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

Pavel Begunkov
2025-11-19T00:02:30Z

On 8/22/25 14:59, David Hildenbrand wrote:
> 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.

Exactly

>> 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)

Makes sense. No objection, hopefully it won't cause too many conflicts.

>> 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.

If we go with this version:

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>

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Pavel Begunkov