linux-kernel - Re: [PATCH net-next] page_pool: add debug for release to cache from wrong CPU

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Message-ID: <20250923083439.60c64f5e@kernel.org>
Date: Tue, 23 Sep 2025 08:34:39 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Dragos Tatulea <dtatulea@...dia.com>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Ilias Apalodimas
 <ilias.apalodimas@...aro.org>, Sebastian Andrzej Siewior
 <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>, Steven
 Rostedt <rostedt@...dmis.org>, netdev@...r.kernel.org, Tariq Toukan
 <tariqt@...dia.com>, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH net-next] page_pool: add debug for release to cache from
 wrong CPU

On Tue, 23 Sep 2025 15:23:02 +0000 Dragos Tatulea wrote:
> On Mon, Sep 22, 2025 at 04:18:27PM -0700, Jakub Kicinski wrote:
> > On Sat, 20 Sep 2025 09:25:31 +0000 Dragos Tatulea wrote:  
> > > The point is not to chase leaks but races from doing a recycle to cache
> > > from the wrong CPU. This is how XDP issue was caught where
> > > xdp_set_return_frame_no_direct() was not set appropriately for cpumap [1].
> > > 
> > > My first approach was to __page_pool_put_page() but then I figured that
> > > the warning should live closer to where the actual assignment happens.
> > > 
> > > [1] https://lore.kernel.org/all/e60404e2-4782-409f-8596-ae21ce7272c4@kernel.org/  
> > 
> > Ah, that thing. I wonder whether the complexity in the driver-facing 
> > xdp_return API is really worth the gain here. IIUC we want to extract
> > the cases where we're doing local recycling and let those cases use
> > the lockless cache. But all those cases should be caught by automatic
> > local recycling detection, so caller can just pass false..
> >  
> This patch was simply adding the debugging code to catch the potential
> misuse from any callers.
> 
> I was planning to send another patch for the xdp_return() API part
> once/if this one got accepted. If it makes more sense I can bundle them
> together in a RFC (as merge window is coming).

Combined RFC would make sense, yes.

But you get what I'm saying right? I'm questioning whether _rx_napi()
flavor of calls even make sense these days. If they don't I'd think
the drivers can't be wrong and the need for the debug check is
diminished?