[RFC][PATCH] memcg: shmem swap cache

From: KAMEZAWA Hiroyuki
Date: Tue Jul 01 2008 - 22:11:16 EST


On Tue, 1 Jul 2008 09:56:45 +0900 (JST)
kamezawa.hiroyu@xxxxxxxxxxxxxx wrote:

> >> - shmem's page is removed by mem_cgroup_uncharge_cache_page(), So,
> >> shmem's swapcache is not charged.
> >
> >Ah, that's interesting: I'd assumed you'd changed that in your
> >no-refcount patches, and had been surprised not to notice a slowdown
> >(waiting for swap to be written and freed before coming under limit).
> >Now you want to make them wait: not entirely an improvement,
> >but I see your point.
> >
> To do this is (maybe) a few line patch. I'll CC you if I wrote some.
>
maybe like this. how do you feel ?
I'll stack this on my queue and test for a while.

==
SwapCache handling fix.

memcg failed to handle shmem's swapcache. This patch tries to fix it.

After this:

Any page marked as SwapCache is not uncharged.
(until delelte_from_swap_cache() delete the flag.)

To check a page is alive shmem-page-cache or not we use
page->mapping && !PageAnon(page) instead of pc->flags & PAGE_CGROUP_FLAG_CACHE.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

Index: test-2.6.26-rc5-mm3++/mm/memcontrol.c
===================================================================
--- test-2.6.26-rc5-mm3++.orig/mm/memcontrol.c 2008-07-02 09:29:52.000000000 +0900
+++ test-2.6.26-rc5-mm3++/mm/memcontrol.c 2008-07-02 10:58:15.000000000 +0900
@@ -685,11 +685,45 @@

VM_BUG_ON(pc->page != page);

- if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
- && ((pc->flags & PAGE_CGROUP_FLAG_CACHE)
- || page_mapped(page)
- || PageSwapCache(page)))
+ /*
+ * File Cache
+ * If called with MEM_CGROUP_CHARGE_TYPE_MAPPED, check page->mapping.
+ * add_to_page_cache() .... charged before inserting radix-tree.
+ * remove_from_page_cache() .... uncharged at removing from radix-tree.
+ * page->mapping && !PageAnon(page) catches file cache.
+ *
+ * Anon/Shmem.....We check PageSwapCache(page).
+ * Anon .... charged before mapped.
+ * Shmem .... charged at add_to_page_cache() as usual File Cache.
+ *
+ * This page will be finally uncharged when removed from swap-cache
+ *
+ * we treat 2 cases here.
+ * A. anonymous page B. shmem.
+ * We never uncharge if page is marked as SwapCache.
+ * add_to_swap_cache() have nothing to do with charge/uncharge.
+ * SwapCache flag is deleted before delete_from_swap_cache() calls this
+ *
+ * shmem's behavior is following. (see shmem.c/swap_state.c also)
+ * at swap-out:
+ * 0. add_to_page_cache()//charged at page creation.
+ * 1. add_to_swap_cache() (marked as SwapCache)
+ * 2. remove_from_page_cache(). (calls this.)
+ * (finally) delete_from_swap_cache(). (calls this.)
+ * at swap-in:
+ * 3. add_to_swap_cache() (no charge here.)
+ * 4. add_to_page_cache() (charged here.)
+ * 5. delete_from_swap_cache() (calls this.)
+ * PageSwapCache(page) catches "2".
+ * page->mapping && !PageAnon() catches "5" and avoid uncharging.
+ */
+ if (PageSwapCache(page))
goto unlock;
+ /* called from unmap or delete_from_swap_cache() */
+ if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
+ && (page_mapped(page)
+ || (page->mapping && !PageAnon(page))))/* alive cache ? */
+ goto unlock;

mz = page_cgroup_zoneinfo(pc);
spin_lock_irqsave(&mz->lru_lock, flags);

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