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

[Alexander Lobakin]

From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
Date: Thu, 15 Feb 2024 13:05:30 +0100

> Alexander Lobakin <aleksander.lobakin@xxxxxxxxx> 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.

> 
> 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