Re: [PATCH RFC 07/39] mm/rmap: convert folio_add_file_rmap_range() into folio_add_file_rmap_[pte|ptes|pmd]()

[Ryan Roberts]

On 04/12/2023 14:21, David Hildenbrand wrote:
> Let's get rid of the compound parameter and instead define implicitly
> which mappings we're adding. That is more future proof, easier to read
> and harder to mess up.
> 
> Use an enum to express the granularity internally. Make the compiler
> always special-case on the granularity by using __always_inline.
> 
> Add plenty of sanity checks with CONFIG_DEBUG_VM. Replace the
> folio_test_pmd_mappable() check by a config check in the caller and
> sanity checks. Convert the single user of folio_add_file_rmap_range().
> 
> This function design can later easily be extended to PUDs and to batch
> PMDs. Note that for now we don't support anything bigger than
> PMD-sized folios (as we cleanly separated hugetlb handling). Sanity checks

Is that definitely true? Don't we support PUD-mapping file-backed DAX memory?


> will catch if that ever changes.
> 
> Next up is removing page_remove_rmap() along with its "compound"
> parameter and smilarly converting all other rmap functions.
> 
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  include/linux/rmap.h | 47 +++++++++++++++++++++++++++--
>  mm/memory.c          |  2 +-
>  mm/rmap.c            | 72 ++++++++++++++++++++++++++++----------------
>  3 files changed, 92 insertions(+), 29 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 77e336f86c72d..a4a30c361ac50 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -186,6 +186,45 @@ typedef int __bitwise rmap_t;
>   */
>  #define RMAP_COMPOUND		((__force rmap_t)BIT(1))
>  
> +/*
> + * Internally, we're using an enum to specify the granularity. Usually,
> + * we make the compiler create specialized variants for the different
> + * granularity.
> + */
> +enum rmap_mode {
> +	RMAP_MODE_PTE = 0,
> +	RMAP_MODE_PMD,
> +};
> +
> +static inline void __folio_rmap_sanity_checks(struct folio *folio,
> +		struct page *page, unsigned int nr_pages, enum rmap_mode mode)
> +{
> +	/* hugetlb folios are handled separately. */
> +	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
> +	VM_WARN_ON_FOLIO(folio_test_large(folio) &&
> +			 !folio_test_large_rmappable(folio), folio);
> +
> +	VM_WARN_ON_ONCE(!nr_pages || nr_pages > folio_nr_pages(folio));

nit: I don't think you technically need the second half of this - its covered by
the test below...

> +	VM_WARN_ON_FOLIO(page_folio(page) != folio, folio);
> +	VM_WARN_ON_FOLIO(page_folio(page + nr_pages - 1) != folio, folio);

...this one.

