Re: [PATCH v2 1/5] mm: Implement folio_remove_rmap_range()

[Ryan Roberts]

On 30/08/2023 15:51, Matthew Wilcox wrote:
> On Wed, Aug 30, 2023 at 10:50:07AM +0100, Ryan Roberts wrote:
>> Like page_remove_rmap() but batch-removes the rmap for a range of pages
>> belonging to a folio. This can provide a small speedup due to less
>> manipuation of the various counters. But more crucially, if removing the
>> rmap for all pages of a folio in a batch, there is no need to
>> (spuriously) add it to the deferred split list, which saves significant
>> cost when there is contention for the split queue lock.
>>
>> All contained pages are accounted using the order-0 folio (or base page)
>> scheme.
>>
>> page_remove_rmap() is refactored so that it forwards to
>> folio_remove_rmap_range() for !compound cases, and both functions now
>> share a common epilogue function. The intention here is to avoid
>> duplication of code.
> 
> What would you think to doing it like this instead?  This probably doesn't
> even compile and it's definitely not sanity checked; just trying to get
> across an idea of the shape of this code.  I think this is more like
> what DavidH was asking for (but he's on holiday this week so won't be
> able to confirm).

I think it was actually Yu Zhou who was arguing for something more like this?

I don't love it, because as I've expressed previously, I don't think an API that
operates on a "range" of pages has any business manipulating a PMD mapping,
because it can't be subdivided. So my preference is for what I've already got.

But certainly your proposal to use nr=-1 to mean compound is the nicest way I've
seen since you no longer make nr redundant in this case.

Given you and Yu both have a lot more experience with this code than me, I'll
follow your advice and do it this way for the next rev.

Thanks,
Ryan


> 
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index a3825ce81102..d442d1e5425d 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -202,6 +202,8 @@ void folio_add_file_rmap_range(struct folio *, struct page *, unsigned int nr,
>  		struct vm_area_struct *, bool compound);
>  void page_remove_rmap(struct page *, struct vm_area_struct *,
>  		bool compound);
> +void folio_remove_rmap_range(struct folio *folio, struct page *page,
> +		int nr, struct vm_area_struct *vma);
>  
>  void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *,
>  		unsigned long address, rmap_t flags);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index ec7f8e6c9e48..2592be47452e 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1380,24 +1380,26 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
>  }
>  
>  /**
> - * page_remove_rmap - take down pte mapping from a page
> - * @page:	page to remove mapping from
> - * @vma:	the vm area from which the mapping is removed
> - * @compound:	uncharge the page as compound or small page
> + * folio_remove_rmap_range - Take down PTE mappings from a range of pages.
> + * @folio:	Folio containing all pages in range.
> + * @page:	First page in range to unmap.
> + * @nr:		Number of pages to unmap.  -1 to unmap a PMD.
> + * @vma:	The VM area containing the range.
>   *
> - * The caller needs to hold the pte lock.
> + * All pages in the range must belong to the same VMA & folio.
> + *
> + * Context: Caller holds the pte lock.
>   */
> -void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
> -		bool compound)
> +void folio_remove_rmap_range(struct folio *folio, struct page *page,
> +			int pages, struct vm_area_struct *vma)
>  {
> -	struct folio *folio = page_folio(page);
>  	atomic_t *mapped = &folio->_nr_pages_mapped;
> +	int nr_unmapped = 0;
> +	int nr_mapped = 0;
>  	int nr = 0, nr_pmdmapped = 0;
>  	bool last;
>  	enum node_stat_item idx;
>  
> -	VM_BUG_ON_PAGE(compound && !PageHead(page), page);
> -
>  	/* Hugetlb pages are not counted in NR_*MAPPED */
>  	if (unlikely(folio_test_hugetlb(folio))) {
>  		/* hugetlb pages are always mapped with pmds */
> @@ -1405,14 +1407,25 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
>  		return;
>  	}
>  
> -	/* Is page being unmapped by PTE? Is this its last map to be removed? */
> -	if (likely(!compound)) {
> -		last = atomic_add_negative(-1, &page->_mapcount);
> -		nr = last;
> -		if (last && folio_test_large(folio)) {
> -			nr = atomic_dec_return_relaxed(mapped);
> -			nr = (nr < COMPOUND_MAPPED);
> +	/* Are we taking down a PMD mapping? */
> +	if (likely(pages > 0)) {
> +		VM_WARN_ON_ONCE(page < folio_page(folio, 0) ||
> +				page + pages > folio_page(folio,
> +						folio_nr_pages(folio)));
> +		while (pages) {
> +			/* Is this the page's last map to be removed? */
> +			last = atomic_add_negative(-1, &page->_mapcount);
> +			if (last)
> +				nr_unmapped++;
> +			pages--;
> +			page++;
>  		}
> +
> +		/* Pages still mapped if folio mapped entirely */
> +		nr_mapped = atomic_sub_return_relaxed(nr_unmapped, mapped);
> +		if (nr_mapped >= COMPOUND_MAPPED)
> +			nr_unmapped = 0;
> +
>  	} else if (folio_test_pmd_mappable(folio)) {
>  		/* That test is redundant: it's for safety or to optimize out */
>  
> @@ -1441,18 +1454,19 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
>  			idx = NR_FILE_PMDMAPPED;
>  		__lruvec_stat_mod_folio(folio, idx, -nr_pmdmapped);
>  	}
> +
>  	if (nr) {
>  		idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
>  		__lruvec_stat_mod_folio(folio, idx, -nr);
>  
>  		/*
> -		 * Queue anon THP for deferred split if at least one
> -		 * page of the folio is unmapped and at least one page
> -		 * is still mapped.
> +		 * Queue large anon folio for deferred split if at least one
> +		 * page of the folio is unmapped and at least one page is still
> +		 * mapped.
>  		 */
> -		if (folio_test_pmd_mappable(folio) && folio_test_anon(folio))
> -			if (!compound || nr < nr_pmdmapped)
> -				deferred_split_folio(folio);
> +		if (folio_test_large(folio) && folio_test_anon(folio) &&
> +		    nr_mapped)
> +			deferred_split_folio(folio);
>  	}
>  
>  	/*
> @@ -1466,6 +1480,20 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
>  	munlock_vma_folio(folio, vma, compound);
>  }
>  
> +/**
> + * page_remove_rmap - take down pte mapping from a page
> + * @page:	page to remove mapping from
> + * @vma:	the vm area from which the mapping is removed
> + * @compound:	uncharge the page as compound or small page
> + *
> + * The caller needs to hold the pte lock.
> + */
> +void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
> +		bool compound)
> +{
> +	folio_remove_rmap_range(page_folio(page), page, compound ? -1 : 1, vma);
> +}
> +
>  /*
>   * @arg: enum ttu_flags will be passed to this argument
>   */