Re: [PATCH 3/4] memcg: clear pc->mem_cgorup if necessary.

Johannes Weiner
2011-12-19T15:37:47Z

On Wed, Dec 14, 2011 at 04:51:24PM +0900, KAMEZAWA Hiroyuki wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> This is a preparation before removing a flag PCG_ACCT_LRU in page_cgroup
> and reducing atomic ops/complexity in memcg LRU handling.
>
> In some cases, pages are added to lru before charge to memcg and pages
> are not classfied to memory cgroup at lru addtion. Now, the lru where
> the page should be added is determined a bit in page_cgroup->flags and
> pc->mem_cgroup. I'd like to remove the check of flag.
>
> To handle the case pc->mem_cgroup may contain stale pointers if pages are
> added to LRU before classification. This patch resets pc->mem_cgroup to
> root_mem_cgroup before lru additions.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

The followup compilation fixes aside, I agree. But the sites where
the owner is actually reset are really not too obvious. How about the
comment patch below?

Otherwise,

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

---
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: memcg: clear pc->mem_cgorup if necessary fix

Add comments to the clearing sites.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---

diff --git a/mm/ksm.c b/mm/ksm.c
index 5c2f0bd..f0ee5bf 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1571,6 +1571,15 @@ struct page *ksm_does_need_to_copy(struct page *page,

new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
if (new_page) {
+ /*
+ * The memcg-specific accounting when moving
+ * pages around the LRU lists relies on the
+ * page's owner (memcg) to be valid. Usually,
+ * pages are assigned to a new owner before
+ * being put on the LRU list, but since this
+ * is not the case here, the stale owner from
+ * a previous allocation cycle must be reset.
+ */
mem_cgroup_reset_owner(new_page);
copy_user_highpage(new_page, page, address, vma);

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 730c4c7..44ccfd2 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -302,6 +302,15 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
new_page = alloc_page_vma(gfp_mask, vma, addr);
if (!new_page)
break; /* Out of memory */
+ /*
+ * The memcg-specific accounting when moving
+ * pages around the LRU lists relies on the
+ * page's owner (memcg) to be valid. Usually,
+ * pages are assigned to a new owner before
+ * being put on the LRU list, but since this
+ * is not the case here, the stale owner from
+ * a previous allocation cycle must be reset.
+ */
mem_cgroup_reset_owner(new_page);
}