Re: issue with inflight pages from page_pool

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> On Mon, 17 Apr 2023 20:17:45 +0200 Lorenzo Bianconi wrote:
> > > 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 :)
> 
> I've seen the page leaks too in my recent testing but I just assumed 
> I fumbled the quick bnxt conversion to page pool. Could it be something
> with page frags? It happened a lot if I used page frags, IIRC mt76 is
> using page frags, too.

my device is allocating a full order 0 page from the pool so it is not related
to fragmented pages as fixed for mt76 by Alex and Felix.

> 
> Is drgn available for your target? You could try to scan the pages on
> the system and see if you can find what's still pointing to the page
> pool (assuming they are indeed leaked and not returned to the page
> allocator without releasing :()

I will test it but since setting sysctl_skb_defer_max to 0 fixes the issue,
I think the pages are still properly linked to the pool, they are just not
returned to it. I proved it using the other patch I posted [0] where I can see
the counter of returned pages incrementing from time to time (in a very long
time slot..).

Unrelated to this issue, but debugging it I think a found a page_pool leak in
skb_condense() [1] where we can reallocate the skb data using kmalloc for a
page_pool recycled skb.

Regards,
Lorenzo

[0] https://lore.kernel.org/netdev/20230417111204.08f19827@kernel.org/T/#t
[1] https://github.com/torvalds/linux/blob/master/net/core/skbuff.c#L6602

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