mm: unmap VM_HUGETLB mappings with optimized path

to #28718400

commit b4cefb36051244bcb5651026d862c332a6cac7df upstream.

When unmapping VM_HUGETLB mappings, vm flags need to be updated.  Since
the vmas have been detached, so it sounds safe to update vm flags with
read mmap_sem.

Link: http://lkml.kernel.org/r/1537376621-51150-3-git-send-email-yang.shi@linux.alibaba.comSigned-off-by: NYang Shi <yang.shi@linux.alibaba.com>
Reviewed-by: NMatthew Wilcox <willy@infradead.org>
Acked-by: NKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: NVlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NXu Yu <xuyu@linux.alibaba.com>
Reviewed-by: NYang Shi <yang.shi@linux.alibaba.com>

mm/mmap.c

@@ -2781,7 +2781,7 @@ static int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
 			 * update vm_flags.
 			 */
 			if (downgrade &&
-			    (tmp->vm_flags & (VM_HUGETLB | VM_PFNMAP)))
+			    (tmp->vm_flags & VM_PFNMAP))
 				downgrade = false;
 
 			tmp = tmp->vm_next;