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

v2.6.34 commit 9d8cebd4 ("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 1444f92c ("mm: merging memory blocks resets mempolicy")
tried to fix that in vma_adjust(), without fully understanding the issue.

v3.11 commit 3964acd0 ("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: 3964acd0

 ("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>