userfaultfd: introduce mfill_get_vma() and mfill_put_vma()

Split the code that finds, locks and verifies VMA from mfill_atomic() into
a helper function.

This function will be used later during refactoring of
mfill_atomic_pte_copy().

Add a counterpart mfill_put_vma() helper that unlocks the VMA and releases
map_changing_lock.

[avagin@google.com: fix lock leak in mfill_get_vma()]
Link: https://lore.kernel.org/20260316173829.1126728-1-avagin@google.com
Link: https://lore.kernel.org/20260402041156.1377214-5-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Andrei Vagin <avagin@google.com>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nikita Kalyazin <kalyazin@amazon.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Carlier <devnexen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: rppt

mm/userfaultfd.c

@@ -157,6 +157,75 @@ static void uffd_mfill_unlock(struct vm_area_struct *vma)
 }
 #endif
 
+static void mfill_put_vma(struct mfill_state *state)
+{
+	if (!state->vma)
+		return;
+
+	up_read(&state->ctx->map_changing_lock);
+	uffd_mfill_unlock(state->vma);
+	state->vma = NULL;
+}
+
+static int mfill_get_vma(struct mfill_state *state)
+{
+	struct userfaultfd_ctx *ctx = state->ctx;
+	uffd_flags_t flags = state->flags;
+	struct vm_area_struct *dst_vma;
+	int err;
+
+	/*
+	 * Make sure the vma is not shared, that the dst range is
+	 * both valid and fully within a single existing vma.
+	 */
+	dst_vma = uffd_mfill_lock(ctx->mm, state->dst_start, state->len);
+	if (IS_ERR(dst_vma))
+		return PTR_ERR(dst_vma);
+
+	/*
+	 * If memory mappings are changing because of non-cooperative
+	 * operation (e.g. mremap) running in parallel, bail out and
+	 * request the user to retry later
+	 */
+	down_read(&ctx->map_changing_lock);
+	state->vma = dst_vma;
+	err = -EAGAIN;
+	if (atomic_read(&ctx->mmap_changing))
+		goto out_unlock;
+
+	err = -EINVAL;
+
+	/*
+	 * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
+	 * it will overwrite vm_ops, so vma_is_anonymous must return false.
+	 */
+	if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
+	    dst_vma->vm_flags & VM_SHARED))
+		goto out_unlock;
+
+	/*
+	 * validate 'mode' now that we know the dst_vma: don't allow
+	 * a wrprotect copy if the userfaultfd didn't register as WP.
+	 */
+	if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
+		goto out_unlock;
+
+	if (is_vm_hugetlb_page(dst_vma))
+		return 0;
+
+	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
+		goto out_unlock;
+	if (!vma_is_shmem(dst_vma) &&
+	    uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
+		goto out_unlock;
+
+	return 0;
+
+out_unlock:
+	mfill_put_vma(state);
+	return err;
+}
+
 static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
 {
 	pgd_t *pgd;
@@ -767,8 +836,6 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
 		.src_addr = src_start,
 		.dst_addr = dst_start,
 	};
-	struct mm_struct *dst_mm = ctx->mm;
-	struct vm_area_struct *dst_vma;
 	long copied = 0;
 	ssize_t err;
 
@@ -783,56 +850,17 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
 	VM_WARN_ON_ONCE(dst_start + len <= dst_start);
 
 retry:
-	/*
-	 * Make sure the vma is not shared, that the dst range is
-	 * both valid and fully within a single existing vma.
-	 */
-	dst_vma = uffd_mfill_lock(dst_mm, dst_start, len);
-	if (IS_ERR(dst_vma)) {
-		err = PTR_ERR(dst_vma);
+	err = mfill_get_vma(&state);
+	if (err)
 		goto out;
-	}
-	state.vma = dst_vma;
-
-	/*
-	 * If memory mappings are changing because of non-cooperative
-	 * operation (e.g. mremap) running in parallel, bail out and
-	 * request the user to retry later
-	 */
-	down_read(&ctx->map_changing_lock);
-	err = -EAGAIN;
-	if (atomic_read(&ctx->mmap_changing))
-		goto out_unlock;
-
-	err = -EINVAL;
-	/*
-	 * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
-	 * it will overwrite vm_ops, so vma_is_anonymous must return false.
-	 */
-	if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
-	    dst_vma->vm_flags & VM_SHARED))
-		goto out_unlock;
-
-	/*
-	 * validate 'mode' now that we know the dst_vma: don't allow
-	 * a wrprotect copy if the userfaultfd didn't register as WP.
-	 */
-	if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
-		goto out_unlock;
 
 	/*
 	 * If this is a HUGETLB vma, pass off to appropriate routine
 	 */
-	if (is_vm_hugetlb_page(dst_vma))
-		return  mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
+	if (is_vm_hugetlb_page(state.vma))
+		return  mfill_atomic_hugetlb(ctx, state.vma, dst_start,
 					     src_start, len, flags);
 
-	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
-		goto out_unlock;
-	if (!vma_is_shmem(dst_vma) &&
-	    uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
-		goto out_unlock;
-
 	while (state.src_addr < src_start + len) {
 		VM_WARN_ON_ONCE(state.dst_addr >= dst_start + len);
 
@@ -851,8 +879,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
 		if (unlikely(err == -ENOENT)) {
 			void *kaddr;
 
-			up_read(&ctx->map_changing_lock);
-			uffd_mfill_unlock(state.vma);
+			mfill_put_vma(&state);
 			VM_WARN_ON_ONCE(!state.folio);
 
 			kaddr = kmap_local_folio(state.folio, 0);
@@ -881,9 +908,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
 			break;
 	}
 
-out_unlock:
-	up_read(&ctx->map_changing_lock);
-	uffd_mfill_unlock(state.vma);
+	mfill_put_vma(&state);
 out:
 	if (state.folio)
 		folio_put(state.folio);