netdev - Re: [BUG] Possible unsafe page

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Date: Wed, 23 Aug 2023 07:49:28 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: netdev@...r.kernel.org, Ratheesh Kannoth <rkannoth@...vell.com>, "David
 S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Geetha sowjanya <gakula@...vell.com>, Ilias Apalodimas
 <ilias.apalodimas@...aro.org>, Jesper Dangaard Brouer <hawk@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Subbaraya Sundeep <sbhatta@...vell.com>,
 Sunil Goutham <sgoutham@...vell.com>, Thomas Gleixner <tglx@...utronix.de>,
 hariprasad <hkelam@...vell.com>
Subject: Re: [BUG] Possible unsafe page_pool usage in octeontx2

On Wed, 23 Aug 2023 11:47:57 +0200 Sebastian Andrzej Siewior wrote:
> The pool can be filled in the same context (within allocation if the
> pool is empty). There is also page_pool_recycle_in_cache() which fills
> the pool from within skb free, for instance:
>  napi_consume_skb() -> skb_release_all() -> skb_release_data() ->
>  napi_frag_unref() -> page_pool_return_skb_page().
> 
> The last one has the following check here:
> |         napi = READ_ONCE(pp->p.napi);
> |         allow_direct = napi_safe && napi &&
> |                 READ_ONCE(napi->list_owner) == smp_processor_id();
> 
> This eventually ends in page_pool_recycle_in_cache() where it adds the
> page to the cache buffer if the check above is true (and BH is disabled). 
> 
> napi->list_owner is set once NAPI is scheduled until the poll callback
> completed. It is safe to add items to list because only one of the two
> can run on a single CPU and the completion of them ensured by having BH
> disabled the whole time.

One clarification - .napi will be NULL for otx2, it's an opt-in for
drivers which allocate from NAPI, and AFAICT otx2 does not opt in.