ANDROID: mm: use raw seqcount variants in vm_write_*

write_seqcount_begin expects to be called from a non-preemptible
context to avoid preemption by a read section that can spin due
to an odd value. But the readers of vm_sequence never retries and
thus writers need not disable preemption. Use the non-lockdep
variant as lockdep checks are now in-built to write_seqcount_begin.

Bug: 161210518
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Change-Id: If4f0cddd7f0a79136495060d4acc1702abb46817

mm/mmap.c

@@ -788,29 +788,9 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 	long adjust_next = 0;
 	int remove_next = 0;
 
-	/*
-	 * Why using vm_raw_write*() functions here to avoid lockdep's warning ?
-	 *
-	 * Locked is complaining about a theoretical lock dependency, involving
-	 * 3 locks:
-	 *   mapping->i_mmap_rwsem --> vma->vm_sequence --> fs_reclaim
-	 *
-	 * Here are the major path leading to this dependency :
-	 *  1. __vma_adjust() mmap_sem  -> vm_sequence -> i_mmap_rwsem
-	 *  2. move_vmap() mmap_sem -> vm_sequence -> fs_reclaim
-	 *  3. __alloc_pages_nodemask() fs_reclaim -> i_mmap_rwsem
-	 *  4. unmap_mapping_range() i_mmap_rwsem -> vm_sequence
-	 *
-	 * So there is no way to solve this easily, especially because in
-	 * unmap_mapping_range() the i_mmap_rwsem is grab while the impacted
-	 * VMAs are not yet known.
-	 * However, the way the vm_seq is used is guarantying that we will
-	 * never block on it since we just check for its value and never wait
-	 * for it to move, see vma_has_changed() and handle_speculative_fault().
-	 */
-	vm_raw_write_begin(vma);
+	vm_write_begin(vma);
 	if (next)
-		vm_raw_write_begin(next);
+		vm_write_begin(next);
 
 	if (next && !insert) {
 		struct vm_area_struct *exporter = NULL, *importer = NULL;
@@ -892,8 +872,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
 			error = anon_vma_clone(importer, exporter);
 			if (error) {
 				if (next && next != vma)
-					vm_raw_write_end(next);
-				vm_raw_write_end(vma);
+					vm_write_end(next);
+				vm_write_end(vma);
 				return error;
 			}
 		}
@@ -1022,7 +1002,7 @@ again:
 		if (next->anon_vma)
 			anon_vma_merge(vma, next);
 		mm->map_count--;
-		vm_raw_write_end(next);
+		vm_write_end(next);
 		put_vma(next);
 		/*
 		 * In mprotect's case 6 (see comments on vma_merge),
@@ -1038,7 +1018,7 @@ again:
 			 */
 			next = vma->vm_next;
 			if (next)
-				vm_raw_write_begin(next);
+				vm_write_begin(next);
 		} else {
 			/*
 			 * For the scope of the comment "next" and
@@ -1086,9 +1066,9 @@ again:
 		uprobe_mmap(insert);
 
 	if (next && next != vma)
-		vm_raw_write_end(next);
+		vm_write_end(next);
 	if (!keep_locked)
-		vm_raw_write_end(vma);
+		vm_write_end(vma);
 
 	validate_mm(mm);
 
@@ -3469,7 +3449,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 		 * that we protect it right now, and let the caller unprotect
 		 * it once the move is done.
 		 */
-		vm_raw_write_begin(new_vma);
+		vm_write_begin(new_vma);
 		vma_link(mm, new_vma, prev, rb_link, rb_parent);
 		*need_rmap_locks = false;
 	}