linux-kernel - Re: [PATCH net-next] page_pool: disable direct recycling based on pool->cpuid on destroy

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Date: Thu, 15 Feb 2024 15:01:55 +0100
From: Lorenzo Bianconi <lorenzo@...nel.org>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] page_pool: disable direct recycling based on
 pool->cpuid on destroy

> From: Lorenzo Bianconi <lorenzo@...nel.org>
> Date: Thu, 15 Feb 2024 14:37:10 +0100
> 
> >> From: Toke Høiland-Jørgensen <toke@...hat.com>
> >> Date: Thu, 15 Feb 2024 13:05:30 +0100
> >>
> >>> Alexander Lobakin <aleksander.lobakin@...el.com> writes:
> >>>
> >>>> Now that direct recycling is performed basing on pool->cpuid when set,
> >>>> memory leaks are possible:
> >>>>
> >>>> 1. A pool is destroyed.
> >>>> 2. Alloc cache is emptied (it's done only once).
> >>>> 3. pool->cpuid is still set.
> >>>> 4. napi_pp_put_page() does direct recycling basing on pool->cpuid.
> >>>> 5. Now alloc cache is not empty, but it won't ever be freed.
> >>>
> >>> Did you actually manage to trigger this? pool->cpuid is only set for the
> >>> system page pool instance which is never destroyed; so this seems a very
> >>> theoretical concern?
> >>
> >> To both Lorenzo and Toke:
> >>
> >> Yes, system page pools are never destroyed, but we might latter use
> >> cpuid in non-persistent PPs. Then there will be memory leaks.
> >> I was able to trigger this by creating bpf/test_run page_pools with the
> >> cpuid set to test direct recycling of live frames.
> > 
> > what about avoiding the page to be destroyed int this case? I do not like the
> 
> I think I didn't get what you wanted to say here :s

My assumption here was cpuid will be set just system page_pool so it is just a
matter of not running page_pool_destroy for them. Anyway in the future we could
allow to set cpuid even for non-system page_pool if the pool is linked to a
given rx-queue and the queue is pinned to a given cpu.

Regards,
Lorenzo

> 
> Rewriting cpuid doesn't introduce any new checks on hotpath. Destroying
> the pool is slowpath and we shouldn't hurt hotpath to handle it.
> 
> > idea of overwriting the cpuid field for it.
> 
> We also overwrite pp->p.napi field a couple lines below. It happens only
> when destroying the pool, we don't care about the fields at this point.
> 
> > 
> > Regards,
> > Lorenzo
> > 
> >>
> >>>
> >>> I guess we could still do this in case we find other uses for setting
> >>> the cpuid; I don't think the addition of the READ_ONCE() will have any
> >>> measurable overhead on the common arches?
> >>
> >> READ_ONCE() is cheap, but I thought it's worth mentioning in the
> >> commitmsg anyway :)
> >>
> >>>
> >>> -Toke
> 
> Thanks,
> Olek

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