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

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Date:   Tue, 5 Dec 2023 14:02:28 +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 13:50, David Hildenbrand wrote:
> On 05.12.23 14:40, Ryan Roberts wrote:
>> On 05/12/2023 13:18, 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.
>>>
>>> Ah, no, I did this for a reason. This function lives in a header, so it will
>>> always be inlined.
>>>
>>
>> Really? It will certainly be duplicated across every compilation unit, but
>> that's separate from being inlined - if the optimizer is off, won't it just end
>> up as an out-of-line function in every compilation unit?
> 
> Good point, I didn't really consider that here, and thinking about it it makes
> perfect sense.
> 
> I think the compiler might even ignore "always_inline". I read that especially
> with recursion the compiler might ignore that. But people can then complain to
> the compiler writers about performance issues here, we told the compiler what we
> think is best.
> 

To be honest, my comment assumed that you had a good reason for using
__always_inline, and in that case then you should be consistent. But if you
don't have a good reason, you should probably just use inline and let the
compiler do what it thinks best?