linux-kernel - Re: [External] Re: [v2 1/6] mm: hugetlb: Skip prep of tail pages when HVO is enabled

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Date:   Wed, 2 Aug 2023 11:06:49 +0100
From:   Usama Arif <usama.arif@...edance.com>
To:     Muchun Song <muchun.song@...ux.dev>
Cc:     linux-kernel@...r.kernel.org, fam.zheng@...edance.com,
        liangma@...ngbit.com, simon.evans@...edance.com,
        punit.agrawal@...edance.com, linux-mm@...ck.org,
        mike.kravetz@...cle.com, rppt@...nel.org
Subject: Re: [External] Re: [v2 1/6] mm: hugetlb: Skip prep of tail pages when
 HVO is enabled



On 01/08/2023 03:04, Muchun Song wrote:
> 
> 
> On 2023/7/30 23:16, Usama Arif wrote:
>> When vmemmap is optimizable, it will free all the duplicated tail
>> pages in hugetlb_vmemmap_optimize while preparing the new hugepage.
>> Hence, there is no need to prepare them.
>>
>> For 1G x86 hugepages, it avoids preparing
>> 262144 - 64 = 262080 struct pages per hugepage.
>>
>> The indirection of using __prep_compound_gigantic_folio is also removed,
>> as it just creates extra functions to indicate demote which can be done
>> with the argument.
>>
>> Signed-off-by: Usama Arif <usama.arif@...edance.com>
>> ---
>>   mm/hugetlb.c         | 32 ++++++++++++++------------------
>>   mm/hugetlb_vmemmap.c |  2 +-
>>   mm/hugetlb_vmemmap.h | 15 +++++++++++----
>>   3 files changed, 26 insertions(+), 23 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 64a3239b6407..541c07b6d60f 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1942,14 +1942,23 @@ static void prep_new_hugetlb_folio(struct 
>> hstate *h, struct folio *folio, int ni
>>       spin_unlock_irq(&hugetlb_lock);
>>   }
>> -static bool __prep_compound_gigantic_folio(struct folio *folio,
>> -                    unsigned int order, bool demote)
>> +static bool prep_compound_gigantic_folio(struct folio *folio, struct 
>> hstate *h, bool demote)
>>   {
>>       int i, j;
>> +    int order = huge_page_order(h);
>>       int nr_pages = 1 << order;
>>       struct page *p;
>>       __folio_clear_reserved(folio);
>> +
>> +    /*
>> +     * No need to prep pages that will be freed later by 
>> hugetlb_vmemmap_optimize.
>> +     * Hence, reduce nr_pages to the pages that will be kept.
>> +     */
>> +    if (IS_ENABLED(CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP) &&
>> +            vmemmap_should_optimize(h, &folio->page))
>> +        nr_pages = HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page);
> 
> We need to initialize the refcount to zero of tail pages (see the big
> comment below in this function), given a situation that someone (maybe
> GUP) could get a ref on the tail pages when the vmemmap is optimizing,
> what prevent this from happening?
> 
> Thanks.
> 

Thanks for pointing this out, will limit to boot time for solving this 
in next version.