Re: [PATCH 3/6] userfaultfd: use userfaultfd_*() helpers instead of open coded flag tests

On Mon, Aug 24, 2026 at 04:10:51PM +0100, Lorenzo Stoakes (ARM) wrote:

On Sun, Aug 23, 2026 at 03:17:40PM +0300, Mike Rapoport (Microsoft) wrote:

quoted

Move userfaultfd_{missing,wp,minor,rwp}() and userfaultfd_protected()
ahead of uffd_disable_huge_pmd_share() and uffd_disable_fault_around()
and make the latter two use the helpers rather than open coded VMA flag
masks.

Convert open coded VMA flag test in mfill_get_vma() to userfaultfd_wp()
as well.

It'd be better to do the moves and the reworks separately. We don't have a limit
on patch count :)

quoted

With every user of the per-VMA uffd modes going through the helpers,
their underlying representation can be changed in the next step.

No functional change.

There is a functional change, or at least seems to be, see below.

quoted

Assisted-by: copilot:claude-opus-5
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
 include/linux/userfaultfd_k.h | 70 +++++++++++++++++++++----------------------
 mm/userfaultfd.c              |  2 +-
 2 files changed, 35 insertions(+), 37 deletions(-)

diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 3396d270b159..d8262e3dc134 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h

@@ -168,42 +168,6 @@ static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,
 	return vma->vm_userfaultfd_ctx.ctx == vm_ctx.ctx;
 }

-/*
- * Never enable huge pmd sharing on some uffd registered vmas:
- *
- * - VM_UFFD_WP and VM_UFFD_RWP VMAs, because the write protect / access
- *   tracking information is per pgtable entry.
- *
- * - VM_UFFD_MINOR VMAs, because otherwise we would never get minor faults for
- *   VMAs which share huge pmds. (If you have two mappings to the same
- *   underlying pages, and fault in the non-UFFD-registered one with a write,
- *   with huge pmd sharing this would *also* setup the second UFFD-registered
- *   mapping, and we'd not get minor faults.)
- */
-static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
-{
-	return vma_test_any_mask(vma,
-		mk_vma_flags_from_masks(VMA_UFFD_WP, VMA_UFFD_RWP,
-					VMA_UFFD_MINOR));
-}
-
-/*
- * Don't do fault around for WP, RWP or MINOR registered uffd range.  For
- * MINOR registered range, fault around will be a total disaster and ptes can
- * be installed without notifications; for WP it should mostly be fine as long
- * as the fault around checks for pte_none() before the installation, however
- * to be super safe we just forbid it; for RWP, pre-faulted neighbours would
- * be indistinguishable from accessed pages in PAGEMAP_SCAN (PAGE_IS_ACCESSED)
- * and pollute the tracked working set, so each page must be populated by its
- * own fault.
- */
-static inline bool uffd_disable_fault_around(struct vm_area_struct *vma)
-{
-	return vma_test_any_mask(vma,
-		mk_vma_flags_from_masks(VMA_UFFD_WP, VMA_UFFD_RWP,
-					VMA_UFFD_MINOR));
-}
-
 static inline bool userfaultfd_missing(const struct vm_area_struct *vma)
 {
 	return vma_test_any_mask(vma, VMA_UFFD_MISSING);

@@ -235,6 +199,40 @@ static inline bool userfaultfd_protected(const struct vm_area_struct *vma)
 	return userfaultfd_wp(vma) || userfaultfd_rwp(vma);
 }

+/*
+ * Never enable huge pmd sharing on some uffd registered vmas:
+ *
+ * - uffd-WP and uffd-RWP VMAs, because the write protect / access tracking
+ *   information is per pgtable entry.
+ *
+ * - uffd-MINOR VMAs, because otherwise we would never get minor faults for
+ *   VMAs which share huge pmds. (If you have two mappings to the same
+ *   underlying pages, and fault in the non-UFFD-registered one with a write,
+ *   with huge pmd sharing this would *also* setup the second UFFD-registered
+ *   mapping, and we'd not get minor faults.)
+ */
+static inline bool uffd_disable_huge_pmd_share(struct vm_area_struct *vma)
+{
+	return userfaultfd_minor(vma) || userfaultfd_wp(vma) ||
+	       userfaultfd_rwp(vma);
+}
+
+/*
+ * Don't do fault around for WP, RWP or MINOR registered uffd range.  For
+ * MINOR registered range, fault around will be a total disaster and ptes can
+ * be installed without notifications; for WP it should mostly be fine as long
+ * as the fault around checks for pte_none() before the installation, however
+ * to be super safe we just forbid it; for RWP, pre-faulted neighbours would
+ * be indistinguishable from accessed pages in PAGEMAP_SCAN (PAGE_IS_ACCESSED)
+ * and pollute the tracked working set, so each page must be populated by its
+ * own fault.
+ */
+static inline bool uffd_disable_fault_around(struct vm_area_struct *vma)
+{
+	return userfaultfd_minor(vma) || userfaultfd_wp(vma) ||
+	       userfaultfd_rwp(vma);

This is changing the logic.

Before we were testing only the flags, now we have:

static inline bool userfaultfd_rwp(const struct vm_area_struct *vma)
{
	/*
	 * Callers gate PAGE_NONE usage on this; PAGE_NONE is a BUILD_BUG()
	 * without CONFIG_ARCH_HAS_PTE_PROTNONE, so fold to false.
	 */
	if (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_PROTNONE))
		return false;
	return vma_test_single_mask(vma, VMA_UFFD_RWP);
}

I.e. adding in a CONFIG_ARCH_HAS_PTE_PROTNONE check.

Without CONFIG_ARCH_HAS_PTE_PROTNONE VMA_UFFD_RWP is hardwired to VM_NONE
so it's functionally the same ;-)
 

BTW side-note these:

static inline bool userfaultfd_missing(const struct vm_area_struct *vma)
{
	return vma_test_any_mask(vma, VMA_UFFD_MISSING);
}

static inline bool userfaultfd_wp(const struct vm_area_struct *vma)
{
	return vma_test_any_mask(vma, VMA_UFFD_WP);
}

static inline bool userfaultfd_minor(const struct vm_area_struct *vma)
{
	return vma_test_any_mask(vma, VMA_UFFD_MINOR);
}

Should all use vma_test_single_mask() really :)

These are changed anyway in a later patch.
 

--
Cheers, Lorenzo

-- 
Sincerely yours,
Mike.