Re: [RFC][BUGFIX][PATCH] memcg: fix shrink_usage

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 26 Mar 2009 13:08:21 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> This is another bug I've working on recently.
> 
> I want this (and the stale swapcache problem) to be fixed for 2.6.30.
> 
> Any comments?
> 
> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> Current mem_cgroup_shrink_usage has two problems.
> 
> 1. It doesn't call mem_cgroup_out_of_memory and doesn't update last_oom_jiffies,
>    so pagefault_out_of_memory invokes global OOM.
> 2. Considering hierarchy, shrinking has to be done from the mem_over_limit,
>    not from the memcg where the page to be charged to.
> 

Ah, i see. good cacth. 
But it seems to be the patch is a bit big and includes duplications.
Can't we divide this patch into 2 and reduce modification ?

mem_cgroup_shrink_usage() should do something proper...
My brief thinking is a patch like this, how do you think ?

Maybe renaming this function is appropriate...
==
mem_cgroup_shrink_usage() is called by shmem, but its purpose is
not different from try_charge().

In current behavior, it ignores upward hierarchy and doesn't update
OOM status of memcg. That's bad. We can simply call try_charge()
and drop charge later.

Reported-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

Index: test/mm/memcontrol.c
===================================================================
--- test.orig/mm/memcontrol.c
+++ test/mm/memcontrol.c
@@ -1655,16 +1655,16 @@ int mem_cgroup_shrink_usage(struct page 
 	if (unlikely(!mem))
 		return 0;
 
-	do {
-		progress = mem_cgroup_hierarchical_reclaim(mem,
-					gfp_mask, true, false);
-		progress += mem_cgroup_check_under_limit(mem);
-	} while (!progress && --retry);
+	ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, mem, true);
 
+	if (!ret) {
+		css_put(&mem->css); /* refcnt by charge *//
+		res_counter_uncharge(&mem->res, PAGE_SIZE);
+		if (do_swap_account)
+			res_counter_uncharge(&mem->memsw, PAGE_SIZE);
+	}
 	css_put(&mem->css);
-	if (!retry)
-		return -ENOMEM;
-	return 0;
+	return ret;
 }
 
 static DEFINE_MUTEX(set_limit_mutex);

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