Re: Unending loop in __alloc_pages_slowpath following OOM-kill; rfc: patch.

[Minchan Kim]

On Tue, May 24, 2011 at 1:54 PM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
>>>From 8bd3f16736548375238161d1bd85f7d7c381031f Mon Sep 17 00:00:00 2001
>> From: Minchan Kim <minchan.kim@xxxxxxxxx>
>> Date: Sat, 21 May 2011 01:37:41 +0900
>> Subject: [PATCH] Prevent unending loop in __alloc_pages_slowpath
>>
>> From: Andrew Barry <abarry@xxxxxxxx>
>>
>> I believe I found a problem in __alloc_pages_slowpath, which allows a process to
>> get stuck endlessly looping, even when lots of memory is available.
>>
>> Running an I/O and memory intensive stress-test I see a 0-order page allocation
>> with __GFP_IO and __GFP_WAIT, running on a system with very little free memory.
>> Right about the same time that the stress-test gets killed by the OOM-killer,
>> the utility trying to allocate memory gets stuck in __alloc_pages_slowpath even
>> though most of the systems memory was freed by the oom-kill of the stress-test.
>>
>> The utility ends up looping from the rebalance label down through the
>> wait_iff_congested continiously. Because order=0, __alloc_pages_direct_compact
>> skips the call to get_page_from_freelist. Because all of the reclaimable memory
>> on the system has already been reclaimed, __alloc_pages_direct_reclaim skips the
>> call to get_page_from_freelist. Since there is no __GFP_FS flag, the block with
>> __alloc_pages_may_oom is skipped. The loop hits the wait_iff_congested, then
>> jumps back to rebalance without ever trying to get_page_from_freelist. This loop
>> repeats infinitely.
>>
>> The test case is pretty pathological. Running a mix of I/O stress-tests that do
>> a lot of fork() and consume all of the system memory, I can pretty reliably hit
>> this on 600 nodes, in about 12 hours. 32GB/node.
>>
>> Signed-off-by: Andrew Barry <abarry@xxxxxxxx>
>> Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
>> Cc: Mel Gorman <mgorman@xxxxxxx>
>> ---
>> Âmm/page_alloc.c | Â Â2 +-
>> Â1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 3f8bce2..e78b324 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2064,6 +2064,7 @@ restart:
>> Â Â Â Â Â Â Â first_zones_zonelist(zonelist, high_zoneidx, NULL,
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â &preferred_zone);
>>
>> +rebalance:
>> Â Â Â /* This is the last chance, in general, before the goto nopage. */
>> Â Â Â page = get_page_from_freelist(gfp_mask, nodemask, order, zonelist,
>> Â Â Â Â Â Â Â Â Â Â Â high_zoneidx, alloc_flags & ~ALLOC_NO_WATERMARKS,
>> @@ -2071,7 +2072,6 @@ restart:
>> Â Â Â if (page)
>> Â Â Â Â Â Â Â goto got_pg;
>>
>> -rebalance:
>> Â Â Â /* Allocate without watermarks if the context allows */
>> Â Â Â if (alloc_flags & ALLOC_NO_WATERMARKS) {
>> Â Â Â Â Â Â Â page = __alloc_pages_high_priority(gfp_mask, order,
>
> I'm sorry I missed this thread long time.

No problem. It would be better than not review.

>
> In this case, I think we should call drain_all_pages(). then following
> patch is better.

Strictly speaking, this problem isn't related to drain_all_pages.
This problem caused by lru empty but I admit it could work well if
your patch applied.
So yours could help, too.

> However I also think your patch is valuable. because while the task is
> sleeping in wait_iff_congested(), an another task may free some pages.
> thus, rebalance path should try to get free pages. iow, you makes sense.

Yes.
Off-topic.
I would like to move cond_resched below get_page_from_freelist in
__alloc_pages_direct_reclaim. Otherwise, it is likely we can be stolen
pages to other processes.
One more benefit is that if it's apparently OOM path(ie,
did_some_progress = 0), we can reduce OOM kill latency due to remove
unnecessary cond_resched.

>
> So, I'd like to propose to merge both your and my patch.

Recently, there was discussion on drain_all_pages with Wu.
He saw much overhead in 8-core system, AFAIR.
I Cced Wu.

How about checking per-cpu before calling drain_all_pages() than
unconditional calling?
if (per_cpu_ptr(zone->pageset, smp_processor_id())
    drain_all_pages();

Of course, It can miss other CPU free pages. But above routine assume
local cpu direct reclaim is successful but it failed by per-cpu. So I
think it works.

Thanks for good suggestion and Reviewed-by, KOSAKI.
-- 
Kind regards,
Minchan Kim
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