[PATCH] mm: fix madvise vma merging

Better late than never, I've at last reviewed the madvise vma merging
going into 2.6.13.  Remove a pointless check and fix two little bugs -
a simple test (with /proc/<pid>/maps hacked to show ReadHints) showed
both mismerges in practice: though being madvise, neither was disastrous.

1. Correct placement of the success label in madvise_behavior: as in
   mprotect_fixup and mlock_fixup, it is necessary to update vm_flags
   when vma_merge succeeds (to handle the exceptional Case 8 noted in
   the comments above vma_merge itself).

2. Correct initial value of prev when starting part way into a vma: as
   in sys_mprotect and do_mlock, it needs to be set to vma in this case
   (vma_merge handles only that minimum of cases shown in its comments).

3. If find_vma_prev sets prev, then the vma it returns is prev->vm_next,
   so it's pointless to make that same assignment again in sys_madvise.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/madvise.c

@@ -37,7 +37,7 @@ static long madvise_behavior(struct vm_area_struct * vma,
 
 	if (new_flags == vma->vm_flags) {
 		*prev = vma;
-		goto success;
+		goto out;
 	}
 
 	pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT);
@@ -62,6 +62,7 @@ static long madvise_behavior(struct vm_area_struct * vma,
 			goto out;
 	}
 
+success:
 	/*
 	 * vm_flags is protected by the mmap_sem held in write mode.
 	 */
@@ -70,7 +71,6 @@ static long madvise_behavior(struct vm_area_struct * vma,
 out:
 	if (error == -ENOMEM)
 		error = -EAGAIN;
-success:
 	return error;
 }
 
@@ -237,8 +237,9 @@ asmlinkage long sys_madvise(unsigned long start, size_t len_in, int behavior)
 	 * - different from the way of handling in mlock etc.
 	 */
 	vma = find_vma_prev(current->mm, start, &prev);
-	if (!vma && prev)
-		vma = prev->vm_next;
+	if (vma && start > vma->vm_start)
+		prev = vma;
+
 	for (;;) {
 		/* Still start < end. */
 		error = -ENOMEM;