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

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Message-ID: <CAHS8izMJx=+229iLt7GphUwioeAK5=CL0Fxi7TVywS2D+c-PKw@mail.gmail.com>
Date: Fri, 9 May 2025 12:04:37 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
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 9, 2025 at 11:11 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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.
>

Right, all I'm saying is that if it's at all possible to keep net_iov
something that can be extended with fields unrelated to struct page,
lets do that. net_iov already has fields that should not belong in
struct page like net_iov_owner and I think more will be added.

I'm thinking netmem_desc can be the fields that are shared between
struct net_iov and struct page (but both can have more specific to the
different memory types). As you say, for now netmem_desc can currently
overlap fields in struct page and struct net_iov, and a follow up
change can replace it with something that gets kmalloced and (I
guess?) there is a pointer in struct page or struct net_iov that
refers to the netmem_desc that contains the shared fields.


-- 
Thanks,
Mina