ksm: mem cgroup charge swapin copy

But ksm swapping does require one small change in mem cgroup handling.
When do_swap_page()'s call to ksm_might_need_to_copy() does indeed
substitute a duplicate page to accommodate a different anon_vma (or a the
!PageSwapCache check in mem_cgroup_try_charge_swapin().

That was returning success without charging, on the assumption that
pte_same() would fail after, which is not the case here.  Originally I
proposed that success, so that an unshrinkable mem cgroup at its limit
would not fail unnecessarily; but that's a minor point, and there are
plenty of other places where we may fail an overallocation which might
later prove unnecessary.  So just go ahead and do what all the other
exceptions do: proceed to charge current mm.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Chris Wright <chrisw@redhat.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1737,11 +1737,12 @@ int mem_cgroup_try_charge_swapin(struct mm_struct *mm,
 		goto charge_cur_mm;
 	/*
 	 * A racing thread's fault, or swapoff, may have already updated
-	 * the pte, and even removed page from swap cache: return success
-	 * to go on to do_swap_page()'s pte_same() test, which should fail.
+	 * the pte, and even removed page from swap cache: in those cases
+	 * do_swap_page()'s pte_same() test will fail; but there's also a
+	 * KSM case which does need to charge the page.
 	 */
 	if (!PageSwapCache(page))
-		return 0;
+		goto charge_cur_mm;
 	mem = try_get_mem_cgroup_from_swapcache(page);
 	if (!mem)
 		goto charge_cur_mm;