Re: [RFC v3 00/18] Split netmem from struct page

[Byungchul Park <byungchul@sk.com>]

On Thu, May 29, 2025 at 12:10:29PM +0900, Byungchul Park wrote:
> The MM subsystem is trying to reduce struct page to a single pointer.
> 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 netmem which is used for page pools.
> 
> Matthew Wilcox tried and stopped the same work, you can see in:
> 
>    https://lore.kernel.org/linux-mm/20230111042214.907030-1-willy@infradead.org/
> 
> Mina Almasry already has done a lot fo prerequisite works by luck, he
> said :).  I stacked my patches on the top of his work e.i. netmem.
> 
> 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 submitted once
> all the converting work of page to netmem are done:
> 
>    1. converting of libeth_fqe by Tony Nguyen.
>    2. converting of mlx5 by Tariq Toukan.

+cc Tony Nguyen
+cc Tariq Toukan

	Byungchul

>    3. converting of prueth_swdata (on me).
>    4. converting of freescale driver (on me).
> 
> For our discussion, I'm sharing what the final patch looks like the
> following.
> 
> 	Byungchul
> --8<--
> commit 86be39ea488df859cff6bc398a364f1dc486f2f9
> 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 56d07edd01f9..5a7864eb9d76 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -119,17 +119,6 @@ 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 9e4ed3530788..e88e299dd0f0 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -39,11 +39,8 @@ 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 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 (18):
>   netmem: introduce struct netmem_desc mirroring struct page
>   netmem: introduce netmem alloc APIs to wrap page alloc APIs
>   page_pool: use netmem alloc/put APIs in __page_pool_alloc_page_order()
>   page_pool: rename __page_pool_alloc_page_order() to
>     __page_pool_alloc_netmem_order()
>   page_pool: use netmem alloc/put APIs in __page_pool_alloc_pages_slow()
>   page_pool: rename page_pool_return_page() to page_pool_return_netmem()
>   page_pool: use netmem put API in page_pool_return_netmem()
>   page_pool: rename __page_pool_release_page_dma() to
>     __page_pool_release_netmem_dma()
>   page_pool: rename __page_pool_put_page() to __page_pool_put_netmem()
>   page_pool: rename __page_pool_alloc_pages_slow() to
>     __page_pool_alloc_netmems_slow()
>   mlx4: use netmem descriptor and APIs for page pool
>   netmem: use _Generic to cover const casting for page_to_netmem()
>   netmem: remove __netmem_get_pp()
>   page_pool: make page_pool_get_dma_addr() just wrap
>     page_pool_get_dma_addr_netmem()
>   netdevsim: use netmem descriptor and APIs for page pool
>   netmem: introduce a netmem API, virt_to_head_netmem()
>   mt76: use netmem descriptor and APIs for page pool
>   page_pool: access ->pp_magic through struct netmem_desc in
>     page_pool_page_is_pp()
> 
>  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                          | 145 +++++++++++++-----
>  include/net/page_pool/helpers.h               |   7 +-
>  mm/page_alloc.c                               |   1 +
>  net/core/netmem_priv.h                        |  14 ++
>  net/core/page_pool.c                          | 101 ++++++------
>  15 files changed, 239 insertions(+), 174 deletions(-)
> 
> 
> base-commit: d09a8a4ab57849d0401d7c0bc6583e367984d9f7
> -- 
> 2.17.1