mm: remove VM_LOCK_RMAP code

When a VMA is in an inconsistent state during setup or teardown, the worst
that can happen is that the rmap code will not be able to find the page.

The mapping is in the process of being torn down (PTEs just got
invalidated by munmap), or set up (no PTEs have been instantiated yet).

It is also impossible for the rmap code to follow a pointer to an already
freed VMA, because the rmap code holds the anon_vma->lock, which the VMA
teardown code needs to take before the VMA is removed from the anon_vma
chain.

Hence, we should not need the VM_LOCK_RMAP locking at all.

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Larry Woodman <lwoodman@redhat.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -554,9 +554,7 @@ again:			remove_next = 1 + (end > next->vm_end);
 		 */
 		if (importer && !importer->anon_vma) {
 			/* Block reverse map lookups until things are set up. */
-			importer->vm_flags |= VM_LOCK_RMAP;
 			if (anon_vma_clone(importer, vma)) {
-				importer->vm_flags &= ~VM_LOCK_RMAP;
 				return -ENOMEM;
 			}
 			importer->anon_vma = anon_vma;
@@ -618,11 +616,6 @@ again:			remove_next = 1 + (end > next->vm_end);
 		__vma_unlink(mm, next, vma);
 		if (file)
 			__remove_shared_vm_struct(next, file, mapping);
-		/*
-		 * This VMA is now dead, no need for rmap to follow it.
-		 * Call anon_vma_merge below, outside of i_mmap_lock.
-		 */
-		next->vm_flags |= VM_LOCK_RMAP;
 	} else if (insert) {
 		/*
 		 * split_vma has split insert from vma, and needs
@@ -635,20 +628,12 @@ again:			remove_next = 1 + (end > next->vm_end);
 	if (mapping)
 		spin_unlock(&mapping->i_mmap_lock);
 
-	/*
-	 * The current VMA has been set up. It is now safe for the
-	 * rmap code to get from the pages to the ptes.
-	 */
-	if (anon_vma && importer)
-		importer->vm_flags &= ~VM_LOCK_RMAP;
-
 	if (remove_next) {
 		if (file) {
 			fput(file);
 			if (next->vm_flags & VM_EXECUTABLE)
 				removed_exe_file_vma(mm);
 		}
-		/* Protected by mmap_sem and VM_LOCK_RMAP. */
 		if (next->anon_vma)
 			anon_vma_merge(vma, next);
 		mm->map_count--;