linux-kernel - Re: [PATCH RFC 34/39] mm/rmap: introduce folio_try_dup_anon_rmap

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Message-ID: <183cf38c-476e-44c4-a8c7-22ed43122b41@arm.com>
Date:   Tue, 5 Dec 2023 13:40:51 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Hugh Dickins <hughd@...gle.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH RFC 34/39] mm/rmap: introduce
 folio_try_dup_anon_rmap_[pte|ptes|pmd]()

On 05/12/2023 13:32, David Hildenbrand wrote:
> On 05.12.23 14:17, David Hildenbrand wrote:
>> On 05.12.23 14:12, Ryan Roberts wrote:
>>> On 04/12/2023 14:21, David Hildenbrand wrote:
>>>> The last user of page_needs_cow_for_dma() and __page_dup_rmap() are gone,
>>>> remove them.
>>>>
>>>> Add folio_try_dup_anon_rmap_ptes() right away, we want to perform rmap
>>>> baching during fork() soon.
>>>>
>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>> ---
>>>>    include/linux/mm.h   |   6 --
>>>>    include/linux/rmap.h | 145 +++++++++++++++++++++++++++++--------------
>>>>    2 files changed, 100 insertions(+), 51 deletions(-)
>>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index 24c1c7c5a99c0..f7565b35ae931 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -1964,12 +1964,6 @@ static inline bool folio_needs_cow_for_dma(struct
>>>> vm_area_struct *vma,
>>>>        return folio_maybe_dma_pinned(folio);
>>>>    }
>>>>    -static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
>>>> -                      struct page *page)
>>>> -{
>>>> -    return folio_needs_cow_for_dma(vma, page_folio(page));
>>>> -}
>>>> -
>>>>    /**
>>>>     * is_zero_page - Query if a page is a zero page
>>>>     * @page: The page to query
>>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>>>> index 21d72cc602adc..84439f7720c62 100644
>>>> --- a/include/linux/rmap.h
>>>> +++ b/include/linux/rmap.h
>>>> @@ -354,68 +354,123 @@ static inline void folio_dup_file_rmap_pmd(struct
>>>> folio *folio,
>>>>    #endif
>>>>    }
>>>>    -static inline void __page_dup_rmap(struct page *page, bool compound)
>>>> +static inline int __folio_try_dup_anon_rmap(struct folio *folio,
>>>
>>> __always_inline?
>>
>> Yes.
>>
>>>
>>>> +        struct page *page, unsigned int nr_pages,
>>>> +        struct vm_area_struct *src_vma, enum rmap_mode mode)
>>>>    {
>>>> -    VM_WARN_ON(folio_test_hugetlb(page_folio(page)));
>>>> +    int i;
>>>>    -    if (compound) {
>>>> -        struct folio *folio = (struct folio *)page;
>>>> +    VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
>>>>    -        VM_BUG_ON_PAGE(compound && !PageHead(page), page);
>>>> -        atomic_inc(&folio->_entire_mapcount);
>>>> -    } else {
>>>> -        atomic_inc(&page->_mapcount);
>>>> +    /*
>>>> +     * No need to check+clear for already shared PTEs/PMDs of the folio.
>>>> +     * This includes PTE mappings of (order-0) KSM folios.
>>>> +     */
>>>> +    if (likely(mode == RMAP_MODE_PTE)) {
>>>
>>> Presumbly if __always_inline then the compiler will remove this if/else and just
>>> keep the part indicated by mode? In which case "likely" is pretty useless? Same
>>> for all similar sites in the other patches.
>>
>> Yes, also had this in mind. As long as we use __always_inline it
>> shouldn't ever matter.
> 
> It seems to be cleanest to just do:
> 
> switch (mode) {
> case RMAP_MODE_PTE:
>     ...
>     break;
> case RMAP_MODE_PMD:
>     ...
>     break;
> }
> 

Agreed.