Re: [PATCH v3 01/15] mm: Batch-copy PTE ranges during fork()

[David Hildenbrand]

> With rmap batching from [1] -- rebased+changed on top of that -- we could turn
> that into an effective (untested):
>
>           if (page && folio_test_anon(folio)) {
> +               nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end,
> +                                               pte, enforce_uffd_wp, &nr_dirty,
> +                                               &nr_writable);
>                   /*
>                    * If this page may have been pinned by the parent process,
>                    * copy the page immediately for the child so that we'll always
>                    * guarantee the pinned page won't be randomly replaced in the
>                    * future.
>                    */
> -               folio_get(folio);
> -               if (unlikely(folio_try_dup_anon_rmap_pte(folio, page, src_vma))) {
> +               folio_ref_add(folio, nr);
> +               if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page, nr, src_vma))) {
>                           /* Page may be pinned, we have to copy. */
> -                       folio_put(folio);
> -                       return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
> -                                                addr, rss, prealloc, page);
> +                       folio_ref_sub(folio, nr);
> +                       ret = copy_present_page(dst_vma, src_vma, dst_pte,
> +                                               src_pte, addr, rss, prealloc,
> +                                               page);
> +                       return ret == 0 ? 1 : ret;
>                   }
> -               rss[MM_ANONPAGES]++;
> +               rss[MM_ANONPAGES] += nr;
>           } else if (page) {
> -               folio_get(folio);
> -               folio_dup_file_rmap_pte(folio, page);
> -               rss[mm_counter_file(page)]++;
> +               nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end,
> +                                               pte, enforce_uffd_wp, &nr_dirty,
> +                                               &nr_writable);
> +               folio_ref_add(folio, nr);
> +               folio_dup_file_rmap_ptes(folio, page, nr);
> +               rss[mm_counter_file(page)] += nr;
>           }
>
>
> We'll have to test performance, but it could be that we want to specialize
> more on !folio_test_large(). That code is very performance-sensitive.
>
>
> [1] https://lkml.kernel.org/r/20231204142146.91437-1-david@xxxxxxxxxx

So, on top of [1] without rmap batching but with a slightly modified 
version of yours (that keeps the existing code structure as pointed out 
and e.g., updates counter updates), running my fork() microbenchmark 
with a 1 GiB of memory:

Compared to [1], with all order-0 pages it gets 13--14% _slower_ and 
with all PTE-mapped THP (order-9) it gets ~29--30% _faster_.

So looks like we really want to have a completely seprate code path for 
"!folio_test_large()" to keep that case as fast as possible. And 
"Likely" we want to use "likely(!folio_test_large()". ;)

Performing rmap batching on top of that code only slightly (another 1% 
or so) improves performance in the PTE-mapped THP (order-9) case right 
now, in contrast to other rmap batching. Reason is as all rmap code gets 
inlined here and we're only doing subpage mapcount updates + PAE handling.

--
Cheers,

David / dhildenb