netdev - Re: [PATCH net-next v8 11/17] io_uring/zcrx: implement zerocopy receive pp memory provider

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Message-ID: <aa20a0fd-75fb-4859-bd0e-74d0098daae8@gmail.com>
Date: Tue, 10 Dec 2024 04:45:23 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Jakub Kicinski <kuba@...nel.org>, David Wei <dw@...idwei.uk>
Cc: io-uring@...r.kernel.org, netdev@...r.kernel.org,
 Jens Axboe <axboe@...nel.dk>, 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>,
 Mina Almasry <almasrymina@...gle.com>,
 Stanislav Fomichev <stfomichev@...il.com>, Joe Damato <jdamato@...tly.com>,
 Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH net-next v8 11/17] io_uring/zcrx: implement zerocopy
 receive pp memory provider

On 12/10/24 04:01, Jakub Kicinski wrote:
> On Wed,  4 Dec 2024 09:21:50 -0800 David Wei wrote:
>> Then, either the buffer is dropped and returns back to the page pool
>> into the ->freelist via io_pp_zc_release_netmem, in which case the page
>> pool will match hold_cnt for us with ->pages_state_release_cnt. Or more
>> likely the buffer will go through the network/protocol stacks and end up
>> in the corresponding socket's receive queue. From there the user can get
>> it via an new io_uring request implemented in following patches. As
>> mentioned above, before giving a buffer to the user we bump the refcount
>> by IO_ZC_RX_UREF.
>>
>> Once the user is done with the buffer processing, it must return it back
>> via the refill queue, from where our ->alloc_netmems implementation can
>> grab it, check references, put IO_ZC_RX_UREF, and recycle the buffer if
>> there are no more users left. As we place such buffers right back into
>> the page pools fast cache and they didn't go through the normal pp
>> release path, they are still considered "allocated" and no pp hold_cnt
>> is required. For the same reason we dma sync buffers for the device
>> in io_zc_add_pp_cache().
> 
> Can you say more about the IO_ZC_RX_UREF bias? net_iov is not the page
> struct, we can add more fields. In fact we have 8B of padding in it
> that can be allocated without growing the struct. So why play with

I guess we can, though it's growing it for everyone not just
io_uring considering how indexing works, i.e. no embedding into
a larger struct.

> biases? You can add a 32b atomic counter for how many refs have been
> handed out to the user.

This set does it in a stupid way, but the bias allows to coalesce
operations with it into a single atomic. Regardless, it can be
placed separately, though we still need a good way to optimise
counting. Take a look at my reply with questions in the v7 thread,
I outlined what can work quite well in terms of performance but
needs a clear api for that from net/

-- 
Pavel Begunkov