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

[Byungchul Park <byungchul@sk.com>]

On Thu, Jul 10, 2025 at 05:27:59PM +0900, Byungchul Park 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.

     3. converting use of the pp fields in strcut page in libeth.

	Byungchul

> For our discussion, I'm sharing what the final patch looks like, in this
> cover letter.
> 
> 	Byungchul
> --8<--
> commit 1847d9890f798456b21ccb27aac7545303048492
> Author: Byungchul Park <byungchul@sk.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@sk.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
>  
>  /*
> ---
> Changes from v8:
> 	1. Rebase on net-next/main as of Jul 10.
> 	2. Exclude non-controversial patches that have already been
> 	   merged to net-next.
> 	3. Re-add the patches that focus on removing accessing the page
> 	   pool fields in struct page.
> 	4. Add utility APIs e.g. casting, to use struct netmem_desc as
> 	   descriptor, to support __netmem_get_pp() that has started to
> 	   be used again e.g. by libeth.
> 
> Changes from v7 (no actual updates):
> 	1. Exclude "netmem: introduce struct netmem_desc mirroring
> 	   struct page" that might be controversial.
> 	2. Exclude "netmem: introduce a netmem API,
> 	   virt_to_head_netmem()" since there are no users.
> 
> Changes from v6 (no actual updates):
> 	1. Rebase on net-next/main as of Jun 25.
> 	2. Supplement a comment describing struct net_iov.
> 	3. Exclude a controversial patch, "page_pool: access ->pp_magic
> 	   through struct netmem_desc in page_pool_page_is_pp()".
> 	4. Exclude "netmem: remove __netmem_get_pp()" since the API
> 	   started to be used again by libeth.
> 
> Changes from v5 (no actual updates):
> 	1. Rebase on net-next/main as of Jun 20.
> 	2. Add given 'Reviewed-by's and 'Acked-by's, thanks to all.
> 	3. Add missing cc's.
> 
> Changes from v4:
> 	1. Add given 'Reviewed-by's, thanks to all.
> 	2. Exclude potentially controversial patches.
> 
> Changes from v3:
> 	1. Relocates ->owner and ->type of net_iov out of netmem_desc
> 	   and make them be net_iov specific.
> 	2. Remove __force when casting struct page to struct netmem_desc.
> 
> Changes from v2:
> 	1. Introduce a netmem API, virt_to_head_netmem(), and use it
> 	   when it's needed.
> 	2. Introduce struct netmem_desc as a new struct and union'ed
> 	   with the existing fields in struct net_iov.
> 	3. Make page_pool_page_is_pp() access ->pp_magic through struct
> 	   netmem_desc instead of struct page.
> 	4. Move netmem alloc APIs from include/net/netmem.h to
> 	   net/core/netmem_priv.h.
> 	5. Apply trivial feedbacks, thanks to Mina, Pavel, and Toke.
> 	6. Add given 'Reviewed-by's, thanks to Mina.
> 
> Changes from v1:
> 	1. Rebase on net-next's main as of May 26.
> 	2. Check checkpatch.pl, feedbacked by SJ Park.
> 	3. Add converting of page to netmem in mt76.
> 	4. Revert 'mlx5: use netmem descriptor and APIs for page pool'
> 	   since it's on-going by Tariq Toukan.  I will wait for his
> 	   work to be done.
> 	5. Revert 'page_pool: use netmem APIs to access page->pp_magic
> 	   in page_pool_page_is_pp()' since we need more discussion.
> 	6. Revert 'mm, netmem: remove the page pool members in struct
> 	   page' since there are some prerequisite works to remove the
> 	   page pool fields from struct page.  I can submit this patch
> 	   separatedly later.
> 	7. Cancel relocating a page pool member in struct page.
> 	8. Modify static assert for offests and size of struct
> 	   netmem_desc.
> 
> Changes from rfc:
> 	1. Rebase on net-next's main branch.
> 	   https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/
> 	2. Fix a build error reported by kernel test robot.
> 	   https://lore.kernel.org/all/202505100932.uzAMBW1y-lkp@intel.com/
> 	3. Add given 'Reviewed-by's, thanks to Mina and Ilias.
> 	4. Do static_assert() on the size of struct netmem_desc instead
> 	   of placing place-holder in struct page, feedbacked by
> 	   Matthew.
> 	5. Do struct_group_tagged(netmem_desc) on struct net_iov instead
> 	   of wholly renaming it to strcut netmem_desc, feedbacked by
> 	   Mina and Pavel.
> 
> Byungchul Park (8):
>   netmem: introduce struct netmem_desc mirroring struct page
>   netmem: introduce utility APIs to use struct netmem_desc
>   page_pool: access ->pp_magic through struct netmem_desc in
>     page_pool_page_is_pp()
>   netmem: use netmem_desc instead of page to access ->pp in
>     __netmem_get_pp()
>   netmem: introduce a netmem API, virt_to_head_netmem()
>   mlx4: use netmem descriptor and APIs for page pool
>   netdevsim: use netmem descriptor and APIs for page pool
>   mt76: use netmem descriptor and APIs for page pool
> 
>  drivers/net/ethernet/mellanox/mlx4/en_rx.c    |  48 ++---
>  drivers/net/ethernet/mellanox/mlx4/en_tx.c    |   8 +-
>  drivers/net/ethernet/mellanox/mlx4/mlx4_en.h  |   4 +-
>  drivers/net/netdevsim/netdev.c                |  19 +-
>  drivers/net/netdevsim/netdevsim.h             |   2 +-
>  drivers/net/wireless/mediatek/mt76/dma.c      |   6 +-
>  drivers/net/wireless/mediatek/mt76/mt76.h     |  12 +-
>  .../net/wireless/mediatek/mt76/sdio_txrx.c    |  24 +--
>  drivers/net/wireless/mediatek/mt76/usb.c      |  10 +-
>  include/linux/mm.h                            |  12 --
>  include/net/netmem.h                          | 183 +++++++++++++++---
>  mm/page_alloc.c                               |   1 +
>  12 files changed, 231 insertions(+), 98 deletions(-)
> 
> 
> base-commit: c65d34296b2252897e37835d6007bbd01b255742
> -- 
> 2.17.1