netdev - Re: [PATCH v7 04/15] net: prepare for non devmem TCP memory providers

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Message-ID: <b966d211-26de-4796-9e54-5b464a565b7c@gmail.com>
Date: Mon, 4 Nov 2024 21:24:35 +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>,
 Stanislav Fomichev <stfomichev@...il.com>, Joe Damato <jdamato@...tly.com>,
 Pedro Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH v7 04/15] net: prepare for non devmem TCP memory providers

On 11/4/24 20:20, Mina Almasry wrote:
> On Fri, Nov 1, 2024 at 10:41 AM Pavel Begunkov <asml.silence@...il.com> wrote:
...
>> io_uring originated netmem that are marked unreadable as well
>> and so will end up in tcp_recvmsg_dmabuf(), then we reject and
>> fail since they should not be fed to devmem TCP. It should be
>> fine from correctness perspective.
>>
>> We need to check frags, and that's the place where we iterate
>> frags. Another option is to add a loop in tcp_recvmsg_locked
>> walking over all frags of an skb and doing the checks, but
>> that's an unnecessary performance burden to devmem.
>>
> 
> Checking each frag in tcp_recvmsg_dmabuf (and the equivalent io_uring
> function) is not ideal really. Especially when you're dereferencing
> nio->pp to do the check which IIUC will pull a cache line not normally
> needed in this code path and may have a performance impact.

Even if it's cold, all net_iov processed are expected to come
from the same page pool and would be cached. And it should be of
a comparable hotness as dmabuf_genpool_chunk_owner accesses below,
considering that there is enough of processing happening around,
it should be of a worry and can be improve upon later.

-- 
Pavel Begunkov