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Message-Id: <07192BE2-C66E-4F74-8F76-05F57777C6B7@linux.dev>
Date: Tue, 19 Sep 2023 16:41:44 +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 16:26, 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?
>
> Unless this isn't something worth handling
>
> Joao
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