linux-kernel - Re: [PATCH net-next v3 2/3] net: introduce abstraction for network memory

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Date: Thu, 4 Jan 2024 13:44:24 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Shakeel Butt <shakeelb@...gle.com>, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, kvm@...r.kernel.org,
 virtualization@...ts.linux.dev, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Stefan
 Hajnoczi <stefanha@...hat.com>, Stefano Garzarella <sgarzare@...hat.com>,
 David Howells <dhowells@...hat.com>, Jason Gunthorpe <jgg@...dia.com>,
 Christian König <christian.koenig@....com>, Yunsheng Lin
 <linyunsheng@...wei.com>, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH net-next v3 2/3] net: introduce abstraction for network
 memory

On Thu, 21 Dec 2023 15:44:22 -0800 Mina Almasry wrote:
> The warning is like so:
> 
> ./include/net/page_pool/helpers.h: In function ‘page_pool_alloc’:
> ./include/linux/stddef.h:8:14: warning: returning ‘void *’ from a
> function with return type ‘netmem_ref’ {aka ‘long unsigned int’} makes
> integer from pointer without a cast [-Wint-conversion]
>     8 | #define NULL ((void *)0)
>       |              ^
> ./include/net/page_pool/helpers.h:132:24: note: in expansion of macro
> ‘NULL’
>   132 |                 return NULL;
>       |                        ^~~~
> 
> And happens in all the code where:
> 
> netmem_ref func()
> {
>     return NULL;
> }
> 
> It's fixable by changing the return to `return (netmem_ref NULL);` or
> `return 0;`, but I feel like netmem_ref should be some type which
> allows a cast from NULL implicitly.

Why do you think we should be able to cast NULL implicitly?
netmem_ref is a handle, it could possibly be some form of 
an ID in the future, rather than a pointer. Or have more low
bits stolen for specific use cases.

unsigned long, and returning 0 as "no handle" makes perfect sense to me.

Note that 0 is a special case, bitwise types are allowed to convert
to 0/bool and 0 is implicitly allowed to become a bitwise type.
This will pass without a warning:

typedef unsigned long __bitwise netmem_ref;

netmem_ref some_code(netmem_ref ref)
{
	// direct test is fine
	if (!ref)
		// 0 "upgrades" without casts
		return 0;
	// 1 does not, we need __force
	return (__force netmem_ref)1 | ref;
}

The __bitwise annotation will make catching people trying
to cast to struct page * trivial.

You seem to be trying hard to make struct netmem a thing.
Perhaps you have a reason I'm not getting?