linux-kernel - Re: [PATCH net-next v6 1/9] netmem: introduce struct netmem

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Message-ID: <CAHS8izMbtp0dN3+PZsivFD4Zg1DqaL5BJ4cw4jGjs=wCXAns3A@mail.gmail.com>
Date: Mon, 23 Jun 2025 12:09:09 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Byungchul Park <byungchul@...com>, David Hildenbrand <david@...hat.com>, 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, 
	hawk@...nel.org, akpm@...ux-foundation.org, davem@...emloft.net, 
	john.fastabend@...il.com, andrew+netdev@...n.ch, asml.silence@...il.com, 
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	hannes@...xchg.org, ziy@...dia.com, jackmanb@...gle.com
Subject: Re: [PATCH net-next v6 1/9] netmem: introduce struct netmem_desc
 mirroring struct page

On Mon, Jun 23, 2025 at 5:18 AM Harry Yoo <harry.yoo@...cle.com> wrote:
>
> On Mon, Jun 23, 2025 at 07:28:21PM +0900, Byungchul Park wrote:
> > On Mon, Jun 23, 2025 at 11:32:16AM +0200, David Hildenbrand wrote:
> > > On 20.06.25 06:12, Byungchul Park wrote:
> > > > To simplify struct page, the page pool members of struct page should be
> > > > moved to other, allowing these members to be removed from struct page.
> > > >
> > > > Introduce a network memory descriptor to store the members, struct
> > > > netmem_desc, and make it union'ed with the existing fields in struct
> > > > net_iov, allowing to organize the fields of struct net_iov.
> > >
> > > It would be great adding some result from the previous discussions in
> > > here, such as that the layout of "struct net_iov" can be changed because
> > > it is not a "struct page" overlay, what the next steps based on this
> >
> > I think the network folks already know how to use and interpret their
> > data struct, struct net_iov for sure.. but I will add the comment if it
> > you think is needed.  Thanks for the comment.
>
> I agree with David - it's not immediately obvious at first glance.
> That was my feedback on the previous version as well :)
>
> I think it'd be great to add that explanation, since this is where MM and
> networking intersect.
>

I think a lot of people are now saying the same thing: (1) lets keep
net_iov and page/netmem_desc separate, and (2) lets add comments
explaining their relation so this intersection between MM and
networking is not confused in the long term .

For #1, concretely I would recommend removing the union inside struct
net_iov? And also revert all the changes to net_iov for that matter.
They are all to bring netmem_desc and net_iov closer together, but the
feedback is that we should keep them separate, and I kinda agree with
that. The fact that net_iov includes a netmem_desc in your patch makes
readers think they're very closely related.

For #2, add this comment (roughly) on top of struct net_iov? Untested
with kdoc and spell checker:

diff --git a/include/net/netmem.h b/include/net/netmem.h
index 7a1dafa3f080..8fb2b294e5f2 100644
--- a/include/net/netmem.h
+++ b/include/net/netmem.h
@@ -30,6 +30,25 @@ enum net_iov_type {
        NET_IOV_MAX = ULONG_MAX
 };

+/* A memory descriptor representing abstract networking I/O vectors.
+ *
+ * net_iovs are allocated by networking code, and generally represent some
+ * abstract form of non-paged memory used by the networking stack. The size
+ * of the chunk is PAGE_SIZE.
+ *
+ * This memory can be any form of non-struct paged memory. Examples include
+ * imported dmabuf memory and imported io_uring memory. See
net_iov_type for all
+ * the supported types.
+ *
+ * @type: the type of the memory. Different types of net_iovs are supported.
+ * @pp_magic: pp field, similar to the one in struct page/struct netmem_desc.
+ * @pp: the pp this net_iov belongs to, if any.
+ * @owner: the net_iov_area this net_iov belongs to, if any.
+ * @dma_addr: the dma addrs of the net_iov. Needed for the network card to
+ * send/receive this net_iov.
+ * @pp_ref_count:  the pp ref count of this net_iov, exactly the same usage as
+ * struct page/struct netmem_desc.
+ */





--
Thanks,
Mina