Re: [RFC PATCH v2 3/7] mm: migrate: allocate the right size of non hugetlb or THP compound pages.

[Zi Yan <ziy@nvidia.com>]

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On 10 Dec 2021, at 2:53, Eric Ren wrote:

> Hi,
>
> On 2021/12/10 07:04, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> alloc_migration_target() is used by alloc_contig_range() and non-LRU
>> movable compound pages can be migrated. Current code does not allocate the
>> right page size for such pages. Check THP precisely using
>> is_transparent_huge() and add allocation support for non-LRU compound
>> pages.
> Could you elaborate on why the current code doesn't get the right size?  how this patch fixes it.

The current code only check PageHuge() and PageTransHuge(). Non-LRU compound
pages will be regarded as PageTransHuge() and an order-9 page is always allocated
regardless of the actual page order. This patch makes the exact check for
THPs using is_transparent_huge() instead of PageTransHuge() and checks PageCompound()
after PageHuge() and is_transparent_huge() for non-LRU compound pages.

>
> The description sounds like it's an existing bug, if so, the patch subject should be changed to
> "Fixes ..."?

I have not seen any related bug report.

>
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>>   mm/migrate.c | 8 ++++++--
>>   1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index d487a399253b..2ce3c771b1de 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1563,7 +1563,7 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
>>   		return alloc_huge_page_nodemask(h, nid, mtc->nmask, gfp_mask);
>>   	}
>>  -	if (PageTransHuge(page)) {
>> +	if (is_transparent_hugepage(page)) {
>>   		/*
>>   		 * clear __GFP_RECLAIM to make the migration callback
>>   		 * consistent with regular THP allocations.
>> @@ -1572,13 +1572,17 @@ struct page *alloc_migration_target(struct page *page, unsigned long private)
> if (PageTransHuge(page)) {  // just give more code context
> ...
>>   		gfp_mask |= GFP_TRANSHUGE;
>>   		order = HPAGE_PMD_ORDER;
> order assigned here
>>   	}
>> +	if (PageCompound(page)) {
>> +		gfp_mask |= __GFP_COMP;
>> +		order = compound_order(page);
> re-assinged again as THP is a compound page?

Ah, you are right. Will use else if instead. Thanks.

> Thanks,
> Eric
>> +	}
>>   	zidx = zone_idx(page_zone(page));
>>   	if (is_highmem_idx(zidx) || zidx == ZONE_MOVABLE)
>>   		gfp_mask |= __GFP_HIGHMEM;
>>    	new_page = __alloc_pages(gfp_mask, order, nid, mtc->nmask);
>>  -	if (new_page && PageTransHuge(new_page))
>> +	if (new_page && is_transparent_hugepage(page))
>>   		prep_transhuge_page(new_page);
>>    	return new_page;


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Best Regards,
Yan, Zi

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