Re: [PATCH 18/18] mm, netmem: remove the page pool members in struct page

Byungchul Park
2025-05-26T01:37:50Z

On Fri, May 23, 2025 at 10:55:54AM -0700, Mina Almasry wrote:
> On Thu, May 22, 2025 at 8:26 PM Byungchul Park <byungchul@sk.com> wrote:
> >
> > 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 might still use the space in struct
> > page, the size of struct netmem_desc should be checked, until struct
> > netmem_desc has its own instance from slab, to avoid conficting with
> > other members within struct page.
> >
> > Remove the page pool members in struct page and add a static checker for
> > the size.
> >
> > Signed-off-by: Byungchul Park <byungchul@sk.com>
> > ---
> > include/linux/mm_types.h | 11 -----------
> > include/net/netmem.h | 28 +++++-----------------------
> > 2 files changed, 5 insertions(+), 34 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 873e820e1521..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 */
> > - 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 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 c63a7e20f5f3..257c22398d7a 100644
> > --- a/include/net/netmem.h
> > +++ b/include/net/netmem.h
> > @@ -77,30 +77,12 @@ struct net_iov_area {
> > unsigned long base_virtual;
> > };
> >
> > -/* These fields in struct page are used by the page_pool and net stack:
> > - *
> > - * struct {
> > - * unsigned long _pp_mapping_pad;
> > - * unsigned long pp_magic;
> > - * struct page_pool *pp;
> > - * unsigned long dma_addr;
> > - * atomic_long_t pp_ref_count;
> > - * };
> > - *
> > - * We mirror the page_pool fields here so the page_pool can access these fields
> > - * without worrying whether the underlying fields belong to a page or net_iov.
> > - *
> > - * The non-net stack fields of struct page are private to the mm stack and must
> > - * never be mirrored to net_iov.
> > +/* XXX: The page pool fields in struct page have been removed but they
> > + * might still use the space in struct page. Thus, the size of struct
> > + * netmem_desc should be under control until struct netmem_desc has its
> > + * own instance from slab.
> > */
> > -#define NET_IOV_ASSERT_OFFSET(pg, iov) \
> > - static_assert(offsetof(struct page, pg) == \
> > - offsetof(struct net_iov, iov))
> > -NET_IOV_ASSERT_OFFSET(pp_magic, pp_magic);
> > -NET_IOV_ASSERT_OFFSET(pp, pp);
> > -NET_IOV_ASSERT_OFFSET(dma_addr, dma_addr);
> > -NET_IOV_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
> > -#undef NET_IOV_ASSERT_OFFSET
> > +static_assert(sizeof(struct netmem_desc) <= offsetof(struct page, _refcount));
> >
>
> Removing these asserts is actually a bit dangerous. Functions like
> netmem_or_pp_magic() rely on the fact that the offsets are the same
> between struct page and struct net_iov to access these fields without

Worth noting this patch removes the page pool fields from struct page.

However, yes, I will keep necessary assertions with some changes applied
so that it can work even after removing the page pool fields like:

NET_IOV_ASSERT_OFFSET(lru, pp_magic);
NET_IOV_ASSERT_OFFSET(mapping, _pp_mapping_pad);

> worrying about the type of the netmem. What we do in these helpers is
> we we clear the least significant bit of the netmem, and then access
> the field. This works only because we verified at build time that the
> offset is the same.
>
> I think we have 3 options here:
>
> 1. Keep the asserts as-is, then in the follow up patch where we remove
> netmem_desc from struct page, we update the asserts to make sure
> struct page and struct net_iov can grab the netmem_desc in a uniform

Ah. It's worth noting that I'm removing the page pool fields all the
way from strcut page, instead of placing a place-holder that I did in
RFC as Matthew requested.

> way.
>
> 2. We remove the asserts, but all the helpers that rely on
> __netmem_clear_lsb need to be modified to do custom handling of
> net_iov vs page. Something like:
>
> static inline void netmem_or_pp_magic(netmem_ref netmem, unsigned long pp_magic)
> {
> if (netmem_is_net_iov(netmem)
> netmem_to_net_iov(netmem)->pp_magic |= pp_magic;
> else
> netmem_to_page(netmem)->pp_magic |= pp_magic;

struct page should not have pp_magic field once the page pool fields are
gone.

Byungchul
> }
>
> Option #2 requires extra checks, which may affect the performance
> reported by page_pool_bench_simple that I pointed you to before.
>
> 3. We could swap out all the individual asserts for one assert, if
> both page and net_iov have a netmem_desc subfield. This will also need
> to be reworked when netmem_desc is eventually moved out of struct page
> and is slab allocated:
>
> NET_IOV_ASSERT_OFFSET(netmem_desc, netmem_desc);
>
> --
> Thanks,
> Mina