mm/hugetlb: introduce hugetlb_walk()

huge_pte_offset() is the main walker function for hugetlb pgtables.  The
name is not really representing what it does, though.

Instead of renaming it, introduce a wrapper function called hugetlb_walk()
which will use huge_pte_offset() inside.  Assert on the locks when walking
the pgtable.

Note, the vma lock assertion will be a no-op for private mappings.

Document the last special case in the page_vma_mapped_walk() path where we
don't need any more lock to call hugetlb_walk().

Taking vma lock there is not needed because either: (1) potential callers
of hugetlb pvmw holds i_mmap_rwsem already (from one rmap_walk()), or (2)
the caller will not walk a hugetlb vma at all so the hugetlb code path not
reachable (e.g.  in ksm or uprobe paths).

It's slightly implicit for future page_vma_mapped_walk() callers on that
lock requirement.  But anyway, when one day this rule breaks, one will get
a straightforward warning in hugetlb_walk() with lockdep, then there'll be
a way out.

[akpm@linux-foundation.org: coding-style cleanups]
Link: https://lkml.kernel.org/r/20221216155229.2043750-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 3 insertions, 7 deletions

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index fdb16246f46e..48f1a8ad2243 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -388,9 +388,7 @@ static bool hugetlb_vma_maps_page(struct vm_area_struct *vma,
 {
 	pte_t *ptep, pte;
 
-	ptep = huge_pte_offset(vma->vm_mm, addr,
-			huge_page_size(hstate_vma(vma)));
-
+	ptep = hugetlb_walk(vma, addr, huge_page_size(hstate_vma(vma)));
 	if (!ptep)
 		return false;
 
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 3b1797e0448a..15a5bf765d43 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -252,14 +252,12 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
 					 unsigned long flags,
 					 unsigned long reason)
 {
-	struct mm_struct *mm = ctx->mm;
 	pte_t *ptep, pte;
 	bool ret = true;
 
-	mmap_assert_locked(mm);
-
-	ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
+	mmap_assert_locked(ctx->mm);
 
+	ptep = hugetlb_walk(vma, address, vma_mmu_pagesize(vma));
 	if (!ptep)
 		goto out;