netdev - Re: issue with inflight pages from page

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Message-ID: <20230417120837.6f1e0ef6@kernel.org>
Date:   Mon, 17 Apr 2023 12:08:37 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        hawk@...nel.org, ilias.apalodimas@...aro.org, davem@...emloft.net,
        pabeni@...hat.com, bpf@...r.kernel.org,
        lorenzo.bianconi@...hat.com, nbd@....name
Subject: Re: issue with inflight pages from page_pool

On Mon, 17 Apr 2023 20:42:39 +0200 Lorenzo Bianconi wrote:
> > 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..).

If it's that then I'm with Eric. There are many ways to keep the pages
in use, no point working around one of them and not the rest :(

> 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.

I don't see a problem having pp_recycle = 1 and head in slab is legal.
pp_recycle just means that *if* a page is from the page pool we own 
the recycling reference. A page from slab will not be treated as a PP
page cause it doesn't have pp_magic set to the correct pattern.