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

Byungchul Park
2025-05-29T03:29:09Z

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