linux-kernel - Re: [PATCH v3 2/3] mm: Implement folio_remove_rmap

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Date:   Wed, 26 Jul 2023 07:42:25 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Yu Zhao <yuzhao@...gle.com>, Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 2/3] mm: Implement folio_remove_rmap_range()

On 26/07/2023 06:53, Yu Zhao wrote:
> On Thu, Jul 20, 2023 at 5:30 AM Ryan Roberts <ryan.roberts@....com> 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.
>>
>> page_remove_rmap() is refactored so that it forwards to
>> folio_remove_rmap_range() for !compound cases, and both functions now
>> share a common epilogue function. The intention here is to avoid
>> duplication of code.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>  include/linux/rmap.h |   2 +
>>  mm/rmap.c            | 125 ++++++++++++++++++++++++++++++++-----------
>>  2 files changed, 97 insertions(+), 30 deletions(-)
>>
>> 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);
> 
> I prefer folio_remove_rmap_range(page, nr, vma). Passing both the
> folio and the starting page seems redundant to me.

I prefer to pass folio explicitly because it makes it clear that all pages in
the range must belong to the same folio.

> 
> Matthew, is there a convention (function names, parameters, etc.) for
> operations on a range of pages within a folio?
> 
> And regarding the refactor, what I have in mind is that
> folio_remove_rmap_range() is the core API and page_remove_rmap() is
> just a wrapper around it, i.e., folio_remove_rmap_range(page, 1, vma).

I tried to do it that way, but the existing page_remove_rmap() also takes a
'compound' parameter; it can operate on compound, thp pages and uses the
alternative accounting scheme in this case.

I could add a compound parameter to folio_remove_rmap_range() but in that case
the range parameters don't make sense - when compound is true we are implicitly
operating on the whole folio due to the way the accounting is done. So I felt it
was clearer for folio_remove_rmap_range() to deal with small page accounting
only. page_remove_rmap() forwards to folio_remove_rmap_range() when
compound=false and page_remove_rmap() directly deals with the thp accounting
when compound=true.

> 
> Let me post a diff later and see if it makes sense to you.