Re: [resend][PATCH] mm, vmscan: fix do_try_to_free_pages() livelock

[Minchan Kim <minchan@kernel.org>]

On 06/20/2012 07:17 AM, KOSAKI Motohiro wrote:

> (6/17/12 8:43 PM), Minchan Kim wrote:
>> On 06/17/2012 02:48 AM, Aaditya Kumar wrote:
>>
>>> On Fri, Jun 15, 2012 at 12:57 PM, Minchan Kim <minchan@kernel.org> wrote:
>>>
>>>>>
>>>>> pgdat_balanced() doesn't recognized zone. Therefore kswapd may sleep
>>>>> if node has multiple zones. Hm ok, I realized my descriptions was
>>>>> slightly misleading. priority 0 is not needed. bakance_pddat() calls
>>>>> pgdat_balanced()
>>>>> every priority. Most easy case is, movable zone has a lot of free pages and
>>>>> normal zone has no reclaimable page.
>>>>>
>>>>> btw, current pgdat_balanced() logic seems not correct. kswapd should
>>>>> sleep only if every zones have much free pages than high water mark
>>>>> _and_ 25% of present pages in node are free.
>>>>>
>>>>
>>>>
>>>> Sorry. I can't understand your point.
>>>> Current kswapd doesn't sleep if relevant zones don't have free pages above high watermark.
>>>> It seems I am missing your point.
>>>> Please anybody correct me.
>>>
>>> Since currently direct reclaim is given up based on
>>> zone->all_unreclaimable flag,
>>> so for e.g in one of the scenarios:
>>>
>>> Lets say system has one node with two zones (NORMAL and MOVABLE) and we
>>> hot-remove the all the pages of the MOVABLE zone.
>>>
>>> While migrating pages during memory hot-unplugging, the allocation function
>>> (for new page to which the page in MOVABLE zone would be moved)  can end up
>>> looping in direct reclaim path for ever.
>>>
>>> This is so because when most of the pages in the MOVABLE zone have
>>> been migrated,
>>> the zone now contains lots of free memory (basically above low watermark)
>>> BUT all are in MIGRATE_ISOLATE list of the buddy list.
>>>
>>> So kswapd() would not balance this zone as free pages are above low watermark
>>> (but all are in isolate list). So zone->all_unreclaimable flag would
>>> never be set for this zone
>>> and allocation function would end up looping forever. (assuming the
>>> zone NORMAL is
>>> left with no reclaimable memory)
>>>
>>
>>
>> Thanks a lot, Aaditya! Scenario you mentioned makes perfect.
>> But I don't see it's a problem of kswapd.
>>
>> a5d76b54 made new migration type 'MIGRATE_ISOLATE' which is very irony type because there are many free pages in free list
>> but we can't allocate it. :(
>> It doesn't reflect right NR_FREE_PAGES while many places in the kernel use NR_FREE_PAGES to trigger some operation.
>> Kswapd is just one of them confused.
>> As right fix of this problem, we should fix hot plug code, IMHO which can fix CMA, too. 
>>
>> This patch could make inconsistency between NR_FREE_PAGES and SumOf[free_area[order].nr_free]
>> and it could make __zone_watermark_ok confuse so we might need to fix move_freepages_block itself to reflect
>> free_area[order].nr_free exactly. 
>>
>> Any thought?
>>
>> Side Note: I still need KOSAKI's patch with fixed description regardless of this problem because set zone->all_unreclaimable of only kswapd is very fragile.
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 4403009..19de56c 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -5593,8 +5593,10 @@ int set_migratetype_isolate(struct page *page)
>>  
>>  out:
>>         if (!ret) {
>> +               int pages_moved;
>>                 set_pageblock_migratetype(page, MIGRATE_ISOLATE);
>> -               move_freepages_block(zone, page, MIGRATE_ISOLATE);
>> +               pages_moved = move_freepages_block(zone, page, MIGRATE_ISOLATE);
>> +               __mod_zone_page_state(zone, NR_FREE_PAGES, -pages_moved);
>>         }   
>>  
>>         spin_unlock_irqrestore(&zone->lock, flags);
>> @@ -5607,12 +5609,14 @@ void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>>  {
>>         struct zone *zone;
>>         unsigned long flags;
>> +       int pages_moved;
>>         zone = page_zone(page);
>>         spin_lock_irqsave(&zone->lock, flags);
>>         if (get_pageblock_migratetype(page) != MIGRATE_ISOLATE)
>>                 goto out;
>>         set_pageblock_migratetype(page, migratetype);
>> -       move_freepages_block(zone, page, migratetype);
>> +       pages_moved = move_freepages_block(zone, page, migratetype);
>> +       __mod_zone_page_state(zone, NR_FREE_PAGES, pages_moved);
>>  out:
>>         spin_unlock_irqrestore(&zone->lock, flags);
>>  }
> 
> Unfortunately, this doesn't work. there are two reasons. 1) when memory hotplug occue, we have
> two scenarios. a) free page -> page block change into isolate b) page block change into isolate
> -> free page. The above patch only care scenario (a). Thus it lead to confusing NR_FREE_PAGES value.
> _if_ we put a new branch free page hotpath, we can solve scenario (b). but I don't like it. because of,
> zero hotpath overhead is one of memory hotplug design principle. 2) event if we can solve above issue,


Yeb. Aaditya already pointed out.
And I just sent other patch.
Let's talk about this problem on another thread because it's not a direct/background reclaim problem.
http://lkml.org/lkml/2012/6/20/30


> all_unreclaimable logic still broken. because of, __alloc_pages_slowpath() wake up kswapd only once and
> don't wake up when "goto rebalance" path. But, wake_all_kswapd() is racy and no guarantee to wake up
> kswapd. It mean direct reclaim should work fine w/o background reclaim.


We can fix it easily in direct reclaim path but I think your approach still make sense
because current scheme of zone_unreclaimable setting is very fragile on livelock.
So if you send your patch again with rewritten description, I have no objection.

Thanks.
-- 
Kind regards,
Minchan Kim

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