linux-kernel - Re: [PATCH 11/12] hugetlb: batch TLB flushes when freeing vmemmap

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Date:   Wed, 30 Aug 2023 12:17:35 +0100
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Muchun Song <muchun.song@...ux.dev>,
        Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun Duan <duanxiongchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/12] hugetlb: batch TLB flushes when freeing vmemmap



On 30/08/2023 09:23, Muchun Song wrote:
> 
> 
> On 2023/8/26 03:04, Mike Kravetz wrote:
>> From: Joao Martins <joao.m.martins@...cle.com>
>>
>> Now that a list of pages is deduplicated at once, the TLB
>> flush can be batched for all vmemmap pages that got remapped.
>>
>> Add a flags field and pass whether it's a bulk allocation or
>> just a single page to decide to remap.
>>
>> The TLB flush is global as we don't have guarantees from caller
>> that the set of folios is contiguous, or to add complexity in
>> composing a list of kVAs to flush.
>>
>> Modified by Mike Kravetz to perform TLB flush on single folio if an
>> error is encountered.
>>
>> Signed-off-by: Joao Martins <joao.m.martins@...cle.com>
>> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
>> ---
>>   mm/hugetlb_vmemmap.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index 904a64fe5669..a2fc7b03ac6b 100644
>> --- a/mm/hugetlb_vmemmap.c
>> +++ b/mm/hugetlb_vmemmap.c
>> @@ -36,6 +36,7 @@ struct vmemmap_remap_walk {
>>       unsigned long        reuse_addr;
>>       struct list_head    *vmemmap_pages;
>>   #define VMEMMAP_REMAP_ONLY_SPLIT    BIT(0)
>> +#define    VMEMMAP_REMAP_BULK_PAGES    BIT(1)
> 
> We could reuse the flag (as I suggest VMEMMAP_SPLIT_WITHOUT_FLUSH)
> proposed in the patch 10. When I saw this patch, I think the name
> is not suitable, maybe VMEMMAP_WITHOUT_TLB_FLUSH is better.
> 

As mentioned in the previous patch, yeah makes sense to have a bit just for
no TLB flush and perhaps we don't even BIT(1). We can use remap_pte to tell PTE
vs PMD flush "skipping"

> Thanks.
> 
>>       unsigned long        flags;
>>   };
>>   @@ -211,7 +212,8 @@ static int vmemmap_remap_range(unsigned long start,
>> unsigned long end,
>>               return ret;
>>       } while (pgd++, addr = next, addr != end);
>>   -    if (!(walk->flags & VMEMMAP_REMAP_ONLY_SPLIT))
>> +    if (!(walk->flags &
>> +          (VMEMMAP_REMAP_ONLY_SPLIT | VMEMMAP_REMAP_BULK_PAGES)))
>>           flush_tlb_kernel_range(start, end);
>>         return 0;
>> @@ -377,7 +379,7 @@ static int vmemmap_remap_free(unsigned long start,
>> unsigned long end,
>>           .remap_pte    = vmemmap_remap_pte,
>>           .reuse_addr    = reuse,
>>           .vmemmap_pages    = &vmemmap_pages,
>> -        .flags        = 0,
>> +        .flags        = !bulk_pages ? 0 : VMEMMAP_REMAP_BULK_PAGES,
>>       };
>>       int nid = page_to_nid((struct page *)start);
>>       gfp_t gfp_mask = GFP_KERNEL | __GFP_THISNODE | __GFP_NORETRY |
>> @@ -427,6 +429,7 @@ static int vmemmap_remap_free(unsigned long start,
>> unsigned long end,
>>               .remap_pte    = vmemmap_restore_pte,
>>               .reuse_addr    = reuse,
>>               .vmemmap_pages    = &vmemmap_pages,
>> +            .flags        = 0,
>>           };
>>             vmemmap_remap_range(reuse, end, &walk);
>> @@ -700,6 +703,8 @@ void hugetlb_vmemmap_optimize_folios(struct hstate *h,
>> struct list_head *folio_l
>>       list_for_each_entry(folio, folio_list, lru)
>>           hugetlb_vmemmap_optimize_bulk(h, &folio->page, &vmemmap_pages);
>>   +    flush_tlb_kernel_range(0, TLB_FLUSH_ALL);
>> +
>>       free_vmemmap_page_list(&vmemmap_pages);
>>   }
>>   
>