Re: Accounting problem of MIGRATE_ISOLATED freed page

[Aaditya Kumar <aaditya.kumar.30@gmail.com>]

On Fri, Jun 22, 2012 at 12:52 PM, KOSAKI Motohiro
<kosaki.motohiro@gmail.com> wrote:
>> Let me summary again.
>>
>> The problem:
>>
>> when hotplug offlining happens on zone A, it starts to freed page as MIGRATE_ISOLATE type in buddy.
>> (MIGRATE_ISOLATE is very irony type because it's apparently on buddy but we can't allocate them)
>> When the memory shortage happens during hotplug offlining, current task starts to reclaim, then wake up kswapd.
>> Kswapd checks watermark, then go sleep BECAUSE current zone_watermark_ok_safe doesn't consider
>> MIGRATE_ISOLATE freed page count. Current task continue to reclaim in direct reclaim path without kswapd's help.
>> The problem is that zone->all_unreclaimable is set by only kswapd so that current task would be looping forever
>> like below.
>>
>> __alloc_pages_slowpath
>> restart:
>>        wake_all_kswapd
>> rebalance:
>>        __alloc_pages_direct_reclaim
>>                do_try_to_free_pages
>>                        if global_reclaim && !all_unreclaimable
>>                                return 1; /* It means we did did_some_progress */
>>        skip __alloc_pages_may_oom
>>        should_alloc_retry
>>                goto rebalance;
>>
>> If we apply KOSAKI's patch[1] which doesn't depends on kswapd about setting zone->all_unreclaimable,
>> we can solve this problem by killing some task. But it doesn't wake up kswapd, still.
>> It could be a problem still if other subsystem needs GFP_ATOMIC request.
>> So kswapd should consider MIGRATE_ISOLATE when it calculate free pages before going sleep.
>
> I agree. And I believe we should remove rebalance label and alloc
> retrying should always wake up kswapd.
> because wake_all_kswapd is unreliable, it have no guarantee to success
> to wake up kswapd. then this
> micro optimization is NOT optimization. Just trouble source. Our
> memory reclaim logic has a lot of race
> by design. then any reclaim code shouldn't believe some one else works fine.
>

I think this is a better approach, since MIGRATE_ISLOATE is really a
temporary phenomenon, it makes sense to just retry allocation.
One issue however, with this approach is that it does not exactly work
for PAGE_ALLOC_COSTLY_ORDER, But well, given the
frequency of such allocation, I think may be it is an acceptable
compromise to handle such request by OOM in case of many
MIGRATE_ISOLATE
pages present.

what do you think ?

>
>> Firstly I tried to solve this problem by this.
>> https://lkml.org/lkml/2012/6/20/30
>> The patch's goal was to NOT increase nr_free and NR_FREE_PAGES when we free page into MIGRATE_ISOLATED.
>> But it increases little overhead in higher order free page but I think it's not a big deal.
>> More problem is duplicated codes for handling only MIGRATE_ISOLATE freed page.
>>
>> Second approach which is suggested by KOSAKI is what you mentioned.
>> But the concern about second approach is how to make sure matched count increase/decrease of nr_isolated_areas.
>> I mean how to make sure nr_isolated_areas would be zero when isolation is done.
>> Of course, we can investigate all of current caller and make sure they don't make mistake
>> now. But it's very error-prone if we consider future's user.
>> So we might need test_set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>>
>> IMHO, ideal solution is that we remove MIGRATE_ISOLATE type totally in buddy.
>> For it, there is no problem to isolate already freed page in buddy allocator but the concern is how to handle
>> freed page later by do_migrate_range in memory_hotplug.c.
>> We can create custom putback_lru_pages
>>
>> put_page_hotplug(page)
>> {
>>        int migratetype = get_pageblock_migratetype(page)
>>        VM_BUG_ON(migratetype != MIGRATE_ISOLATE);
>>        __page_cache_release(page);
>>        free_one_page(zone, page, 0, MIGRATE_ISOLATE);
>> }
>>
>> putback_lru_pages_hotplug(&source)
>> {
>>        foreach page from source
>>                put_page_hotplug(page)
>> }
>>
>> do_migrate_range()
>> {
>>        migrate_pages(&source);
>>        putback_lru_pages_hotplug(&source);
>> }
>>
>> I hope this summary can help you, Kame and If I miss something, please let me know it.
>
> I disagree this. Because of, memory hotplug intentionally don't use
> stopmachine. It is because
> we don't stop any system service when memory is being unpluged. That's
> said various subsystem
> try to allocate memory during page migration for memory unplug. IOW,
> we shouldn't do_migrate_page()
> is only one caller.

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