Re: [PATCH RFC 34/39] mm/rmap: introduce folio_try_dup_anon_rmap_[pte|ptes|pmd]()

[David Hildenbrand]

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@xxxxxxxxxx>
> > ---
> >   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.

> > +		for (i = 0; i < nr_pages; i++) {
> > +			if (PageAnonExclusive(page + i))
> > +				goto clear;
> > +		}
> > +	} else if (mode == RMAP_MODE_PMD) {
> > +		if (PageAnonExclusive(page))
> > +			goto clear;
> >   	}
> > +	goto dup;
> > +
> > +clear:
> > +	/*
> > +	 * If this folio may have been pinned by the parent process,
> > +	 * don't allow to duplicate the mappings but instead require to e.g.,
> > +	 * copy the subpage immediately for the child so that we'll always
> > +	 * guarantee the pinned folio won't be randomly replaced in the
> > +	 * future on write faults.
> > +	 */
> > +	if (likely(!folio_is_device_private(folio) &&
> > +	    unlikely(folio_needs_cow_for_dma(src_vma, folio))))
> > +		return -EBUSY;
> > +
> > +	if (likely(mode == RMAP_MODE_PTE)) {
> > +		for (i = 0; i < nr_pages; i++)
>
> Do you really need to reset i=0 here? You have already checked that lower pages
> are shared in the above loop, so can't you just start from the first exclusive
> page here?

It's best to check the updated patch I sent.

--
Cheers,

David / dhildenb