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

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Message-ID: <CAHS8izMsKaP66A1peCHEMxaqf0SV-O6uRQ9Q6MDNpnMbJ+XLUA@mail.gmail.com>
Date: Fri, 13 Jun 2025 19:19:07 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Byungchul Park <byungchul@...com>
Cc: Jakub Kicinski <kuba@...nel.org>, willy@...radead.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, kernel_team@...ynix.com, 
	ilias.apalodimas@...aro.org, harry.yoo@...cle.com, hawk@...nel.org, 
	akpm@...ux-foundation.org, davem@...emloft.net, john.fastabend@...il.com, 
	andrew+netdev@...n.ch, asml.silence@...il.com, toke@...hat.com, 
	tariqt@...dia.com, edumazet@...gle.com, pabeni@...hat.com, saeedm@...dia.com, 
	leon@...nel.org, ast@...nel.org, daniel@...earbox.net, david@...hat.com, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz, 
	rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, horms@...nel.org, 
	linux-rdma@...r.kernel.org, bpf@...r.kernel.org, vishal.moola@...il.com
Subject: Re: [PATCH net-next 1/9] netmem: introduce struct netmem_desc
 mirroring struct page

On Thu, Jun 12, 2025 at 6:13 PM Byungchul Park <byungchul@...com> wrote:
>
> On Wed, Jun 11, 2025 at 06:55:42PM -0700, Jakub Kicinski wrote:
> > On Tue, 10 Jun 2025 10:30:01 +0900 Byungchul Park wrote:
> > > > What's the intended relation between the types?
> > >
> > > One thing I'm trying to achieve is to remove pp fields from struct page,
> > > and make network code use struct netmem_desc { pp fields; } instead of
> > > sturc page for that purpose.
> > >
> > > The reason why I union'ed it with the existing pp fields in struct
> > > net_iov *temporarily* for now is, to fade out the existing pp fields
> > > from struct net_iov so as to make the final form like:
> >
> > I see, I may have mixed up the complaints there. I thought the effort
> > was also about removing the need for the ref count. And Rx is
> > relatively light on use of ref counting.
> >
> > > > netmem_ref exists to clearly indicate that memory may not be readable.
> > > > Majority of memory we expect to allocate from page pool must be
> > > > kernel-readable. What's the plan for reading the "single pointer"
> > > > memory within the kernel?
> > > >
> > > > I think you're approaching this problem from the easiest and least
> > >
> > > No, I've never looked for the easiest way.  My bad if there are a better
> > > way to achieve it.  What would you recommend?
> >
> > Sorry, I don't mean that the approach you took is the easiest way out.
> > I meant that between Rx and Tx handling Rx is the easier part because
> > we already have the suitable abstraction. It's true that we use more
> > fields in page struct on Rx, but I thought Tx is also more urgent
> > as there are open reports for networking taking references on slab
> > pages.
> >
> > In any case, please make sure you maintain clear separation between
> > readable and unreadable memory in the code you produce.
>
> Do you mean the current patches do not?  If yes, please point out one
> as example, which would be helpful to extract action items.
>

I think one thing we could do to improve separation between readable
(pages/netmem_desc) and unreadable (net_iov) is to remove the struct
netmem_desc field inside the net_iov, and instead just duplicate the
pp/pp_ref_count/etc fields. The current code gives off the impression
that net_iov may be a container of netmem_desc which is not really
accurate.

But I don't think that's a major blocker. I think maybe the real issue
is that there are no reviews from any mm maintainers? So I'm not 100%
sure this is in line with their memdesc plans. I think probably
patches 2->8 are generic netmem-ifications that are good to merge
anyway, but I would say patch 1 and 9 need a reviewed by from someone
on the mm side. Just my 2 cents.

Btw, this series has been marked as changes requested on patchwork, so
it is in need of a respin one way or another:

https://patchwork.kernel.org/project/netdevbpf/list/?series=&submitter=byungchul&state=*&q=&archive=&delegate=

https://docs.kernel.org/process/maintainer-netdev.html#patch-status

-- 
Thanks,
Mina