Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner

[Vlastimil Babka <vbabka@suse.cz>]

On 06/05/2014 02:02 AM, David Rientjes wrote:
> On Wed, 4 Jun 2014, Vlastimil Babka wrote:
>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index ae7db5f..3dce5a7 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -640,11 +640,18 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>>   		}
>>
>>   		/*
>> -		 * Skip if free. page_order cannot be used without zone->lock
>> -		 * as nothing prevents parallel allocations or buddy merging.
>> +		 * Skip if free. We read page order here without zone lock
>> +		 * which is generally unsafe, but the race window is small and
>> +		 * the worst thing that can happen is that we skip some
>> +		 * potential isolation targets.
>
> Should we only be doing the low_pfn adjustment based on the order for
> MIGRATE_ASYNC?  It seems like sync compaction, including compaction that
> is triggered from the command line, would prefer to scan over the
> following pages.

I thought even sync compaction would benefit from the skipped 
iterations. I'd say the probability of this race is smaller than 
probability of somebody allocating what compaction just freed.

>>   		 */
>> -		if (PageBuddy(page))
>> +		if (PageBuddy(page)) {
>> +			unsigned long freepage_order = page_order_unsafe(page);
>
> I don't assume that we want a smp_wmb() in set_page_order() for this
> little race and to recheck PageBuddy() here after smp_rmb().

Hm right, barriers didn't came up last time a patch like this was 
posted. Rechecking PageBuddy() did came up but I thought the range 
checks on the order are enough for this case.

> I think this is fine for MIGRATE_ASYNC.
>
>> +
>> +			if (freepage_order > 0 && freepage_order < MAX_ORDER)
>> +				low_pfn += (1UL << freepage_order) - 1;
>>   			continue;
>> +		}
>>
>>   		/*
>>   		 * Check may be lockless but that's ok as we recheck later.
>> @@ -733,6 +740,13 @@ next_pageblock:
>>   		low_pfn = ALIGN(low_pfn + 1, pageblock_nr_pages) - 1;
>>   	}
>>
>> +	/*
>> +	 * The PageBuddy() check could have potentially brought us outside
>> +	 * the range to be scanned.
>> +	 */
>> +	if (unlikely(low_pfn > end_pfn))
>> +		end_pfn = low_pfn;
>> +
>>   	acct_isolated(zone, locked, cc);
>>
>>   	if (locked)
>> diff --git a/mm/internal.h b/mm/internal.h
>> index 1a8a0d4..6aa1f74 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -164,7 +164,8 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
>>    * general, page_zone(page)->lock must be held by the caller to prevent the
>>    * page from being allocated in parallel and returning garbage as the order.
>>    * If a caller does not hold page_zone(page)->lock, it must guarantee that the
>> - * page cannot be allocated or merged in parallel.
>> + * page cannot be allocated or merged in parallel. Alternatively, it must
>> + * handle invalid values gracefully, and use page_order_unsafe() below.
>>    */
>>   static inline unsigned long page_order(struct page *page)
>>   {
>> @@ -172,6 +173,23 @@ static inline unsigned long page_order(struct page *page)
>>   	return page_private(page);
>>   }
>>
>> +/*
>> + * Like page_order(), but for callers who cannot afford to hold the zone lock,
>> + * and handle invalid values gracefully. ACCESS_ONCE is used so that if the
>> + * caller assigns the result into a local variable and e.g. tests it for valid
>> + * range  before using, the compiler cannot decide to remove the variable and
>> + * inline the function multiple times, potentially observing different values
>> + * in the tests and the actual use of the result.
>> + */
>> +static inline unsigned long page_order_unsafe(struct page *page)
>> +{
>> +	/*
>> +	 * PageBuddy() should be checked by the caller to minimize race window,
>> +	 * and invalid values must be handled gracefully.
>> +	 */
>> +	return ACCESS_ONCE(page_private(page));
>> +}
>> +
>>   /* mm/util.c */
>>   void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
>>   		struct vm_area_struct *prev, struct rb_node *rb_parent);
>
> I don't like this change at all, I don't think we should have header
> functions that imply the context in which the function will be called.  I
> think it would make much more sense to just do
> ACCESS_ONCE(page_order(page)) in the migration scanner with a comment.

But that won't compile. It would have to be converted to a #define, 
unless there's some trick I don't know. Sure I would hope this could be 
done cleaner somehow.

> These are __attribute__((pure)) semantics for page_order().

Not sure I understand what you mean here. Would adding that attribute 
change anything?


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