memcg: fix Bad page state after replace_page_cache

My 9ce70c0 "memcg: fix deadlock by inverting lrucare nesting" put a
nasty little bug into v3.3's version of mem_cgroup_replace_page_cache(),
sometimes used for FUSE.  Replacing __mem_cgroup_commit_charge_lrucare()
by __mem_cgroup_commit_charge(), I used the "pc" pointer set up earlier:
but it's for oldpage, and needs now to be for newpage.  Once oldpage was
freed, its PageCgroupUsed bit (cleared above but set again here) caused
"Bad page state" messages - and perhaps worse, being missed from newpage.
(I didn't find this by using FUSE, but in reusing the function for tmpfs.)

Signed-off-by: Hugh Dickins <[email protected]>
Cc: [email protected] [v3.3 only]
Signed-off-by: Linus Torvalds <[email protected]>

mm/memcontrol.c

@@ -3392,6 +3392,7 @@ void mem_cgroup_replace_page_cache(struct page *oldpage,
 	 * the newpage may be on LRU(or pagevec for LRU) already. We lock
 	 * LRU while we overwrite pc->mem_cgroup.
 	 */
+	pc = lookup_page_cgroup(newpage);
 	__mem_cgroup_commit_charge(memcg, newpage, 1, pc, type, true);
 }