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.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
---
 include/linux/mm.h               | 26 ++++++++++++++++++++++++++
 mm/memory.c                      | 18 ++++--------------
 tools/testing/vma/vma_internal.h | 26 ++++++++++++++++++++++++++
 3 files changed, 56 insertions(+), 14 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index fea113d1d723..af2904aeb163 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -555,6 +555,32 @@ extern unsigned int kobjsize(const void *objp);
  */
 #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..
diff --git a/mm/memory.c b/mm/memory.c
index a520720702f0..732414852570 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1463,25 +1463,15 @@ copy_p4d_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
 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 modified 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
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index 73c2025777e6..233819a9e7ee 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -145,6 +145,32 @@ 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
 
-- 
2.51.2

On 18.11.25 11:17, Lorenzo Stoakes wrote:
> 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.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Reviewed-by: Pedro Falcato <pfalcato@suse.de>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> ---

Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>

-- 
Cheers

David