linux-kernel - Re: [PATCH 01/18] netmem: introduce struct netmem_desc struct_group_tagged()'ed on struct net

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Message-ID: <20250526005634.GA74632@system.software.com>
Date: Mon, 26 May 2025 09:56:34 +0900
From: Byungchul Park <byungchul@...com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: willy@...radead.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, kuba@...nel.org, almasrymina@...gle.com,
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	horms@...nel.org, linux-rdma@...r.kernel.org, bpf@...r.kernel.org,
	vishal.moola@...il.com
Subject: Re: [PATCH 01/18] netmem: introduce struct netmem_desc
 struct_group_tagged()'ed on struct net_iov

On Fri, May 23, 2025 at 11:01:01AM +0200, Toke Høiland-Jørgensen wrote:
> Byungchul Park <byungchul@...com> writes:
> 
> > To simplify struct page, the page pool members of struct page should be
> > moved to other, allowing these members to be removed from struct page.
> >
> > Introduce a network memory descriptor to store the members, struct
> > netmem_desc, reusing struct net_iov that already mirrored struct page.
> >
> > While at it, relocate _pp_mapping_pad to group struct net_iov's fields.
> >
> > Signed-off-by: Byungchul Park <byungchul@...com>
> > ---
> >  include/linux/mm_types.h |  2 +-
> >  include/net/netmem.h     | 43 +++++++++++++++++++++++++++++++++-------
> >  2 files changed, 37 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 56d07edd01f9..873e820e1521 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -120,13 +120,13 @@ struct page {
> >  			unsigned long private;
> >  		};
> >  		struct {	/* page_pool used by netstack */
> > +			unsigned long _pp_mapping_pad;
> >  			/**
> >  			 * @pp_magic: magic value to avoid recycling non
> >  			 * page_pool allocated pages.
> >  			 */
> >  			unsigned long pp_magic;
> >  			struct page_pool *pp;
> > -			unsigned long _pp_mapping_pad;
> >  			unsigned long dma_addr;
> >  			atomic_long_t pp_ref_count;
> >  		};
> 
> The reason that field is called "_pp_mapping_pad" is that it's supposed
> to overlay the page->mapping field, so that none of the page_pool uses
> set a value here. Moving it breaks that assumption. Once struct

Right.  I will fix it.  Thanks.

	Byungchul

> netmem_desc is completely decoupled from struct page this obviously
> doesn't matter, but I think it does today? At least, trying to use that
> field for the DMA index broke things, which is why we ended up with the
> bit-stuffing in pp_magic...
> 
> -Toke
>