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

[Pavel Begunkov <asml.silence@gmail.com>]

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.

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?

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;
  			src_offset = offset_in_page(src_offset);
  			n = min(PAGE_SIZE - src_offset, PAGE_SIZE - dst_offset);
  			n = min(n, len);

-- 
Pavel Begunkov