linux-kernel - Re: [PATCH RFC net-next v2 7/7] net: skbuff: always try to recycle PP pages directly when in softirq

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Message-ID: <20230719135150.4da2f0ff@kernel.org>
Date:   Wed, 19 Jul 2023 13:51:50 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Alexander Duyck <alexanderduyck@...com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        "Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC net-next v2 7/7] net: skbuff: always try to recycle
 PP pages directly when in softirq

On Wed, 19 Jul 2023 18:34:46 +0200 Alexander Lobakin wrote:
> > What if we got here from netpoll? napi budget was 0, so napi_safe is
> > false, but in_softirq() can be true or false.  
> 
> If we're on the same CPU where the NAPI would run and in the same
> context, i.e. softirq, in which the NAPI would run, what is the problem?
> If there really is a good one, I can handle it here.

#define SOFTIRQ_BITS		8
#define SOFTIRQ_MASK		(__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
# define softirq_count()	(preempt_count() & SOFTIRQ_MASK)
#define in_softirq()		(softirq_count())

I don't know what else to add beyond that and the earlier explanation.

AFAIK pages as allocated by page pool do not benefit from the usual
KASAN / KMSAN checkers, so if we were to double-recycle a page once
a day because of a netcons race - it's going to be a month long debug
for those of us using Linux in production.