           We always copy pgtables when dst_vma has uffd-wp
+ *                           enabled even if it's file-backed
+ *                           (e.g. shmem). Because when uffd-wp is enabled,
+ *                           pgtable contains uffd-wp protection information,
+ *                           that's something we can't retrieve from page cache,
+ *                           and skip copying will lose those info.
+ *
+ *          VM_MAYBE_GUARD - Could contain page guard region markers which
+ *                           by design are a property of the page tables
+ *                           only and thus cannot be reconstructed on page
+ *                           fault.
+ */
+#define VM_COPY_ON_FORK (VM_PFNMAP | VM_MIXEDMAP | VM_UFFD_WP | VM_MAYBE_GUARD)
+
+/*
  * mapping from the currently active vm_flags protection bits (the
  * low four bits) to a page protection mask..
  */
--- a/mm/memory.c~mm-introduce-copy-on-fork-vmas-and-make-vm_maybe_guard-one
+++ a/mm/memory.c
@@ -1465,25 +1465,15 @@ copy_p4d_range(struct vm_area_struct *ds
 static bool
 vma_needs_copy(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
 {
+	if (src_vma->vm_flags & VM_COPY_ON_FORK)
+		return true;
 	/*
-	 * Always copy pgtables when dst_vma has uffd-wp enabled even if it's
-	 * file-backed (e.g. shmem). Because when uffd-wp is enabled, pgtable
-	 * contains uffd-wp protection information, that's something we can't
-	 * retrieve from page cache, and skip copying will lose those info.
+	 * The presence of an anon_vma indicates an anonymous VMA has page
+	 * tables which naturally cannot be reconstituted on page fault.
 	 */
-	if (userfaultfd_wp(dst_vma))
-		return true;
-
-	if (src_vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
-		return true;
-
 	if (src_vma->anon_vma)
 		return true;
 
-	/* Guard regions have momdified page tables that require copying. */
-	if (src_vma->vm_flags & VM_MAYBE_GUARD)
-		return true;
-
 	/*
 	 * Don't copy ptes where a page fault will fill them correctly.  Fork
 	 * becomes much lighter when there are big shared or private readonly
--- a/tools/testing/vma/vma_internal.h~mm-introduce-copy-on-fork-vmas-and-make-vm_maybe_guard-one
+++ a/tools/testing/vma/vma_internal.h
@@ -146,6 +146,32 @@ extern unsigned long dac_mmap_min_addr;
  */
 #define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
 
+/*
+ * Flags which should result in page tables being copied on fork. These are
+ * flags which indicate that the VMA maps page tables which cannot be
+ * reconsistuted upon page fault, so necessitate page table copying upon
+ *
+ * VM_PFNMAP / VM_MIXEDMAP - These contain kernel-mapped data which cannot be
+ *                           reasonably reconstructed on page fault.
+ *
+ *              VM_UFFD_WP - Encodes metadata about an installed uffd
+ *                           write protect handler, which cannot be
+ *                           reconstructed on page fault.
+ *
+ *                           We always copy pgtables when dst_vma has uffd-wp
+ *                           enabled even if it's file-backed
+ *                           (e.g. shmem). Because when uffd-wp is enabled,
+ *                           pgtable contains uffd-wp protection information,
+ *                           that's something we can't retrieve from page cache,
+ *                           and skip copying will lose those info.
+ *
+ *          VM_MAYBE_GUARD - Could contain page guard region markers which
+ *                           by design are a property of the page tables
+ *                           only and thus cannot be reconstructed on page
+ *                           fault.
+ */
+#define VM_COPY_ON_FORK (VM_PFNMAP | VM_MIXEDMAP | VM_UFFD_WP | VM_MAYBE_GUARD)
+
 #define FIRST_USER_ADDRESS	0UL
 #define USER_PGTABLES_CEILING	0UL
 
_

Patches currently in -mm which might be from lorenzo.stoakes@oracle.com are

mm-shmem-update-shmem-to-use-mmap_prepare.patch
device-dax-update-devdax-to-use-mmap_prepare.patch
mm-vma-remove-unused-function-make-internal-functions-static.patch
mm-add-vma_desc_size-vma_desc_pages-helpers.patch
relay-update-relay-to-use-mmap_prepare.patch
mm-vma-rename-__mmap_prepare-function-to-avoid-confusion.patch
mm-add-remap_pfn_range_prepare-remap_pfn_range_complete.patch
mm-abstract-io_remap_pfn_range-based-on-pfn.patch
mm-introduce-io_remap_pfn_range_.patch
mm-add-ability-to-take-further-action-in-vm_area_desc.patch
doc-update-porting-vfs-documentation-for-mmap_prepare-actions.patch
mm-hugetlbfs-update-hugetlbfs-to-use-mmap_prepare.patch
mm-add-shmem_zero_setup_desc.patch
mm-update-mem-char-driver-to-use-mmap_prepare.patch
mm-update-resctl-to-use-mmap_prepare.patch
mm-vma-small-vma-lock-cleanups.patch
mm-correctly-handle-uffd-pte-markers.patch
mm-introduce-leaf-entry-type-and-use-to-simplify-leaf-entry-logic.patch
mm-introduce-leaf-entry-type-and-use-to-simplify-leaf-entry-logic-fix.patch
mm-avoid-unnecessary-uses-of-is_swap_pte.patch
mm-eliminate-uses-of-is_swap_pte-when-leafent_from_pte-suffices.patch
mm-use-leaf-entries-in-debug-pgtable-remove-is_swap_pte.patch
fs-proc-task_mmu-refactor-pagemap_pmd_range.patch
mm-avoid-unnecessary-use-of-is_swap_pmd.patch
mm-huge_memory-refactor-copy_huge_pmd-non-present-logic.patch
mm-huge_memory-refactor-change_huge_pmd-non-present-logic.patch
mm-replace-pmd_to_swp_entry-with-leafent_from_pmd.patch
mm-replace-pmd_to_swp_entry-with-leafent_from_pmd-fix.patch
mm-introduce-pmd_is_huge-and-use-where-appropriate.patch
mm-remove-remaining-is_swap_pmd-users-and-is_swap_pmd.patch
mm-remove-non_swap_entry-and-use-leaf-entry-helpers-instead.patch
mm-remove-non_swap_entry-and-use-leaf-entry-helpers-instead-fix.patch
mm-remove-is_hugetlb_entry_.patch
mm-eliminate-further-swapops-predicates.patch
mm-replace-remaining-pte_to_swp_entry-with-leafent_from_pte.patch
mm-introduce-vm_maybe_guard-and-make-visible-in-proc-pid-smaps.patch
mm-add-atomic-vma-flags-and-set-vm_maybe_guard-as-such.patch
mm-implement-sticky-vma-flags.patch
mm-introduce-copy-on-fork-vmas-and-make-vm_maybe_guard-one.patch
mm-set-the-vm_maybe_guard-flag-on-guard-region-install.patch
tools-testing-vma-add-vma-sticky-userland-tests.patch
tools-testing-selftests-mm-add-madv_collapse-test-case.patch
tools-testing-selftests-mm-add-smaps-visibility-guard-region-test.patch