Subject: Re: [PATCH net-next v16 04/13] netdev: netdevice devmem allocator

From: Jakub Kicinski <[email-masked]>
Date: Wed, 10 Jul 2024 12:55:33 -0700

On Wed, 10 Jul 2024 12:29:58 -0700 Mina Almasry wrote:
> On Wed, Jul 10, 2024 at 9:37 AM Jakub Kicinski <[email-masked]> wrote:
> > On Wed, 10 Jul 2024 00:17:37 +0000 Mina Almasry wrote:  
> > > +     net_devmem_dmabuf_binding_get(binding);  
> >
> > Why does every iov need to hold a ref? pp holds a ref and does its own
> > accounting, so it won't disappear unless all the pages are returned.  
> 
> I guess it doesn't really need to, but this is the design/approach I
> went with, and I actually prefer it a bit. The design is borrowed from
> how struct dev_pagemap does this, IIRC. Every page allocated from the
> pgmap holds a reference to the pgmap to ensure the pgmap doesn't go
> away while some page that originated from it is out in the wild, and
> similarly I did so in the binding here.

Oh, you napi_pp_put_page() on the other end! I can see how that could
be fine.

> We could assume that the page_pool is accounting iovs for us, but that
> is not always true, right? page_pool_return_page() disconnects a
> netmem from the page_pool and AFAIU the page_pool can go away while
> there is such a netmem still in use in the net stack. Currently this
> can't happen with iovs because I currently don't support non-pp
> refcounting for iovs (so they're always recyclable), but you have a
> comment on the other patch asking why that works; depending on how we
> converge on that conversation, the details of how the pp refcounting
> could change.

Even then - we could take the ref as the page "leaks" out of the pool,
rather than doing it on the fast path, right? Or just BUG_ON() 'cause
that reference ain't coming back ;)

> It's nice to know that the binding refcounting will work regardless of
> the details of how the pp refcounting works. IMHO having the binding
> rely on the pp refcounting to ensure all the iovs are freed introduces
> some fragility.
> 
> Additionally IMO the net_devmem_dmabuf_binding_get/put aren't so
> expensive to want to optimize out, right? The allocation is a slow
> path anyway and the fast path recycles netmem.

Yes, I should have read patch 10. I think it's avoidable :) but with
recycling it can indeed perform just fine (do you happen to have
recycling rate stats from prod runs?)