netdev - Re: [PATCH v1 03/15] net: generalise net

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Message-ID: <6ccc5207-51b1-423b-807f-4ad9ae3a0b92@gmail.com>
Date: Wed, 9 Oct 2024 23:13:42 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Mina Almasry <almasrymina@...gle.com>, David Wei <dw@...idwei.uk>
Cc: io-uring@...r.kernel.org, netdev@...r.kernel.org,
 Jens Axboe <axboe@...nel.dk>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jesper Dangaard Brouer
 <hawk@...nel.org>, David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH v1 03/15] net: generalise net_iov chunk owners

On 10/9/24 21:44, Mina Almasry wrote:
> On Mon, Oct 7, 2024 at 3:16 PM David Wei <dw@...idwei.uk> wrote:
>>
>> From: Pavel Begunkov <asml.silence@...il.com>
>>
>> Currently net_iov stores a pointer to struct dmabuf_genpool_chunk_owner,
>> which serves as a useful abstraction to share data and provide a
>> context. However, it's too devmem specific, and we want to reuse it for
>> other memory providers, and for that we need to decouple net_iov from
>> devmem. Make net_iov to point to a new base structure called
>> net_iov_area, which dmabuf_genpool_chunk_owner extends.
> 
> 
> Similar feeling to Stan initially. I also thought you'd reuse
> dmabuf_genpool_chunk_owner. Seems like you're doing that but also
> renaming it to net_iov_area almost, which seems fine.
> 
> I guess, with this patch, there is no way to tell, given just a
> net_iov whether it's dmabuf or something else, right? I wonder if

By intention there is no good/clear way to tell if it's a dmabuf
or page backed net_iov in the generic path, but you can easily
check if it's devmem or io_uring by comparing page pool's ops.
net_iov::pp should always be available when it's in the net stack.
5/15 does exactly that in the devmem tcp portion of tcp.c.

> that's an issue. In my mind when an skb is in tcp_recvmsg() we need to
> make sure it's a dmabuf net_iov specifically to call
> tcp_recvmsg_dmabuf for example. I'll look deeper here.

Mentioned above, patch 5/15 handles that.

>>   static inline struct dmabuf_genpool_chunk_owner *
>> -net_iov_owner(const struct net_iov *niov)
>> +net_devmem_iov_to_chunk_owner(const struct net_iov *niov)
>>   {
>> -       return niov->owner;
>> -}
>> +       struct net_iov_area *owner = net_iov_owner(niov);
>>
>> -static inline unsigned int net_iov_idx(const struct net_iov *niov)
>> -{
>> -       return niov - net_iov_owner(niov)->niovs;
>> +       return container_of(owner, struct dmabuf_genpool_chunk_owner, area);
> 
> Couldn't this end up returning garbage if the net_iov is not actually
> a dmabuf one? Is that handled somewhere in a later patch that I
> missed?

Surely it will if someone manages to use it with non-devmem net_iovs,
which is why I renamed it to "devmem*".

-- 
Pavel Begunkov