On Mon, Jul 14, 2025 at 12:37 PM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
>
> On Mon, Jul 14, 2025 at 5:01 AM Byungchul Park <byungchul@xxxxxx> wrote:
> >
> > To eliminate the use of the page pool fields in struct page, the page
> > pool code should use netmem descriptor and APIs instead.
> >
> > However, __netmem_get_pp() still accesses ->pp via struct page. So
> > change it to use struct netmem_desc instead, since ->pp no longer will
> > be available in struct page.
> >
> > While at it, add a helper, pp_page_to_nmdesc(), that can be used to
> > extract netmem_desc from page only if it's pp page. For now that
> > netmem_desc overlays on page, it can be achieved by just casting.
> >
> > Signed-off-by: Byungchul Park <byungchul@xxxxxx>
> > ---
> > include/net/netmem.h | 13 ++++++++++++-
> > 1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/net/netmem.h b/include/net/netmem.h
> > index 535cf17b9134..2b8a7b51ac99 100644
> > --- a/include/net/netmem.h
> > +++ b/include/net/netmem.h
> > @@ -267,6 +267,17 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> > return (struct net_iov *)((__force unsigned long)netmem & ~NET_IOV);
> > }
> >
> > +static inline struct netmem_desc *pp_page_to_nmdesc(struct page *page)
> > +{
> > + DEBUG_NET_WARN_ON_ONCE(!page_pool_page_is_pp(page));
> > +
> > + /* XXX: How to extract netmem_desc from page must be changed,
> > + * once netmem_desc no longer overlays on page and will be
> > + * allocated through slab.
> > + */
> > + return (struct netmem_desc *)page;
> > +}
> > +
>
> Same thing. Do not create a generic looking pp_page_to_nmdesc helper
> which does not check that the page is the correct type. The
> DEBUG_NET... is not good enough.
>
> You don't need to add a generic helper here. There is only one call
> site. Open code this in the callsite. The one callsite is marked as
> unsafe, only called by code that knows that the netmem is specifically
> a pp page. Open code this in the unsafe callsite, instead of creating
> a generic looking unsafe helper and not even documenting it's unsafe.
>
On second read through the series, I actually now think this is a
great idea :-) Adding this helper has simplified the series greatly. I
did not realize you were converting entire drivers to netmem just to
get rid of page->pp accesses. Adding a pp_page_to_nmdesc helper makes
the entire series simpler.
You're also calling it only from code paths like drivers that already
assumed that the page is a pp page and did page->pp deference without
a check, so this should be safe.
Only thing I would change is add a comment explaining that the calling
code needs to check the page is pp page or know it's a pp page (like a
driver that supports pp).
> > /**
> > * __netmem_get_pp - unsafely get pointer to the &page_pool backing @netmem
> > * @netmem: netmem reference to get the pointer from
> > @@ -280,7 +291,7 @@ static inline struct net_iov *__netmem_clear_lsb(netmem_ref netmem)
> > */
> > static inline struct page_pool *__netmem_get_pp(netmem_ref netmem)
> > {
> > - return __netmem_to_page(netmem)->pp;
> > + return pp_page_to_nmdesc(__netmem_to_page(netmem))->pp;
> > }
>
> This makes me very sad. Casting from netmem -> page -> nmdesc...
>
> Instead, we should be able to go from netmem directly to nmdesc. I
> would suggest rename __netmem_clear_lsb to netmem_to_nmdesc and have
> it return netmem_desc instead of net_iov. Then use it here.
>
> We could have an unsafe version of netmem_to_nmdesc which converts the
> netmem to netmem_desc without clearing the lsb and mark it unsafe.
>
This, I think, we should address to keep some sanity in the code and
reduce the casts and make it a bit more maintainable.
--
Thanks,
Mina
