[PATCH v4 03/15] userfaultfd: introduce mfill_establish_pmd() helper

Mike Rapoport
2026-04-02T04:12:27Z

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

There is a lengthy code chunk in mfill_atomic() that establishes the PMD
for UFFDIO operations. This code may be called twice: first time when the
copy is performed with VMA/mm locks held and the other time after the copy
is retried with locks dropped.

Move the code that establishes a PMD into a helper function so it can be
reused later during refactoring of mfill_atomic_pte_copy().

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
---
mm/userfaultfd.c | 102 ++++++++++++++++++++++++-----------------------
1 file changed, 52 insertions(+), 50 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index c4074b6f4aca..f653fb3f74df 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -157,6 +157,56 @@ static void uffd_mfill_unlock(struct vm_area_struct *vma)
}
#endif

+static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
+{
+ pgd_t *pgd;
+ p4d_t *p4d;
+ pud_t *pud;
+
+ pgd = pgd_offset(mm, address);
+ p4d = p4d_alloc(mm, pgd, address);
+ if (!p4d)
+ return NULL;
+ pud = pud_alloc(mm, p4d, address);
+ if (!pud)
+ return NULL;
+ /*
+ * Note that we didn't run this because the pmd was
+ * missing, the *pmd may be already established and in
+ * turn it may also be a trans_huge_pmd.
+ */
+ return pmd_alloc(mm, pud, address);
+}
+
+static int mfill_establish_pmd(struct mfill_state *state)
+{
+ struct mm_struct *dst_mm = state->ctx->mm;
+ pmd_t *dst_pmd, dst_pmdval;
+
+ dst_pmd = mm_alloc_pmd(dst_mm, state->dst_addr);
+ if (unlikely(!dst_pmd))
+ return -ENOMEM;
+
+ dst_pmdval = pmdp_get_lockless(dst_pmd);
+ if (unlikely(pmd_none(dst_pmdval)) &&
+ unlikely(__pte_alloc(dst_mm, dst_pmd)))
+ return -ENOMEM;
+
+ dst_pmdval = pmdp_get_lockless(dst_pmd);
+ /*
+ * If the dst_pmd is THP don't override it and just be strict.
+ * (This includes the case where the PMD used to be THP and
+ * changed back to none after __pte_alloc().)
+ */
+ if (unlikely(!pmd_present(dst_pmdval) || pmd_leaf(dst_pmdval)))
+ return -EEXIST;
+ if (unlikely(pmd_bad(dst_pmdval)))
+ return -EFAULT;
+
+ state->pmd = dst_pmd;
+ return 0;
+}
+
/* Check if dst_addr is outside of file's size. Must be called with ptl held. */
static bool mfill_file_over_size(struct vm_area_struct *dst_vma,
unsigned long dst_addr)
@@ -489,27 +539,6 @@ static int mfill_atomic_pte_poison(struct mfill_state *state)
return ret;
}

-static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
-{
- pgd_t *pgd;
- p4d_t *p4d;
- pud_t *pud;
-
- pgd = pgd_offset(mm, address);
- p4d = p4d_alloc(mm, pgd, address);
- if (!p4d)
- return NULL;
- pud = pud_alloc(mm, p4d, address);
- if (!pud)
- return NULL;
- /*
- * Note that we didn't run this because the pmd was
- * missing, the *pmd may be already established and in
- * turn it may also be a trans_huge_pmd.
- */
- return pmd_alloc(mm, pud, address);
-}
-
#ifdef CONFIG_HUGETLB_PAGE
/*
* mfill_atomic processing for HUGETLB vmas. Note that this routine is
@@ -742,7 +771,6 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
struct vm_area_struct *dst_vma;
long copied = 0;
ssize_t err;
- pmd_t *dst_pmd;

/*
* Sanitize the command parameters:
@@ -808,41 +836,15 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
while (state.src_addr < src_start + len) {
VM_WARN_ON_ONCE(state.dst_addr >= dst_start + len);

- pmd_t dst_pmdval;
-
- dst_pmd = mm_alloc_pmd(dst_mm, state.dst_addr);
- if (unlikely(!dst_pmd)) {
- err = -ENOMEM;
+ err = mfill_establish_pmd(&state);
+ if (err)
break;
- }

- dst_pmdval = pmdp_get_lockless(dst_pmd);
- if (unlikely(pmd_none(dst_pmdval)) &&
- unlikely(__pte_alloc(dst_mm, dst_pmd))) {
- err = -ENOMEM;
- break;
- }
- dst_pmdval = pmdp_get_lockless(dst_pmd);
- /*
- * If the dst_pmd is THP don't override it and just be strict.
- * (This includes the case where the PMD used to be THP and
- * changed back to none after __pte_alloc().)
- */
- if (unlikely(!pmd_present(dst_pmdval) ||
- pmd_trans_huge(dst_pmdval))) {
- err = -EEXIST;
- break;
- }
- if (unlikely(pmd_bad(dst_pmdval))) {
- err = -EFAULT;
- break;
- }
/*
* For shmem mappings, khugepaged is allowed to remove page
* tables under us; pte_offset_map_lock() will deal with that.
*/

- state.pmd = dst_pmd;
err = mfill_atomic_pte(&state);
cond_resched();

--
2.53.0