From:   Zi Yan <ziy@nvidia.com>
To:     Eric Ren <renzhengeek@gmail.com>
Cc:     David Hildenbrand <david@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christoph Hellwig <hch@lst.de>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        linuxppc-dev@lists.ozlabs.org,
        virtualization@lists.linux-foundation.org,
        iommu@lists.linux-foundation.org, Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [RFC PATCH v2 1/7] mm: page_alloc: avoid merging non-fallbackable pageblocks with others.
Date:   Fri, 10 Dec 2021 10:39:58 -0500
Message-ID: <4640ADF8-F5F3-4CEF-A849-8F8C28F17DD0@nvidia.com>
In-Reply-To: <2926b360-5204-45f5-e1a0-e260214f8dbf@gmail.com>
References: <20211209230414.2766515-1-zi.yan@sent.com>
 <20211209230414.2766515-2-zi.yan@sent.com>
 <2926b360-5204-45f5-e1a0-e260214f8dbf@gmail.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@techsingula=
rity.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 =3D=3D 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.=C2=A0 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 =3D 0; order < MAX_ORDER; order++) \
>>   		for (type =3D 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) !=3D 0 ||=

>> +		zone->nr_reserved_highatomic !=3D 0;
>
> Make zone->nr_reserved_highatomic !=3D 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 >=3D 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 w=
ould 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 =3D __find_buddy_pfn(pfn, order);
>> @@ -1131,8 +1138,8 @@ static inline void __free_one_page(struct page *=
page,
>>   			buddy_mt =3D get_pageblock_migratetype(buddy);
>>    			if (migratetype !=3D 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 =3D order + 1;
>> @@ -2483,6 +2490,7 @@ static int fallbacks[MIGRATE_TYPES][3] =3D {
>>   	[MIGRATE_UNMOVABLE]   =3D { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE,  =
 MIGRATE_TYPES },
>>   	[MIGRATE_MOVABLE]     =3D { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE,=
 MIGRATE_TYPES },
>>   	[MIGRATE_RECLAIMABLE] =3D { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE,  =
 MIGRATE_TYPES },
>> +	[MIGRATE_HIGHATOMIC] =3D { MIGRATE_TYPES }, /* Never used */
>>   #ifdef CONFIG_CMA
>>   	[MIGRATE_CMA]         =3D { MIGRATE_TYPES }, /* Never used */
>>   #endif
>> @@ -2794,8 +2802,8 @@ static void reserve_highatomic_pageblock(struct =
page *page, struct zone *zone,
>>    	/* Yoink! */
>>   	mt =3D 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 +=3D 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, unsig=
ned int order)
>>   		struct page *endpage =3D page + (1 << order) - 1;
>>   		for (; page < endpage; page +=3D pageblock_nr_pages) {
>>   			int mt =3D 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);
>>   		}


--
Best Regards,
Yan, Zi

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