Re: [RFC PATCH v3 1/8] mm: page_alloc: avoid merging non-fallbackable pageblocks with others.

[Zi Yan <ziy@nvidia.com>]

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On 12 Jan 2022, at 5:54, David Hildenbrand wrote:

> On 05.01.22 22:47, 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 aed44e9b5d89..0aa549653e4e 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;
>> +}
>> +
>>  #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 8dd6399bafb5..5193c953dbf8 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1042,6 +1042,12 @@ 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;
>> +}
>
> Due to zone_cma_pages(), the unlikely() below will be very wrong on many
> setups. Previously, isolation really was a corner case. CMA and
> highatomic are less of a corner case ...

Got it.

>
> I'm not even sure if this check is worth having around anymore at all,
> or if it would be easier and cheaper to just always check the both
> migration types unconditionally. Would certainly simplify the code.

I will remove the if check below, since, like you said, the check is
no longer a corner case with added highatomic and CMA check.

>
> Side node: we actually care about has_free_non_fallback_pageblock(), we
> can only merge with free pageblocks. But that might not necessarily be
> cheaper to test/track/check.
>

I agree that what we are actually looking for is free pageblocks of these
migratetypes. But tracking them is nontrivial.

>> +
>>  /*
>>   * Freeing function for a buddy system allocator.
>>   *
>> @@ -1117,14 +1123,15 @@ 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))) {
>>  			int buddy_mt;
>>
>>  			buddy_pfn = __find_buddy_pfn(pfn, order);
>> @@ -1132,8 +1139,8 @@ 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;
>> @@ -2484,6 +2491,7 @@ 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
>> @@ -2795,8 +2803,8 @@ 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);
>> @@ -3545,8 +3553,8 @@ 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);
>>  		}
>
> That part is a nice cleanup IMHO. Although the "has fallback" part is a
> bit imprecise. "migratetype_is_mergable()" might be a bit clearer.
> ideally "migratetype_is_mergable_with_other_types()". Can we come up
> with a nice name for that?

Sure. Will change the name.

Thank you for the comments.


--
Best Regards,
Yan, Zi

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