mm/msync: exit early when the flags is an MS_ASYNC and start < vm_start

If an unmapped region was found and the flag is MS_ASYNC (without
MS_INVALIDATE) there is nothing to do and the result would be always
-ENOMEM, so return immediately.

Link: https://lkml.kernel.org/r/20201025092901.56399-1-sh1r4s3@mail.si-head.nl
Signed-off-by: Nikita Ermakov <sh1r4s3@mail.si-head.nl>
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/msync.c

@@ -55,7 +55,9 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
 		goto out;
 	/*
 	 * If the interval [start,end) covers some unmapped address ranges,
-	 * just ignore them, but return -ENOMEM at the end.
+	 * just ignore them, but return -ENOMEM at the end. Besides, if the
+	 * flag is MS_ASYNC (w/o MS_INVALIDATE) the result would be -ENOMEM
+	 * anyway and there is nothing left to do, so return immediately.
 	 */
 	mmap_read_lock(mm);
 	vma = find_vma(mm, start);
@@ -69,6 +71,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
 			goto out_unlock;
 		/* Here start < vma->vm_end. */
 		if (start < vma->vm_start) {
+			if (flags == MS_ASYNC)
+				goto out_unlock;
 			start = vma->vm_start;
 			if (start >= end)
 				goto out_unlock;