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

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Message-ID: <d1c4ee80-2096-4949-af9f-d55dcac071cb@oracle.com>
Date:   Tue, 19 Sep 2023 09:55:21 +0100
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Muchun Song <muchun.song@...ux.dev>
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 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.

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