Re: [RFC PATCH] net: Fix one page_pool page leak from skb_frag_unref

From: Mina Almasry <almasrymina@google.com>
To: Jakub Kicinski <kuba@kernel.org>
Cc: Dragos Tatulea <dtatulea@nvidia.com>,
	"davem@davemloft.net" <davem@davemloft.net>,
	 "pabeni@redhat.com" <pabeni@redhat.com>,
	 "ilias.apalodimas@linaro.org" <ilias.apalodimas@linaro.org>,
	 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	 "jacob.e.keller@intel.com" <jacob.e.keller@intel.com>,
	 "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Jianbo Liu <jianbol@nvidia.com>,
	 "edumazet@google.com" <edumazet@google.com>
Subject: Re: [RFC PATCH] net: Fix one page_pool page leak from skb_frag_unref
Date: Fri, 26 Apr 2024 21:24:09 -0700	[thread overview]
Message-ID: <CAHS8izNs-LV=6FE39sjF3V7qVfveOsOAOJ_X62TSzWpvamsS0Q@mail.gmail.com> (raw)
In-Reply-To: <20240426160859.0d85908e@kernel.org>

On Fri, Apr 26, 2024 at 4:09 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Thu, 25 Apr 2024 12:20:59 -0700 Mina Almasry wrote:
> > -       if (recycle && napi_pp_get_page(page))
> > +       if (napi_pp_get_page(page))
>
> Pretty sure you can't do that. The "recycle" here is a concurrency
> guarantee. A guarantee someone is holding a pp ref on that page,
> a ref which will not go away while napi_pp_get_page() is executing.

I don't mean to argue, but I think the get_page()/put_page() pair we
do in the page ref path is susceptible to the same issue. AFAIU it's
not safe to get_page() if another CPU can be dropping the last ref,
get_page_unless_zero() should be used instead.

Since get_page() is good in the page ref path without some guarantee,
it's not obvious to me why we need this guarantee in the pp ref path,
but I could be missing some subtlety. At any rate, if you prefer us
going down the road of reverting commit 2cc3aeb5eccc ("skbuff: Fix a
potential race while recycling page_pool packets"), I think that could
also fix the issue.

-- 
Thanks,
Mina