linux-kernel - Re: [PATCH RFC 18/35] io_uring/zcrx: remove "struct io_copy_cache" and one nth

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Message-ID: <473f3576-ddf3-4388-aeec-d486f639950a@redhat.com>
Date: Fri, 22 Aug 2025 15:59:57 +0200
From: David Hildenbrand <david@...hat.com>
To: Pavel Begunkov <asml.silence@...il.com>, linux-kernel@...r.kernel.org
Cc: Jens Axboe <axboe@...nel.dk>, Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
 Dennis Zhou <dennis@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
 iommu@...ts.linux.dev, io-uring@...r.kernel.org,
 Jason Gunthorpe <jgg@...dia.com>, Johannes Weiner <hannes@...xchg.org>,
 John Hubbard <jhubbard@...dia.com>, kasan-dev@...glegroups.com,
 kvm@...r.kernel.org, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
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 linux-scsi@...r.kernel.org, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
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 Michal Hocko <mhocko@...e.com>, Mike Rapoport <rppt@...nel.org>,
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 Tejun Heo <tj@...nel.org>, virtualization@...ts.linux.dev,
 Vlastimil Babka <vbabka@...e.cz>, wireguard@...ts.zx2c4.com, x86@...nel.org,
 Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH RFC 18/35] io_uring/zcrx: remove "struct io_copy_cache"
 and one nth_page() usage

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