netdev - Re: [RFC PATCH net-next v1 2/4] net: introduce abstraction for network memory

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Date: Mon, 18 Dec 2023 14:38:58 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: David Ahern <dsahern@...nel.org>, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, bpf@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
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Subject: Re: [RFC PATCH net-next v1 2/4] net: introduce abstraction for
 network memory

On Mon, Dec 18, 2023 at 2:06 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Sun, 17 Dec 2023 00:14:59 -0800 Mina Almasry wrote:
> > > > Sure thing I can do that. Is it better to do something like:
> > > >
> > > > struct netmem_ref;
> > > >
> > > > like in this patch:
> > > >
> > > > https://lore.kernel.org/linux-mm/20221108194139.57604-1-torvalds@linux-foundation.org/
> > > >
> > > > Asking because checkpatch tells me not to add typedefs to the kernel,
> > > > but checkpatch can be ignored if you think it's OK.
> > > >
> > > > Also with this approach I can't use container_of and I need to do a
> > > > cast, I assume that's fine.
> > > >
> > >
> > > Isn't that the whole point of this set - to introduce a new data type
> > > and avoid casts?
>
> I don't see how we can avoid casts if the type of the referenced object
> is encoded on the low bits of the pointer. If we had a separate member
> we could so something like:
>
> struct netmem_ref {
>         enum netmem_type type;
>         union {
>                 struct page *p;
>                 struct page_pool_iov *pi;
>         };
> };
>
> barring crazy things with endian-aware bitfields, we need at least one
> cast.
>
> > My understanding here the requirements from Jason are:
> >
> > 1. Never pass a non-page to an mm api.
> > 2. If a mangle a pointer to indicate it's not a page, then I must not
> > call it mm's struct page*, I must add a new type.
> >
> > I think both requirements are met regardless of whether
> > netmem_to_page() is implemented using union/container_of or straight
> > casts. folios implemented something similar being unioned with struct
> > page to avoid casts.
>
> Folios overlay a real struct page. It's completely different.
>
> > I honestly could go either way on this. The union
> > provides some self documenting code and avoids casts.
>
> Maybe you guys know some trick to mask out the bottom bit :S
>
> > The implementation without the union obfuscates the type and makes it much
> > more opaque.
>
> Some would say that that's the damn point of the wrapping..
>
> You don't want non-core code futzing with the inside of the struct.
>
> > I finished addressing the rest of the comments and I have this series
> > and the next devmem TCP series ready to go, so I fired v2 of this
> > patchset. If one feels strongly about this let me know and I will
> > re-spin.
>
> You didn't address my feedback :|
>
> struct netmem which contains struct page by value is almost as bad
> as passing around pretend struct page pointers.

Sorry about that. I misread your original request as 'here is
something else you can do if you want', not something that you feel is
critical. Honestly I missed the subtlety and the approaches seemed
roughly equivalent to me. I will respin after the 24hr cooldown.

-- 
Thanks,
Mina