Re: issue with inflight pages from page_pool

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> On 17.04.23 20:17, Lorenzo Bianconi wrote:
> > > On Mon, Apr 17, 2023 at 7:53 PM Lorenzo Bianconi <lorenzo@kernel.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 ?
> > 
> > The main issue in my tests (and even in mt76 I think) is the pages are not returned
> > to the pool for a very long time (even hours) and doing so the pool is like in a
> > 'limbo' state where it is not actually deallocated and page_pool_release_retry
> > continues complaining about it. I think this is because we do not have more tcp
> > traffic to deallocate them, but I am not so familiar with tcp codebase :)
> > 
> > Regards,
> > Lorenzo
> > 
> > > 
> > > Or buffers transferred to another socket or pipe (splice() and friends)
> 
> I'm not absolutely sure that it is the same problem, but I also saw some
> problems with page_pool destroying and page_pool_release_retry(). I did
> post it, but I did not get any reply:
> 
> https://lore.kernel.org/bpf/20230311213709.42625-1-gerhard@engleder-embedded.com/T/
> 
> Could this be a similar issue?

I am not sure too. In order to prove it is the same issue, I would say you can try to
run the test applying my poc patch or setting sysctl_skb_defer_max to 0.

Regards,
Lorenzo

> 
> Gerhard

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