Re: [RFC PATCH v2 1/7] mm: page_alloc: avoid merging non-fallbackable pageblocks with others.

[Zi Yan <ziy@nvidia.com>]

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Hi Eric,

Thanks for looking into my patch.

On 10 Dec 2021, at 2:43, Eric Ren wrote:

> Hi,
>
> On 2021/12/10 07:04, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> This is done in addition to MIGRATE_ISOLATE pageblock merge avoidance.
>> It prepares for the upcoming removal of the MAX_ORDER-1 alignment
>> requirement for CMA and alloc_contig_range().
>>
>> MIGRARTE_HIGHATOMIC should not merge with other migratetypes like
>> MIGRATE_ISOLATE and MIGRARTE_CMA[1], so this commit prevents that too.
>> Also add MIGRARTE_HIGHATOMIC to fallbacks array for completeness.
>>
>> [1] https://lore.kernel.org/linux-mm/20211130100853.GP3366@techsingularity.net/
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   include/linux/mmzone.h |  6 ++++++
>>   mm/page_alloc.c        | 28 ++++++++++++++++++----------
>>   2 files changed, 24 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 58e744b78c2c..b925431b0123 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -83,6 +83,12 @@ static inline bool is_migrate_movable(int mt)
>>   	return is_migrate_cma(mt) || mt == MIGRATE_MOVABLE;
>>   }
>>  +/* See fallbacks[MIGRATE_TYPES][3] in page_alloc.c */
>> +static inline bool migratetype_has_fallback(int mt)
>> +{
>> +	return mt < MIGRATE_PCPTYPES;
>> +}
>> +
>
> I would suggest spliting the patch into 2 parts.  The first part: no functioning change, just introduce migratetype_has_fallback()
> and replace where it applys to.

OK. I can do that.

>
>>   #define for_each_migratetype_order(order, type) \
>>   	for (order = 0; order < MAX_ORDER; order++) \
>>   		for (type = 0; type < MIGRATE_TYPES; type++)
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index edfd6c81af82..107a5f186d3b 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1041,6 +1041,12 @@ buddy_merge_likely(unsigned long pfn, unsigned long buddy_pfn,
>>   	return page_is_buddy(higher_page, higher_buddy, order + 1);
>>   }
>>  +static inline bool has_non_fallback_pageblock(struct zone *zone)
>> +{
>> +	return has_isolate_pageblock(zone) || zone_cma_pages(zone) != 0 ||
>> +		zone->nr_reserved_highatomic != 0;
>
> Make zone->nr_reserved_highatomic != 0 a helper as zone_cma_pages()?

I am not sure. We have zone_cma_pages() because when CMA is not enabled, 0 can be
simply returned. But MIGRATE_HIGHATOMIC is always present, then an helper function
is not that useful.

>> +}
>> +
>>   /*
>>    * Freeing function for a buddy system allocator.
>>    *
>> @@ -1116,14 +1122,15 @@ static inline void __free_one_page(struct page *page,
>>   	}
>>   	if (order < MAX_ORDER - 1) {
>>   		/* If we are here, it means order is >= pageblock_order.
>> -		 * We want to prevent merge between freepages on isolate
>> -		 * pageblock and normal pageblock. Without this, pageblock
>> -		 * isolation could cause incorrect freepage or CMA accounting.
>> +		 * We want to prevent merge between freepages on pageblock
>> +		 * without fallbacks and normal pageblock. Without this,
>> +		 * pageblock isolation could cause incorrect freepage or CMA
>> +		 * accounting or HIGHATOMIC accounting.
>>   		 *
>>   		 * We don't want to hit this code for the more frequent
>>   		 * low-order merging.
>>   		 */
>> -		if (unlikely(has_isolate_pageblock(zone))) {
>> +		if (unlikely(has_non_fallback_pageblock(zone))) {
> I'm not familiar with the code details, just wondering if this change would has side effects on cma
> pageblock merging as it the condition stronger?

No impact on cma pageblock merging, AFAICT.

>
> Thanks,
> Eric
>>   			int buddy_mt;
>>    			buddy_pfn = __find_buddy_pfn(pfn, order);
>> @@ -1131,8 +1138,8 @@ static inline void __free_one_page(struct page *page,
>>   			buddy_mt = get_pageblock_migratetype(buddy);
>>    			if (migratetype != buddy_mt
>> -					&& (is_migrate_isolate(migratetype) ||
>> -						is_migrate_isolate(buddy_mt)))
>> +					&& (!migratetype_has_fallback(migratetype) ||
>> +						!migratetype_has_fallback(buddy_mt)))
>>   				goto done_merging;
>>   		}
>>   		max_order = order + 1;
>> @@ -2483,6 +2490,7 @@ static int fallbacks[MIGRATE_TYPES][3] = {
>>   	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,   MIGRATE_TYPES },
>>   	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE, MIGRATE_TYPES },
>>   	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,   MIGRATE_TYPES },
>> +	[MIGRATE_HIGHATOMIC] = { MIGRATE_TYPES }, /* Never used */
>>   #ifdef CONFIG_CMA
>>   	[MIGRATE_CMA]         = { MIGRATE_TYPES }, /* Never used */
>>   #endif
>> @@ -2794,8 +2802,8 @@ static void reserve_highatomic_pageblock(struct page *page, struct zone *zone,
>>    	/* Yoink! */
>>   	mt = get_pageblock_migratetype(page);
>> -	if (!is_migrate_highatomic(mt) && !is_migrate_isolate(mt)
>> -	    && !is_migrate_cma(mt)) {
>> +	/* Only reserve normal pageblock */
>> +	if (migratetype_has_fallback(mt)) {
>>   		zone->nr_reserved_highatomic += pageblock_nr_pages;
>>   		set_pageblock_migratetype(page, MIGRATE_HIGHATOMIC);
>>   		move_freepages_block(zone, page, MIGRATE_HIGHATOMIC, NULL);
>> @@ -3544,8 +3552,8 @@ int __isolate_free_page(struct page *page, unsigned int order)
>>   		struct page *endpage = page + (1 << order) - 1;
>>   		for (; page < endpage; page += pageblock_nr_pages) {
>>   			int mt = get_pageblock_migratetype(page);
>> -			if (!is_migrate_isolate(mt) && !is_migrate_cma(mt)
>> -			    && !is_migrate_highatomic(mt))
>> +			/* Only change normal pageblock */
>> +			if (migratetype_has_fallback(mt))
>>   				set_pageblock_migratetype(page,
>>   							  MIGRATE_MOVABLE);
>>   		}


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Best Regards,
Yan, Zi

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