mempolicy: mbind_range() set_policy() after vma_merge()

v2.6.34 commit d48f50e31fa4 ("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 344ed2c40a43 ("mm: merging memory blocks resets mempolicy")
tried to fix that in vma_adjust(), without fully understanding the issue.

v3.11 commit e7b0d2d2cc4c ("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.

The user visible effect was that the pages got allocated on the local
node (happened to be 0), after the mbind() caller had specifically
asked for them to be allocated on node 1.  There was not any page
migration involved in the case reported: the pages simply got allocated
on the wrong node.

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.

Link: https://lkml.kernel.org/r/319e4db9-64ae-4bca-92f0-ade85d342ff@google.com
Fixes: e7b0d2d2cc4c ("mm: mempolicy: fix mbind_range() && vma_adjust() interaction")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 340c22c6219336166900e6f1d9674dd5985ed546..a2516d31db6ca8b3646d95344d5fdc534bbdcedc 100644 (file)
--- 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) {