[PATCH mmotm] mempolicy: mbind_range() set_policy() after vma_merge()

[Hugh Dickins]

v2.6.34 commit 9d8cebd4bcd7 ("mm: fix mbind vma merge problem")
introduced vma_merge() to mbind_range(); but unlike madvise, mlock and
mprotect, it put a "continue" to next vma where its precedents go to
update flags on current vma before advancing: that left vma with the
wrong setting in the infamous vma_merge() case 8.

v3.10 commit 1444f92c8498 ("mm: merging memory blocks resets mempolicy")
tried to fix that in vma_adjust(), without fully understanding the issue.

v3.11 commit 3964acd0dbec ("mm: mempolicy: fix mbind_range() &&
vma_adjust() interaction") reverted that, and went about the fix in the
right way, but chose to optimize out an unnecessary mpol_dup() with a
prior mpol_equal() test.  But on tmpfs, that also pessimized out the
vital call to its ->set_policy(), leaving the new mbind unenforced.

Just delete that optimization now (though it could be made conditional
on vma not having a set_policy).  Also remove the "next" variable:
it turned out to be blameless, but also pointless.

Fixes: 3964acd0dbec ("mm: mempolicy: fix mbind_range() && vma_adjust() interaction")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 mm/mempolicy.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -786,7 +786,6 @@ static int vma_replace_policy(struct vm_area_struct *vma,
 static int mbind_range(struct mm_struct *mm, unsigned long start,
 		       unsigned long end, struct mempolicy *new_pol)
 {
-	struct vm_area_struct *next;
 	struct vm_area_struct *prev;
 	struct vm_area_struct *vma;
 	int err = 0;
@@ -801,8 +800,7 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
 	if (start > vma->vm_start)
 		prev = vma;
 
-	for (; vma && vma->vm_start < end; prev = vma, vma = next) {
-		next = vma->vm_next;
+	for (; vma && vma->vm_start < end; prev = vma, vma = vma->vm_next) {
 		vmstart = max(start, vma->vm_start);
 		vmend   = min(end, vma->vm_end);
 
@@ -817,10 +815,6 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
 				 anon_vma_name(vma));
 		if (prev) {
 			vma = prev;
-			next = vma->vm_next;
-			if (mpol_equal(vma_policy(vma), new_pol))
-				continue;
-			/* vma_merge() joined vma && vma->next, case 8 */
 			goto replace;
 		}
 		if (vma->vm_start != vmstart) {