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From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To: Alexander Lobakin <aleksander.lobakin@intel.com>, Lorenzo Bianconi
 <lorenzo@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>, Eric Dumazet
 <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>, Paolo Abeni
 <pabeni@redhat.com>, netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH net-next] page_pool: disable direct recycling based on
 pool->cpuid on destroy
In-Reply-To: <8aa809c0-585f-4750-98d4-e19165c6ff73@intel.com>
References: <20240215113905.96817-1-aleksander.lobakin@intel.com>
 <87v86qc4qd.fsf@toke.dk> <8aa809c0-585f-4750-98d4-e19165c6ff73@intel.com>
Date: Thu, 15 Feb 2024 14:29:03 +0100
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Alexander Lobakin <aleksander.lobakin@intel.com> writes:

> From: Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat.com>
> Date: Thu, 15 Feb 2024 13:05:30 +0100
>
>> Alexander Lobakin <aleksander.lobakin@intel.com> writes:
>>=20
>>> 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.
>>=20
>> 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.
>
>>=20
>> 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 :)

Right. I'm OK with changing this as a form of future-proofing if we end
up finding other uses for setting the cpuid field, so:

Reviewed-by: Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat.com>