netdev - netmem series needs some love and Acks from MM folks

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Message-ID: <aFDTikg1W3Bz_s5E@hyeyoo>
Date: Tue, 17 Jun 2025 11:31:38 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Byungchul Park <byungchul@...com>, 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, 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: netmem series needs some love and Acks from MM folks

On Fri, Jun 13, 2025 at 07:19:07PM -0700, Mina Almasry wrote:
> 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?

Let's try changing the subject to draw some attention from MM people :)

> 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.

As someone who worked on the zpdesc series, I think it is pretty much
in line with the memdesc plans.

I mean, it does differ a bit from the initial idea of generalizing it as
"bump" allocator, but overall, it's still aligned with the memdesc
plans, and looks like a starting point, IMHO.

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

-- 
Cheers,
Harry / Hyeonggon