netdev - Re: issue with inflight pages from page

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Date:   Mon, 17 Apr 2023 20:08:33 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, hawk@...nel.org,
        ilias.apalodimas@...aro.org, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, bpf@...r.kernel.org,
        lorenzo.bianconi@...hat.com, nbd@....name
Subject: Re: issue with inflight pages from page_pool

On Mon, Apr 17, 2023 at 7:53 PM Lorenzo Bianconi <lorenzo@...nel.org> wrote:
>
> Hi all,
>
> I am triggering an issue with a device running the page_pool allocator.
> In particular, the device is running an iperf tcp server receiving traffic
> from a remote client. On the driver I loaded a simple xdp program returning
> xdp_pass. When I remove the ebpf program and destroy the pool, page_pool
> allocator starts complaining in page_pool_release_retry() that not all the pages
> have been returned to the allocator. In fact, the pool is not really destroyed
> in this case.
> Debugging the code it seems the pages are stuck softnet_data defer_list and
> they are never freed in skb_defer_free_flush() since I do not have any more tcp
> traffic. To prove it, I tried to set sysctl_skb_defer_max to 0 and the issue
> does not occur.
> I developed the poc patch below and the issue seems to be fixed:

I do not see why this would be different than having buffers sitting
in some tcp receive
(or out or order) queues for a few minutes ?

Or buffers transferred to another socket or pipe (splice() and friends)