From 3c776e64660028236313f0e54f3a9945764422df Mon Sep 17 00:00:00 2001 From: Daisuke Nishimura Date: Thu, 2 Apr 2009 16:57:43 -0700 Subject: [PATCH] memcg: charge swapcache to proper memcg memcg_test.txt says at 4.1: This swap-in is one of the most complicated work. In do_swap_page(), following events occur when pte is unchanged. (1) the page (SwapCache) is looked up. (2) lock_page() (3) try_charge_swapin() (4) reuse_swap_page() (may call delete_swap_cache()) (5) commit_charge_swapin() (6) swap_free(). Considering following situation for example. (A) The page has not been charged before (2) and reuse_swap_page() doesn't call delete_from_swap_cache(). (B) The page has not been charged before (2) and reuse_swap_page() calls delete_from_swap_cache(). (C) The page has been charged before (2) and reuse_swap_page() doesn't call delete_from_swap_cache(). (D) The page has been charged before (2) and reuse_swap_page() calls delete_from_swap_cache(). memory.usage/memsw.usage changes to this page/swp_entry will be Case (A) (B) (C) (D) Event Before (2) 0/ 1 0/ 1 1/ 1 1/ 1 =========================================== (3) +1/+1 +1/+1 +1/+1 +1/+1 (4) - 0/ 0 - -1/ 0 (5) 0/-1 0/ 0 -1/-1 0/ 0 (6) - 0/-1 - 0/-1 =========================================== Result 1/ 1 1/ 1 1/ 1 1/ 1 In any cases, charges to this page should be 1/ 1. In case of (D), mem_cgroup_try_get_from_swapcache() returns NULL (because lookup_swap_cgroup() returns NULL), so "+1/+1" at (3) means charges to the memcg("foo") to which the "current" belongs. OTOH, "-1/0" at (4) and "0/-1" at (6) means uncharges from the memcg("baa") to which the page has been charged. So, if the "foo" and "baa" is different(for example because of task move), this charge will be moved from "baa" to "foo". I think this is an unexpected behavior. This patch fixes this by modifying mem_cgroup_try_get_from_swapcache() to return the memcg to which the swapcache has been charged if PCG_USED bit is set. IIUC, checking PCG_USED bit of swapcache is safe under page lock. Signed-off-by: Daisuke Nishimura Cc: Balbir Singh Cc: KAMEZAWA Hiroyuki Cc: Li Zefan Cc: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 7bb14fdc780..81b0ae8183d 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -994,13 +994,24 @@ nomem: static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page) { struct mem_cgroup *mem; + struct page_cgroup *pc; swp_entry_t ent; + VM_BUG_ON(!PageLocked(page)); + if (!PageSwapCache(page)) return NULL; - ent.val = page_private(page); - mem = lookup_swap_cgroup(ent); + pc = lookup_page_cgroup(page); + /* + * Used bit of swapcache is solid under page lock. + */ + if (PageCgroupUsed(pc)) + mem = pc->mem_cgroup; + else { + ent.val = page_private(page); + mem = lookup_swap_cgroup(ent); + } if (!mem) return NULL; if (!css_tryget(&mem->css)) -- 2.41.1