Re: [PATCH RFC 18/35] io_uring/zcrx: remove "struct io_copy_cache" and one nth_page() usage

David Hildenbrand
2025-08-22T14:00:15Z

On 22.08.25 13:32, Pavel Begunkov wrote:
> On 8/21/25 21:06, David Hildenbrand wrote:
>> We always provide a single dst page, it's unclear why the io_copy_cache
>> complexity is required.
>
> Because it'll need to be pulled outside the loop to reuse the page for
> multiple copies, i.e. packing multiple fragments of the same skb into
> it. Not finished, and currently it's wasting memory.

Okay, so what you're saying is that there will be follow-up work that
will actually make this structure useful.

>
> Why not do as below? Pages there never cross boundaries of their folios. > Do you want it to be taken into the io_uring tree?

This should better all go through the MM tree where we actually
guarantee contiguous pages within a folio. (see the cover letter)

>
> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
> index e5ff49f3425e..18c12f4b56b6 100644
> --- a/io_uring/zcrx.c
> +++ b/io_uring/zcrx.c
> @@ -975,9 +975,9 @@ static ssize_t io_copy_page(struct io_copy_cache *cc, struct page *src_page,
>
> if (folio_test_partial_kmap(page_folio(dst_page)) ||
> folio_test_partial_kmap(page_folio(src_page))) {
> - dst_page = nth_page(dst_page, dst_offset / PAGE_SIZE);
> + dst_page += dst_offset / PAGE_SIZE;
> dst_offset = offset_in_page(dst_offset);
> - src_page = nth_page(src_page, src_offset / PAGE_SIZE);
> + src_page += src_offset / PAGE_SIZE;

Yeah, I can do that in the next version given that you have plans on
extending that code soon.

--
Cheers

David / dhildenb