linux-kernel - Re: [PATCH net-next v9 0/8] Split netmem from struct page

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Message-ID: <20250711004208.GA40145@system.software.com>
Date: Fri, 11 Jul 2025 09:42:08 +0900
From: Byungchul Park <byungchul@...com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: willy@...radead.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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Subject: Re: [PATCH net-next v9 0/8] Split netmem from struct page

On Thu, Jul 10, 2025 at 11:35:33AM -0700, Mina Almasry wrote:
> On Thu, Jul 10, 2025 at 1:28 AM Byungchul Park <byungchul@...com> wrote:
> >
> > Hi all,
> >
> > The MM subsystem is trying to reduce struct page to a single pointer.
> > See the following link for your information:
> >
> >    https://kernelnewbies.org/MatthewWilcox/Memdescs/Path
> >
> > The first step towards that is splitting struct page by its individual
> > users, as has already been done with folio and slab.  This patchset does
> > that for page pool.
> >
> > Matthew Wilcox tried and stopped the same work, you can see in:
> >
> >    https://lore.kernel.org/linux-mm/20230111042214.907030-1-willy@infradead.org/
> >
> > I focused on removing the page pool members in struct page this time,
> > not moving the allocation code of page pool from net to mm.  It can be
> > done later if needed.
> >
> > The final patch removing the page pool fields will be posted once all
> > the converting of page to netmem are done:
> >
> >    1. converting use of the pp fields in struct page in prueth_swdata.
> >    2. converting use of the pp fields in struct page in freescale driver.
> >
> > For our discussion, I'm sharing what the final patch looks like, in this
> > cover letter.
> >
> >         Byungchul
> > --8<--
> > commit 1847d9890f798456b21ccb27aac7545303048492
> > Author: Byungchul Park <byungchul@...com>
> > Date:   Wed May 28 20:44:55 2025 +0900
> >
> >     mm, netmem: remove the page pool members in struct page
> >
> >     Now that all the users of the page pool members in struct page have been
> >     gone, the members can be removed from struct page.
> >
> >     However, since struct netmem_desc still uses the space in struct page,
> >     the important offsets should be checked properly, until struct
> >     netmem_desc has its own instance from slab.
> >
> >     Remove the page pool members in struct page and modify static checkers
> >     for the offsets.
> >
> >     Signed-off-by: Byungchul Park <byungchul@...com>
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 32ba5126e221..db2fe0d0ebbf 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -120,17 +120,6 @@ struct page {
> >                          */
> >                         unsigned long private;
> >                 };
> > -               struct {        /* page_pool used by netstack */
> > -                       /**
> > -                        * @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;
> > -               };
> >                 struct {        /* Tail pages of compound page */
> >                         unsigned long compound_head;    /* Bit zero is set */
> >                 };
> > diff --git a/include/net/netmem.h b/include/net/netmem.h
> > index 8f354ae7d5c3..3414f184d018 100644
> > --- a/include/net/netmem.h
> > +++ b/include/net/netmem.h
> > @@ -42,11 +42,8 @@ struct netmem_desc {
> >         static_assert(offsetof(struct page, pg) == \
> >                       offsetof(struct netmem_desc, desc))
> >  NETMEM_DESC_ASSERT_OFFSET(flags, _flags);
> > -NETMEM_DESC_ASSERT_OFFSET(pp_magic, pp_magic);
> > -NETMEM_DESC_ASSERT_OFFSET(pp, pp);
> > -NETMEM_DESC_ASSERT_OFFSET(_pp_mapping_pad, _pp_mapping_pad);
> > -NETMEM_DESC_ASSERT_OFFSET(dma_addr, dma_addr);
> > -NETMEM_DESC_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
> > +NETMEM_DESC_ASSERT_OFFSET(lru, pp_magic);
> > +NETMEM_DESC_ASSERT_OFFSET(mapping, _pp_mapping_pad);
> >  #undef NETMEM_DESC_ASSERT_OFFSET
> >
> >  /*
> 
> 
> Can you remove the above patch/diff from the cover letter?

I added the diff for those who might get lost due to the lack of the
final patch.  However, sure, I will remove it.

	Byungchul

> --
> Thanks,
> Mina