netdev - Re: [PATCH v1 13/15] io_uring/zcrx: add copy fallback

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Message-ID: <e7388f1a-82c9-48f9-8490-393b665d6913@kernel.dk>
Date: Wed, 9 Oct 2024 12:38:48 -0600
From: Jens Axboe <axboe@...nel.dk>
To: David Wei <dw@...idwei.uk>, io-uring@...r.kernel.org,
 netdev@...r.kernel.org
Cc: Pavel Begunkov <asml.silence@...il.com>, 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>,
 Mina Almasry <almasrymina@...gle.com>
Subject: Re: [PATCH v1 13/15] io_uring/zcrx: add copy fallback

On 10/7/24 4:16 PM, David Wei wrote:
> From: Pavel Begunkov <asml.silence@...il.com>
> 
> There are scenarios in which the zerocopy path might get a normal
> in-kernel buffer, it could be a mis-steered packet or simply the linear
> part of an skb. Another use case is to allow the driver to allocate
> kernel pages when it's out of zc buffers, which makes it more resilient
> to spikes in load and allow the user to choose the balance between the
> amount of memory provided and performance.
> 
> At the moment we fail such requests. Instead, grab a buffer from the
> page pool, copy data there, and return back to user in the usual way.
> Because the refill ring is private to the napi our page pool is running
> from, it's done by stopping the napi via napi_execute() helper. It grabs
> only one buffer, which is inefficient, and improving it is left for
> follow up patches.

This also looks fine to me. Agree with the sentiment that it'd be nice
to propagate back if copies are happening, and to what extent. But also
agree that this can wait.

-- 
Jens Axboe