hugetlb: add lockdep check for i_mmap_rwsem held in huge_pmd_share

As a debugging aid, huge_pmd_share should make sure i_mmap_rwsem is held
if necessary.  To clarify the 'if necessary', expand the comment block at
the beginning of huge_pmd_share.

No functional change.  The added i_mmap_assert_locked() call is only
enabled if CONFIG_LOCKDEP.

Ideally, this should have been included with commit 34ae204f
("hugetlbfs: remove call to huge_pte_alloc without i_mmap_rwsem").
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lkml.kernel.org/r/20200911201248.88537-1-mike.kravetz@oracle.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -5337,10 +5337,16 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
  * !shared pmd case because we can allocate the pmd later as well, it makes the
  * code much cleaner.
  *
- * This routine must be called with i_mmap_rwsem held in at least read mode.
- * For hugetlbfs, this prevents removal of any page table entries associated
- * with the address space.  This is important as we are setting up sharing
- * based on existing page table entries (mappings).
+ * This routine must be called with i_mmap_rwsem held in at least read mode if
+ * sharing is possible.  For hugetlbfs, this prevents removal of any page
+ * table entries associated with the address space.  This is important as we
+ * are setting up sharing based on existing page table entries (mappings).
+ *
+ * NOTE: This routine is only called from huge_pte_alloc.  Some callers of
+ * huge_pte_alloc know that sharing is not possible and do not take
+ * i_mmap_rwsem as a performance optimization.  This is handled by the
+ * if !vma_shareable check at the beginning of the routine. i_mmap_rwsem is
+ * only required for subsequent processing.
  */
 pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
 {
@@ -5357,6 +5363,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
 	if (!vma_shareable(vma, addr))
 		return (pte_t *)pmd_alloc(mm, pud, addr);
 
+	i_mmap_assert_locked(mapping);
 	vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
 		if (svma == vma)
 			continue;