linux-kernel - Re: [PATCH v4 6/8] hugetlb: batch PMD split for bulk vmemmap dedup

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Message-Id: <83B874B6-FF22-4588-90A9-31644D598032@linux.dev>
Date:   Tue, 19 Sep 2023 16:57:00 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Joao Martins <joao.m.martins@...cle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        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 <21cnbao@...il.com>, Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun Duan <duanxiongchun@...edance.com>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 6/8] hugetlb: batch PMD split for bulk vmemmap dedup



> On Sep 19, 2023, at 16:55, Joao Martins <joao.m.martins@...cle.com> wrote:
> 
> On 19/09/2023 09:41, Muchun Song wrote:
>>> On Sep 19, 2023, at 16:26, Joao Martins <joao.m.martins@...cle.com> wrote:
>>> On 19/09/2023 07:42, Muchun Song wrote:
>>>> On 2023/9/19 07:01, Mike Kravetz wrote:
>>>>> From: Joao Martins <joao.m.martins@...cle.com>
>>>>> 
>>>>> In an effort to minimize amount of TLB flushes, batch all PMD splits
>>>>> belonging to a range of pages in order to perform only 1 (global) TLB
>>>>> flush.
>>>>> 
>>>>> Add a flags field to the walker and pass whether it's a bulk allocation
>>>>> or just a single page to decide to remap. First value
>>>>> (VMEMMAP_SPLIT_NO_TLB_FLUSH) designates the request to not do the TLB
>>>>> flush when we split the PMD.
>>>>> 
>>>>> Rebased and updated by Mike Kravetz
>>>>> 
>>>>> Signed-off-by: Joao Martins <joao.m.martins@...cle.com>
>>>>> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
>>>>> ---
>>>>> mm/hugetlb_vmemmap.c | 79 +++++++++++++++++++++++++++++++++++++++++---
>>>>> 1 file changed, 75 insertions(+), 4 deletions(-)
>>>>> 
>>>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>>>>> index 147ed15bcae4..e8bc2f7567db 100644
>>>>> --- a/mm/hugetlb_vmemmap.c
>>>>> +++ b/mm/hugetlb_vmemmap.c
>>>>> @@ -27,6 +27,7 @@
>>>>>  * @reuse_addr:        the virtual address of the @reuse_page page.
>>>>>  * @vmemmap_pages:    the list head of the vmemmap pages that can be freed
>>>>>  *            or is mapped from.
>>>>> + * @flags:        used to modify behavior in bulk operations
>>>> 
>>>> Better to describe it as "used to modify behavior in vmemmap page table walking
>>>> operations"
>>>> 
>>> OK
>>> 
>>>>> void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head
>>>>> *folio_list)
>>>>> {
>>>>>     struct folio *folio;
>>>>>     LIST_HEAD(vmemmap_pages);
>>>>> +    list_for_each_entry(folio, folio_list, lru)
>>>>> +        hugetlb_vmemmap_split(h, &folio->page);
>>>>> +
>>>>> +    flush_tlb_all();
>>>>> +
>>>>>     list_for_each_entry(folio, folio_list, lru) {
>>>>>         int ret = __hugetlb_vmemmap_optimize(h, &folio->page,
>>>>>                                 &vmemmap_pages);
>>>> 
>>>> This is unlikely to be failed since the page table allocation
>>>> is moved to the above 
>>> 
>>>> (Note that the head vmemmap page allocation
>>>> is not mandatory). 
>>> 
>>> Good point that I almost forgot
>>> 
>>>> So we should handle the error case in the above
>>>> splitting operation.
>>> 
>>> But back to the previous discussion in v2... the thinking was that /some/ PMDs
>>> got split, and say could allow some PTE remapping to occur and free some pages
>>> back (each page allows 6 more splits worst case). Then the next
>>> __hugetlb_vmemmap_optimize() will have to split PMD pages again for those
>>> hugepages that failed the batch PMD split (as we only defer the PTE remap tlb
>>> flush in this stage).
>> 
>> Oh, yes. Maybe we could break the above traversal as early as possible
>> once we enter an ENOMEM?
>> 
> 
> Sounds good -- no point in keep trying to split if we are failing with OOM.
> 
> Perhaps a comment in both of these clauses (the early break on split and the OOM
> handling in batch optimize) could help make this clear.

Make sense.

Thanks.

> 
>>> 
>>> Unless this isn't something worth handling
>>> 
>>> Joao