netdev - Re: [PATCH net-next 2/3] bpf: test_run: Use system page pool for XDP live frame mode

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Message-ID: <59c022bf-4cc4-850f-f8ab-3b8aab36f958@iogearbox.net>
Date: Tue, 20 Feb 2024 10:06:02 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Toke Høiland-Jørgensen <toke@...hat.com>,
 Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 Jakub Kicinski <kuba@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 2/3] bpf: test_run: Use system page pool for XDP
 live frame mode

On 2/15/24 2:26 PM, Toke Høiland-Jørgensen wrote:
> The BPF_TEST_RUN code in XDP live frame mode creates a new page pool
> each time it is called and uses that to allocate the frames used for the
> XDP run. This works well if the syscall is used with a high repetitions
> number, as it allows for efficient page recycling. However, if used with
> a small number of repetitions, the overhead of creating and tearing down
> the page pool is significant, and can even lead to system stalls if the
> syscall is called in a tight loop.
> 
> Now that we have a persistent system page pool instance, it becomes
> pretty straight forward to change the test_run code to use it. The only
> wrinkle is that we can no longer rely on a custom page init callback
> from page_pool itself; instead, we change the test_run code to write a
> random cookie value to the beginning of the page as an indicator that
> the page has been initialised and can be re-used without copying the
> initial data again.
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>

[...]
> -
>   	/* We create a 'fake' RXQ referencing the original dev, but with an
>   	 * xdp_mem_info pointing to our page_pool
>   	 */
>   	xdp_rxq_info_reg(&xdp->rxq, orig_ctx->rxq->dev, 0, 0);
> -	xdp->rxq.mem.type = MEM_TYPE_PAGE_POOL;
> -	xdp->rxq.mem.id = pp->xdp_mem_id;
> +	xdp->rxq.mem.type = MEM_TYPE_PAGE_POOL; /* mem id is set per-frame below */
>   	xdp->dev = orig_ctx->rxq->dev;
>   	xdp->orig_ctx = orig_ctx;
>   
> +	/* We need a random cookie for each run as pages can stick around
> +	 * between runs in the system page pool
> +	 */
> +	get_random_bytes(&xdp->cookie, sizeof(xdp->cookie));
> +

So the assumption is that there is only a tiny chance of collisions with
users outside of xdp test_run. If they do collide however, you'd leak data.
Presumably the 64 bit cookie might suffice.. nit, perhaps makes sense to
explicitly exclude zero cookie?

Acked-by: Daniel Borkmann <daniel@...earbox.net>

Thanks,
Daniel