linux-kernel - Re: [RFC 19/19] mm, netmem: remove the page pool members in struct page

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Message-ID: <aB5FUKRV86Tg92b6@casper.infradead.org>
Date: Fri, 9 May 2025 19:11:28 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Byungchul Park <byungchul@...com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, kuba@...nel.org,
	ilias.apalodimas@...aro.org, harry.yoo@...cle.com, hawk@...nel.org,
	akpm@...ux-foundation.org, ast@...nel.org, daniel@...earbox.net,
	davem@...emloft.net, john.fastabend@...il.com,
	andrew+netdev@...n.ch, edumazet@...gle.com, pabeni@...hat.com,
	vishal.moola@...il.com
Subject: Re: [RFC 19/19] mm, netmem: remove the page pool members in struct
 page

On Fri, May 09, 2025 at 10:32:08AM -0700, Mina Almasry wrote:
> Currently the only restriction on net_iov is that some of its fields
> need to be cache aligned with some of the fields of struct page, but

Cache aligned?  Do you mean alias (ie be at the same offset)?

> What I would suggest here is, roughly:
> 
> 1. Add a new struct:
> 
>                struct netmem_desc {
>                        unsigned long pp_magic;
>                        struct page_pool *pp;
>                        unsigned long _pp_mapping_pad;
>                        unsigned long dma_addr;
>                        atomic_long_t pp_ref_count;
>                };
> 
> 2. Then update struct page to include this entry instead of the definitions:
> 
> struct page {
> ...
>                struct netmem_desc place_holder_1; /* for page pool */
> ...
> }

No, the point is to move these fields out of struct page entirely.

At some point (probably this year), we'll actually kmalloc the netmem_desc
(and shrink struct page), but for now, it'll overlap the other fields
in struct page.

> 3. And update struct net_iov to also include netmem_desc:
> 
> struct net_iov {
>     struct netmem_desc desc;
>     struct net_iov_area *owner;
>     /* More net_iov specific fields in the future */
> };
> 
> And drop patch 1 which does a rename.
> 
> Essentially netmem_desc can be an encapsulation of the shared fields
> between struct page and struct net_iov.

That is not the goal.