linux-kernel - Re: [PATCH v1 2/3] mm: Implement folio_remove_rmap

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Date:   Mon, 17 Jul 2023 11:53:40 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v1 2/3] mm: Implement folio_remove_rmap_range()

On 17 Jul 2023, at 11:51, Ryan Roberts wrote:

> On 17/07/2023 16:09, Zi Yan wrote:
>> On 17 Jul 2023, at 10:31, Ryan Roberts wrote:
>>
>>> Like page_remove_rmap() but batch-removes the rmap for a range of pages
>>> belonging to a folio. This can provide a small speedup due to less
>>> manipuation of the various counters. But more crucially, if removing the
>>> rmap for all pages of a folio in a batch, there is no need to
>>> (spuriously) add it to the deferred split list, which saves significant
>>> cost when there is contention for the split queue lock.
>>>
>>> All contained pages are accounted using the order-0 folio (or base page)
>>> scheme.
>>>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>> ---
>>>  include/linux/rmap.h |  2 ++
>>>  mm/rmap.c            | 65 ++++++++++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 67 insertions(+)
>>>
>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>> index b87d01660412..f578975c12c0 100644
>>> --- a/include/linux/rmap.h
>>> +++ b/include/linux/rmap.h
>>> @@ -200,6 +200,8 @@ void page_add_file_rmap(struct page *, struct vm_area_struct *,
>>>  		bool compound);
>>>  void page_remove_rmap(struct page *, struct vm_area_struct *,
>>>  		bool compound);
>>> +void folio_remove_rmap_range(struct folio *folio, struct page *page,
>>> +		int nr, struct vm_area_struct *vma);
>>>
>>>  void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *,
>>>  		unsigned long address, rmap_t flags);
>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>> index 2baf57d65c23..1da05aca2bb1 100644
>>> --- a/mm/rmap.c
>>> +++ b/mm/rmap.c
>>> @@ -1359,6 +1359,71 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
>>>  	mlock_vma_folio(folio, vma, compound);
>>>  }
>>>
>>> +/*
>>> + * folio_remove_rmap_range - take down pte mappings from a range of pages
>>> + * belonging to a folio. All pages are accounted as small pages.
>>> + * @folio:	folio that all pages belong to
>>> + * @page:       first page in range to remove mapping from
>>> + * @nr:		number of pages in range to remove mapping from
>>
>> We might need some checks to make sure [page, page+nr] is in the range of
>> the folio. Something like:
>>
>> page >= &folio->page && page + nr < (&folio->page + folio_nr_pages(folio))
>
> No problem. Is a VM_WARN_ON() appropriate for something like this?

VM_WARN_ON_ONCE() might be better.

--
Best Regards,
Yan, Zi

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