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SubjectRe: Accounting problem of MIGRATE_ISOLATED freed page
On 06/20/2012 03:32 PM, KOSAKI Motohiro wrote:

> (6/20/12 2:12 AM), Minchan Kim wrote:
>>
>> Hi Aaditya,
>>
>> I want to discuss this problem on another thread.
>>
>> On 06/19/2012 10:18 PM, Aaditya Kumar wrote:
>>> On Mon, Jun 18, 2012 at 6:13 AM, Minchan Kim <minchan@kernel.org> 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.
>>>
>>> Hi Kim,
>>
>> I like called Minchan rather than Kim
>> Never mind. :)
>>
>>>
>>> Yes I agree it is not a problem of kswapd.
>>
>> Yeb.
>>
>>>
>>>> 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]
>>>
>>>
>>> I assume that by the inconsistency you mention above, you mean
>>> temporary inconsistency.
>>>
>>> Sorry, but IMHO as for memory hot plug the main issue with this patch
>>> is that the inconsistency you mentioned above would NOT be a temporary
>>> inconsistency.
>>>
>>> Every time say 'x' number of page frames are off lined, they will
>>> introduce a difference of 'x' pages between
>>> NR_FREE_PAGES and SumOf[free_area[order].nr_free].
>>> (So for e.g. if we do a frequent offline/online it will make
>>> NR_FREE_PAGES negative)
>>>
>>> This is so because, unset_migratetype_isolate() is called from
>>> offlining code (to set the migrate type of off lined pages again back
>>> to MIGRATE_MOVABLE)
>>> after the pages have been off lined and removed from the buddy list.
>>> Since the pages for which unset_migratetype_isolate() is called are
>>> not buddy pages so move_freepages_block() does not move any page, and
>>> thus introducing a permanent inconsistency.
>>
>> Good point. Negative NR_FREE_PAGES is caused by double counting by my patch and __offline_isolated_pages.
>> I think at first MIGRATE_ISOLATE type freed page shouldn't account as free page.
>>
>>>
>>>> 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?
>>>
>>> As for fixing move_freepages_block(), At least for memory hot plug,
>>> the pages stay in MIGRATE_ISOLATE list only for duration
>>> offline_pages() function,
>>> I mean only temporarily. Since fixing move_freepages_block() for will
>>> introduce some overhead, So I am not very sure whether that overhead
>>> is justified
>>> for a temporary condition. What do you think?
>>
>> Yes. I don't like hurt fast path, either.
>> How about this? (Passed just compile test :( )
>> The patch's goal is to NOT increase nr_free and NR_FREE_PAGES about freed page into MIGRATE_ISOLATED.
>>
>> This patch hurts high order page free path but I think it's not critical because higher order allocation
>> is rare than order-0 allocation and we already have done same thing on free_hot_cold_page on order-0 free path
>> which is more hot.
>
> Can't we change zone_water_mark_ok_safe() instead of page allocator? memory hotplug is really rare event.


+1

Firstly, I want to make zone_page_state(z, NR_FREE_PAGES) itself more accurately because it is used by
several places. As I looked over places, I can't find critical places except kswapd forever sleep case.
So it's a nice idea!

In that case, we need zone->lock whenever zone_watermark_ok_safe is called.
Most of cases, it's unnecessary and it might hurt alloc/free performance when memory pressure is high.
But if memory pressure is high, it may be already meaningless alloc/free performance.
So it does make sense, IMHO.

Please raise your hands if anyone has a concern about this.

barrios@bbox:~/linux-next$ git diff
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d2a515d..82cc0a2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1748,16 +1748,38 @@ bool zone_watermark_ok(struct zone *z, int order, unsigned long mark,
zone_page_state(z, NR_FREE_PAGES));
}

-bool zone_watermark_ok_safe(struct zone *z, int order, unsigned long mark,
+bool zone_watermark_ok_safe(struct zone *z, int alloc_order, unsigned long mark,
int classzone_idx, int alloc_flags)
{
+ struct free_area *area;
+ struct list_head *curr;
+ int order;
+ unsigned long flags;
long free_pages = zone_page_state(z, NR_FREE_PAGES);

if (z->percpu_drift_mark && free_pages < z->percpu_drift_mark)
free_pages = zone_page_state_snapshot(z, NR_FREE_PAGES);

- return __zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags,
- free_pages);
+ /*
+ * Memory hotplug/CMA can isolate freed page into MIGRATE_ISOLATE
+ * so that buddy can't allocate it although they are in free list.
+ */
+ spin_lock_irqsave(&z->lock, flags);
+ for (order = 0; order < MAX_ORDER; order++) {
+ int count = 0;
+ area = &(z->free_area[order]);
+ if (unlikely(!list_empty(&area->free_list[MIGRATE_ISOLATE]))) {
+ list_for_each(curr, &area->free_list[MIGRATE_ISOLATE])
+ count++;
+ free_pages -= (count << order);
+ }
+ }
+ if (free_pages < 0)
+ free_pages = 0;
+ spin_unlock_irqrestore(&z->lock, flags);
+
+ return __zone_watermark_ok(z, alloc_order, mark,
+ classzone_idx, alloc_flags, free_pages);
}

#ifdef CONFIG_NUMA





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--
Kind regards,
Minchan Kim


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