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

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Message-ID: <e2db9119-0f8f-42d2-af13-529edb043bc6@redhat.com>
Date:   Tue, 5 Dec 2023 14:32:06 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....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 05.12.23 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;
}

-- 
Cheers,

David / dhildenb