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

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Message-ID: <20250922161827.4c4aebd1@kernel.org>
Date: Mon, 22 Sep 2025 16:18:27 -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 Sat, 20 Sep 2025 09:25:31 +0000 Dragos Tatulea wrote:
> > The patch seems half-baked. If the NAPI local recycling is incorrect
> > the pp will leak a reference and live forever. Which hopefully people
> > would notice. Are you adding this check just to double confirm that
> > any leaks you're chasing are in the driver, and not in the core?  
>
> 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..