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
> */
