netdev - Re: [PATCH v1 06/15] net: page_pool: add ->scrub mem provider callback

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Message-ID: <4d88742a-9e83-464f-9058-85f6777ae13b@gmail.com>
Date: Mon, 4 Nov 2024 21:27:32 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: David Wei <dw@...idwei.uk>, 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 06/15] net: page_pool: add ->scrub mem provider
 callback

On 11/4/24 20:42, Mina Almasry wrote:
> On Fri, Nov 1, 2024 at 2:38 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>>
>> On 11/1/24 19:24, Mina Almasry wrote:
>>> On Fri, Nov 1, 2024 at 11:34 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>> ...
>>>>> Huh, interesting. For devmem TCP we bind a region of memory to the
>>>>> queue once, and after that we can create N connections all reusing the
>>>>> same memory region. Is that not the case for io_uring? There are no
>>>>
>>>> Hmm, I think we already discussed the same question before. Yes, it
>>>> does indeed support arbitrary number of connections. For what I was
>>>> saying above, the devmem TCP analogy would be attaching buffers to the
>>>> netlink socket instead of a tcp socket (that new xarray you added) when
>>>> you give it to user space. Then, you can close the connection after a
>>>> receive and the buffer you've got would still be alive.
>>>>
>>>
>>> Ah, I see. You're making a tradeoff here. You leave the buffers alive
>>> after each connection so the userspace can still use them if it wishes
>>> but they are of course unavailable for other connections.
>>>
>>> But in our case (and I'm guessing yours) the process that will set up
>>> the io_uring memory provider/RSS/flow steering will be a different
>>> process from the one that sends/receive data, no? Because the former
>>> requires CAP_NET_ADMIN privileges while the latter will not. If they
>>> are 2 different processes, what happens when the latter process doing
>>> the send/receive crashes? Does the memory stay unavailable until the
>>> CAP_NET_ADMIN process exits? Wouldn't it be better to tie the lifetime
>>> of the buffers of the connection? Sure, the buffers will become
>>
>> That's the tradeoff google is willing to do in the framework,
>> which is fine, but it's not without cost, e.g. you need to
>> store/erase into the xarray, and it's a design choice in other
>> aspects, like you can't release the page pool if the socket you
>> got a buffer from is still alive but the net_iov hasn't been
>> returned.
>>
>>> unavailable after the connection is closed, but at least you don't
>>> 'leak' memory on send/receive process crashes.
>>>
>>> Unless of course you're saying that only CAP_NET_ADMIN processes will
>>
>> The user can pass io_uring instance itself
>>
> 
> Thanks, but sorry, my point still stands. If the CAP_NET_ADMIN passes
> the io_uring instance to the process doing the send/receive, then the
> latter process crashes, do the io_uring netmems leak until the
> page_pool is destroyed? Or are they cleaned up because the io_uring
> instance is destroyed with the process crashing, and the io_uring will
> destroy the page_pool on exit?

It'll be killed when io_uring dies / closed.

-- 
Pavel Begunkov