[Devel] Re: [RFC] [-mm PATCH] Memory controller fix swap charging context in unuse_pte()

[Balbir Singh]

On Mon, Oct 29, 2007 at 01:57:40AM +0530, Balbir Singh wrote:
Hugh Dickins wrote:

[snip]
 
> Without your mem_cgroup mods in mm/swap_state.c, unuse_pte makes
> the right assignments (I believe).  But I find that swapout (using
> 600M in a 512M machine) from a 200M cgroup quickly OOMs, whereas
> it behaves correctly with your mm/swap_state.c.
> 

On my UML setup, I booted the UML instance with 512M of memory and
used the swapout program that you shared. I tried two things


1. Ran swapout without any changes. The program ran well without
   any OOM condition occuring, lot of reclaim occured.
2. Ran swapout with the changes to mm/swap_state.c removed (diff below)
   and I still did not see any OOM. The reclaim count was much lesser
   since swap cache did not get accounted back to the cgroup from
   which pages were being evicted.

I am not sure why I don't see the OOM that you see, still trying. May be
I missing something obvious at this late hour in the night :-)

Output of the tests
-------------------

balbir at ubuntu:/container/swapout$ cat memory.limit_in_bytes
209715200
balbir at ubuntu:/container/swapout$ cat memory.usage_in_bytes
65536
balbir at ubuntu:/container/swapout$ cat tasks
1815
1847
balbir at ubuntu:/container/swapout$ ps
  PID TTY          TIME CMD
 1815 pts/0    00:00:00 bash
 1848 pts/0    00:00:00 ps
balbir at ubuntu:/container/swapout$ ~/swapout
balbir at ubuntu:/container/swapout$ echo $?
0
balbir at ubuntu:/container/swapout$ cat memory.failcnt
18

Diff to remove mods from swap_state.c (for testing only)
--------------------------------------------------------

--- mm/swap_state.c.org	2007-10-29 01:42:14.000000000 +0530
+++ mm/swap_state.c	2007-10-29 01:52:48.000000000 +0530
@@ -79,10 +79,6 @@ static int __add_to_swap_cache(struct pa
 	BUG_ON(PageSwapCache(page));
 	BUG_ON(PagePrivate(page));
 
-	error = mem_cgroup_cache_charge(page, current->mm, gfp_mask);
-	if (error)
-		goto out;
-
 	error = radix_tree_preload(gfp_mask);
 	if (!error) {
 		write_lock_irq(&swapper_space.tree_lock);
@@ -94,14 +90,11 @@ static int __add_to_swap_cache(struct pa
 			set_page_private(page, entry.val);
 			total_swapcache_pages++;
 			__inc_zone_page_state(page, NR_FILE_PAGES);
-		} else
-			mem_cgroup_uncharge_page(page);
+		}
 
 		write_unlock_irq(&swapper_space.tree_lock);
 		radix_tree_preload_end();
-	} else
-		mem_cgroup_uncharge_page(page);
-out:
+	}
 	return error;
 }
 
@@ -141,7 +134,6 @@ void __delete_from_swap_cache(struct pag
 	BUG_ON(PageWriteback(page));
 	BUG_ON(PagePrivate(page));
 
-	mem_cgroup_uncharge_page(page);
 	radix_tree_delete(&swapper_space.page_tree, page_private(page));
 	set_page_private(page, 0);
 	ClearPageSwapCache(page);
-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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