netdev - Re: [BUG] Possible unsafe page

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Date: Fri, 25 Aug 2023 17:42:58 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jesper Dangaard Brouer <hawk@...nel.org>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>, Sebastian Andrzej
 Siewior <bigeasy@...utronix.de>, 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>, Paolo Abeni <pabeni@...hat.com>,
 Subbaraya Sundeep <sbhatta@...vell.com>, Sunil Goutham
 <sgoutham@...vell.com>, Thomas Gleixner <tglx@...utronix.de>, hariprasad
 <hkelam@...vell.com>, Qingfang DENG <qingfang.deng@...lower.com.cn>
Subject: Re: [BUG] Possible unsafe page_pool usage in octeontx2

On Fri, 25 Aug 2023 19:25:42 +0200 Jesper Dangaard Brouer wrote:
> >> This WQ process is not allowed to use the page_pool_alloc() API this
> >> way (from a work-queue).  The PP alloc-side API must only be used
> >> under NAPI protection.  
> > 
> > Who did say that? If I don't set p.napi, how is Page Pool then tied to NAPI?  
> 
> *I* say that (as the PP inventor) as that was the design and intent,
> that this is tied to a NAPI instance and rely on the NAPI protection to
> make it safe to do lockless access to this cache array.

Absolutely no objection to us making the NAPI / bh context a requirement
past the startup stage, but just to be sure I understand the code -
technically if the driver never recycles direct, does not set the NAPI,
does not use xdp_return_frame_rx_napi etc. - the cache is always empty
so we good?

I wonder if we can add a check like "mark the pool as BH-only on first
BH use, and WARN() on process use afterwards". But I'm not sure what
CONFIG you'd accept that being under ;)