linux-kernel - Re: [RFC 02/19] netmem: introduce netmem alloc/put API to wrap page alloc/put API

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Message-ID: <CAHS8izOVynwxo4ZVG8pxqocThRYYL4EqRHpEtPPFQpLViTUKLA@mail.gmail.com>
Date: Fri, 9 May 2025 06:39:07 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Byungchul Park <byungchul@...com>
Cc: willy@...radead.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, kernel_team@...ynix.com, kuba@...nel.org, 
	ilias.apalodimas@...aro.org, harry.yoo@...cle.com, hawk@...nel.org, 
	akpm@...ux-foundation.org, ast@...nel.org, daniel@...earbox.net, 
	davem@...emloft.net, john.fastabend@...il.com, andrew+netdev@...n.ch, 
	edumazet@...gle.com, pabeni@...hat.com, vishal.moola@...il.com
Subject: Re: [RFC 02/19] netmem: introduce netmem alloc/put API to wrap page
 alloc/put API

On Fri, May 9, 2025 at 4:51 AM Byungchul Park <byungchul@...com> wrote:
>
> To eliminate the use of struct page in page pool, the page pool code
> should use netmem descriptor and API instead.
>
> As part of the work, introduce netmem alloc/put API allowing the code to
> use them rather than struct page things.
>
> Signed-off-by: Byungchul Park <byungchul@...com>
> ---
>  include/net/netmem.h | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/include/net/netmem.h b/include/net/netmem.h
> index 45c209d6cc689..c87a604e980b9 100644
> --- a/include/net/netmem.h
> +++ b/include/net/netmem.h
> @@ -138,6 +138,24 @@ static inline netmem_ref page_to_netmem(struct page *page)
>         return (__force netmem_ref)page;
>  }
>
> +static inline netmem_ref alloc_netmems_node(int nid, gfp_t gfp_mask,
> +               unsigned int order)
> +{
> +       return page_to_netmem(alloc_pages_node(nid, gfp_mask, order));
> +}
> +
> +static inline unsigned long alloc_netmems_bulk_node(gfp_t gfp, int nid,
> +               unsigned long nr_netmems, netmem_ref *netmem_array)
> +{
> +       return alloc_pages_bulk_node(gfp, nid, nr_netmems,
> +                       (struct page **)netmem_array);
> +}
> +
> +static inline void put_netmem(netmem_ref netmem)
> +{
> +       put_page(netmem_to_page(netmem));
> +}

We can't really do this. netmem_ref is an abstraction that can be a
struct page or struct net_iov underneath. We can't be sure when
put_netmem is called that it is safe to convert to a page via
netmem_to_page(). This will crash if put_netmem is called on a
netmem_ref that is a net_iov underneath.

Please read the patch series that introduced netmem_ref to familiarize
yourself with the background here:

https://lkml.iu.edu/hypermail/linux/kernel/2401.2/09140.html

-- 
Thanks,
Mina