> +
> +	switch (mode) {
> +	case RMAP_MODE_PTE:
> +		break;
> +	case RMAP_MODE_PMD:
> +		/*
> +		 * We don't support folios larger than a single PMD yet. So
> +		 * when RMAP_MODE_PMD is set, we assume that we are creating
> +		 * a single "entire" mapping of the folio.
> +		 */
> +		VM_WARN_ON_FOLIO(folio_nr_pages(folio) != HPAGE_PMD_NR, folio);
> +		VM_WARN_ON_FOLIO(nr_pages != HPAGE_PMD_NR, folio);
> +		break;
> +	default:
> +		VM_WARN_ON_ONCE(true);
> +	}
> +}
> +
>  /*
>   * rmap interfaces called when adding or removing pte of page
>   */
> @@ -198,8 +237,12 @@ void folio_add_new_anon_rmap(struct folio *, struct vm_area_struct *,
>  		unsigned long address);
>  void page_add_file_rmap(struct page *, struct vm_area_struct *,
>  		bool compound);
> -void folio_add_file_rmap_range(struct folio *, struct page *, unsigned int nr,
> -		struct vm_area_struct *, bool compound);
> +void folio_add_file_rmap_ptes(struct folio *, struct page *, unsigned int nr,
> +		struct vm_area_struct *);
> +#define folio_add_file_rmap_pte(folio, page, vma) \
> +	folio_add_file_rmap_ptes(folio, page, 1, vma)
> +void folio_add_file_rmap_pmd(struct folio *, struct page *,
> +		struct vm_area_struct *);
>  void page_remove_rmap(struct page *, struct vm_area_struct *,
>  		bool compound);
>  
> diff --git a/mm/memory.c b/mm/memory.c
> index 1f18ed4a54971..15325587cff01 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4414,7 +4414,7 @@ void set_pte_range(struct vm_fault *vmf, struct folio *folio,
>  		folio_add_lru_vma(folio, vma);
>  	} else {
>  		add_mm_counter(vma->vm_mm, mm_counter_file(page), nr);
> -		folio_add_file_rmap_range(folio, page, nr, vma, false);
> +		folio_add_file_rmap_ptes(folio, page, nr, vma);
>  	}
>  	set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr);
>  
> diff --git a/mm/rmap.c b/mm/rmap.c
> index a735ecca47a81..1614d98062948 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1334,31 +1334,19 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>  	SetPageAnonExclusive(&folio->page);
>  }
>  
> -/**
> - * folio_add_file_rmap_range - add pte mapping to page range of a folio
> - * @folio:	The folio to add the mapping to
> - * @page:	The first page to add
> - * @nr_pages:	The number of pages which will be mapped
> - * @vma:	the vm area in which the mapping is added
> - * @compound:	charge the page as compound or small page
> - *
> - * The page range of folio is defined by [first_page, first_page + nr_pages)
> - *
> - * The caller needs to hold the pte lock.
> - */
> -void folio_add_file_rmap_range(struct folio *folio, struct page *page,
> -			unsigned int nr_pages, struct vm_area_struct *vma,
> -			bool compound)
> +static __always_inline void __folio_add_file_rmap(struct folio *folio,
> +		struct page *page, unsigned int nr_pages,
> +		struct vm_area_struct *vma, enum rmap_mode mode)
>  {
>  	atomic_t *mapped = &folio->_nr_pages_mapped;
>  	unsigned int nr_pmdmapped = 0, first;
>  	int nr = 0;
>  
> -	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
> -	VM_WARN_ON_FOLIO(compound && !folio_test_pmd_mappable(folio), folio);
> +	VM_WARN_ON_FOLIO(folio_test_anon(folio), folio);
> +	__folio_rmap_sanity_checks(folio, page, nr_pages, mode);
>  
>  	/* Is page being mapped by PTE? Is this its first map to be added? */
> -	if (likely(!compound)) {
> +	if (likely(mode == RMAP_MODE_PTE)) {
>  		do {
>  			first = atomic_inc_and_test(&page->_mapcount);
>  			if (first && folio_test_large(folio)) {
> @@ -1369,9 +1357,7 @@ void folio_add_file_rmap_range(struct folio *folio, struct page *page,
>  			if (first)
>  				nr++;
>  		} while (page++, --nr_pages > 0);
> -	} else if (folio_test_pmd_mappable(folio)) {
> -		/* That test is redundant: it's for safety or to optimize out */
> -
> +	} else if (mode == RMAP_MODE_PMD) {
>  		first = atomic_inc_and_test(&folio->_entire_mapcount);
>  		if (first) {
>  			nr = atomic_add_return_relaxed(COMPOUND_MAPPED, mapped);
> @@ -1399,6 +1385,43 @@ void folio_add_file_rmap_range(struct folio *folio, struct page *page,
>  		mlock_vma_folio(folio, vma);
>  }
>  
> +/**
> + * folio_add_file_rmap_ptes - add PTE mappings to a page range of a folio
> + * @folio:	The folio to add the mappings to
> + * @page:	The first page to add
> + * @nr_pages:	The number of pages that will be mapped using PTEs
> + * @vma:	The vm area in which the mappings are added
> + *
> + * The page range of the folio is defined by [page, page + nr_pages)
> + *
> + * The caller needs to hold the page table lock.
> + */
> +void folio_add_file_rmap_ptes(struct folio *folio, struct page *page,
> +		unsigned int nr_pages, struct vm_area_struct *vma)
> +{
> +	__folio_add_file_rmap(folio, page, nr_pages, vma, RMAP_MODE_PTE);
> +}
> +
> +/**
> + * folio_add_file_rmap_pmd - add a PMD mapping to a page range of a folio
> + * @folio:	The folio to add the mapping to
> + * @page:	The first page to add
> + * @vma:	The vm area in which the mapping is added
> + *
> + * The page range of the folio is defined by [page, page + HPAGE_PMD_NR)
> + *
> + * The caller needs to hold the page table lock.
> + */
> +void folio_add_file_rmap_pmd(struct folio *folio, struct page *page,
> +		struct vm_area_struct *vma)
> +{
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	__folio_add_file_rmap(folio, page, HPAGE_PMD_NR, vma, RMAP_MODE_PMD);
> +#else
> +	WARN_ON_ONCE(true);
> +#endif
> +}
> +
>  /**
>   * page_add_file_rmap - add pte mapping to a file page
>   * @page:	the page to add the mapping to
> @@ -1411,16 +1434,13 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
>  		bool compound)
>  {
>  	struct folio *folio = page_folio(page);
> -	unsigned int nr_pages;
>  
>  	VM_WARN_ON_ONCE_PAGE(compound && !PageTransHuge(page), page);
>  
>  	if (likely(!compound))
> -		nr_pages = 1;
> +		folio_add_file_rmap_pte(folio, page, vma);
>  	else
> -		nr_pages = folio_nr_pages(folio);
> -
> -	folio_add_file_rmap_range(folio, page, nr_pages, vma, compound);
> +		folio_add_file_rmap_pmd(folio, page, vma);
>  }
>  
>  /**