mm: unmap VM_PFNMAP mappings with optimized path

When unmapping VM_PFNMAP 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/1537376621-51150-4-git-send-email-yang.shi@linux.alibaba.com
Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -2771,15 +2771,6 @@ static int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
 				munlock_vma_pages_all(tmp);
 			}
 
-			/*
-			 * Unmapping vmas, which have VM_HUGETLB or VM_PFNMAP,
-			 * need get done with write mmap_sem held since they may
-			 * update vm_flags.
-			 */
-			if (downgrade &&
-			    (tmp->vm_flags & VM_PFNMAP))
-				downgrade = false;
-
 			tmp = tmp->vm_next;
 		}
 	}