On 17 Jul 2023, at 10:31, 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.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
> include/linux/rmap.h | 2 ++
> mm/rmap.c | 65 ++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 67 insertions(+)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index b87d01660412..f578975c12c0 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -200,6 +200,8 @@ void page_add_file_rmap(struct page *, 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 2baf57d65c23..1da05aca2bb1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1359,6 +1359,71 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
> mlock_vma_folio(folio, vma, compound);
> }
>
> +/*
> + * folio_remove_rmap_range - take down pte mappings from a range of pages
> + * belonging to a folio. All pages are accounted as small pages.
> + * @folio: folio that all pages belong to
> + * @page: first page in range to remove mapping from
> + * @nr: number of pages in range to remove mapping from
We might need some checks to make sure [page, page+nr] is in the range of
the folio. Something like:
page >= &folio->page && page + nr < (&folio->page + folio_nr_pages(folio))
> + * @vma: the vm area from which the mapping is removed
> + *
> + * The caller needs to hold the pte lock.
> + */
> +void folio_remove_rmap_range(struct folio *folio, struct page *page,
> + int nr, struct vm_area_struct *vma)
> +{
> + atomic_t *mapped = &folio->_nr_pages_mapped;
> + int nr_unmapped = 0;
> + int nr_mapped;
> + bool last;
> + enum node_stat_item idx;
> +
> + if (unlikely(folio_test_hugetlb(folio))) {
> + VM_WARN_ON_FOLIO(1, folio);
> + return;
> + }
> +
> + if (!folio_test_large(folio)) {
> + /* Is this the page's last map to be removed? */
> + last = atomic_add_negative(-1, &page->_mapcount);
> + nr_unmapped = last;
> + } else {
> + for (; nr != 0; nr--, page++) {
> + /* Is this the page's last map to be removed? */
> + last = atomic_add_negative(-1, &page->_mapcount);
> + if (last) {
> + /* Page still mapped if folio mapped entirely */
> + nr_mapped = atomic_dec_return_relaxed(mapped);
> + if (nr_mapped < COMPOUND_MAPPED)
> + nr_unmapped++;
> + }
> + }
> + }
> +
> + if (nr_unmapped) {
> + idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
> + __lruvec_stat_mod_folio(folio, idx, -nr_unmapped);
> +
> + /*
> + * Queue anon THP for deferred split if we have just unmapped at
> + * least 1 page, while at least 1 page remains mapped.
> + */
> + if (folio_test_large(folio) && folio_test_anon(folio))
> + if (nr_mapped)
> + deferred_split_folio(folio);
> + }
> +
> + /*
> + * It would be tidy to reset folio_test_anon mapping when fully
> + * unmapped, but that might overwrite a racing page_add_anon_rmap
> + * which increments mapcount after us but sets mapping before us:
> + * so leave the reset to free_pages_prepare, and remember that
> + * it's only reliable while mapped.
> + */
> +
> + munlock_vma_folio(folio, vma, false);
> +}
> +
> /**
> * page_remove_rmap - take down pte mapping from a page
> * @page: page to remove mapping from
> --
> 2.25.1
Everything else looks good to me. Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
--
Best Regards,
Yan, Zi
Attachment:
signature.asc
Description: OpenPGP digital signature
