Re: Found the commit that causes the OOMs

[Minchan Kim]

HI, Wu.

On Sun, Jun 28, 2009 at 8:32 PM, Wu Fengguang<fengguang.wu@xxxxxxxxx> wrote:
> On Sat, Jun 27, 2009 at 08:54:12PM +0800, Johannes Weiner wrote:
>> On Sat, Jun 27, 2009 at 08:12:49AM +0100, David Howells wrote:
>> >
>> > I've managed to bisect things to find the commit that causes the OOMs. ÂIt's:
>> >
>> > Â Â commit 69c854817566db82c362797b4a6521d0b00fe1d8
>> > Â Â Author: MinChan Kim <minchan.kim@xxxxxxxxx>
>> > Â Â Date: Â Tue Jun 16 15:32:44 2009 -0700
>> >
>> > Â Â Â Â vmscan: prevent shrinking of active anon lru list in case of no swap space V3
>> >
>> > Â Â Â Â shrink_zone() can deactivate active anon pages even if we don't have a
>> > Â Â Â Â swap device. ÂMany embedded products don't have a swap device. ÂSo the
>> > Â Â Â Â deactivation of anon pages is unnecessary.
>> >
>> > Â Â Â Â This patch prevents unnecessary deactivation of anon lru pages. ÂBut, it
>> > Â Â Â Â don't prevent aging of anon pages to swap out.
>> >
>> > Â Â Â Â Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
>> > Â Â Â Â Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
>> > Â Â Â Â Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>> > Â Â Â Â Acked-by: Rik van Riel <riel@xxxxxxxxxx>
>> > Â Â Â Â Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> > Â Â Â Â Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> >
>> > This exhibits the problem. ÂThe previous commit:
>> >
>> > Â Â commit 35282a2de4e5e4e173ab61aa9d7015886021a821
>> > Â Â Author: Brice Goglin <Brice.Goglin@xxxxxxxxxxxx>
>> > Â Â Date: Â Tue Jun 16 15:32:43 2009 -0700
>> >
>> > Â Â Â Â migration: only migrate_prep() once per move_pages()
>> >
>> > survives 16 iterations of the LTP syscall testsuite without exhibiting the
>> > problem.
>>
>> Here is the patch in question:
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 7592d8e..879d034 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1570,7 +1570,7 @@ static void shrink_zone(int priority, struct zone *zone,
>> Â Â Â Â* Even if we did not try to evict anon pages at all, we want to
>> Â Â Â Â* rebalance the anon lru active/inactive ratio.
>> Â Â Â Â*/
>> - Â Â if (inactive_anon_is_low(zone, sc))
>> + Â Â if (inactive_anon_is_low(zone, sc) && nr_swap_pages > 0)
>> Â Â Â Â Â Â Â shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
>>
>> Â Â Â throttle_vm_writeout(sc->gfp_mask);
>>
>> When this was discussed, I think we missed that nr_swap_pages can
>> actually get zero on swap systems as well and this should have been
>> total_swap_pages - otherwise we also stop balancing the two anon lists
>> when swap is _full_ which was not the intention of this change at all.
>
> Exactly. In Jesse's OOM case, the swap is exhausted.
> total_swap_pages is the better choice in this situation.
>
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426766] Active_anon:290797 active_file:28 inactive_anon:97034
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426767] Âinactive_file:61 unevictable:11322 dirty:0 writeback:0 unstable:0
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426768] Âfree:3341 slab:13776 mapped:5880 pagetables:6851 bounce:0
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426772] DMA free:7776kB min:40kB low:48kB high:60kB active_anon:556kB inactive_anon:524kB
> +active_file:16kB inactive_file:0kB unevictable:0kB present:15340kB pages_scanned:30 all_unreclaimable? no
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426775] lowmem_reserve[]: 0 1935 1935 1935
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426781] DMA32 free:5588kB min:5608kB low:7008kB high:8412kB active_anon:1162632kB
> +inactive_anon:387612kB active_file:96kB inactive_file:256kB unevictable:45288kB present:1982128kB pages_scanned:980
> +all_unreclaimable? no
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426784] lowmem_reserve[]: 0 0 0 0
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426787] DMA: 64*4kB 77*8kB 45*16kB 18*32kB 4*64kB 2*128kB 2*256kB 3*512kB 1*1024kB
> +1*2048kB 0*4096kB = 7800kB
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426796] DMA32: 871*4kB 149*8kB 1*16kB 2*32kB 1*64kB 0*128kB 1*256kB 1*512kB 0*1024kB
> +0*2048kB 0*4096kB = 5588kB
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426804] 151250 total pagecache pages
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426806] 18973 pages in swap cache
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426808] Swap cache stats: add 610640, delete 591667, find 144356/181468
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426810] Free swap Â= 0kB
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.426811] Total swap = 979956kB
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.434828] 507136 pages RAM
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.434831] 23325 pages reserved
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.434832] 190892 pages shared
> Jun 18 07:44:53 jbarnes-g45 kernel: [64377.434833] 248816 pages non-shared
>
>
> In David's OOM case, there are two symptoms:
> 1) 70000 unaccounted/leaked pages as found by Andrew
> Â (plus rather big number of PG_buddy and pagetable pages)
> 2) almost zero active_file/inactive_file; small inactive_anon;
> Â many slab and active_anon pages.
>
> In the situation of (2), the slab cache is _under_ scanned. So David
> got OOM when vmscan should have squeezed some free pages from the slab
> cache. Which is one important side effect of MinChan's patch?

My patch's side effect is (2).

My guessing is following as.

1. The number of page scanned in shrink_slab is increased in shrink_page_list.
And it is doubled for mapped page or swapcache.
2. shrink_page_list is called by shrink_inactive_list
3. shrink_inactive_list is called by shrink_list

Look at the shrink_list.
If inactive lru list is low, it always call shrink_active_list not
shrink_inactive_list in case of anon.
It means it doesn't increased sc->nr_scanned.
Then shrink_slab can't shrink enough slab pages.
So, David OOM have a lot of slab pages and active anon pages.

Does it make sense ?
If it make sense, we have to change shrink_slab's pressure method.
What do you think ?


-- 
Kinds regards,
Minchan Kim
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