linux-kernel - Re: Problem in pfmemalloc skb handling in net/core/dev.c

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Message-ID: <20210409084436.GK3697@techsingularity.net>
Date:   Fri, 9 Apr 2021 09:44:36 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Xie He <xie.he.0141@...il.com>
Cc:     Mel Gorman <mgorman@...e.de>, jslaby@...e.cz,
        Neil Brown <neilb@...e.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Mike Christie <michaelc@...wisc.edu>,
        Eric B Munson <emunson@...bm.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Sebastian Andrzej Siewior <sebastian@...akpoint.cc>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Problem in pfmemalloc skb handling in net/core/dev.c

On Fri, Apr 09, 2021 at 01:33:24AM -0700, Xie He wrote:
> On Fri, Apr 9, 2021 at 12:30 AM Mel Gorman <mgorman@...hsingularity.net> wrote:
> >
> > Under what circumstances do you expect sk_memalloc_socks() to be false
> > and skb_pfmemalloc() to be true that would cause a problem?
> 
> For example, if at the time the skb is allocated,
> "sk_memalloc_socks()" was true, then the skb might be allocated as a
> pfmemalloc skb. However, if after this skb is allocated and before
> this skb reaches "__netif_receive_skb", "sk_memalloc_socks()" has
> changed from "true" to "false", then "__netif_receive_skb" will see
> "sk_memalloc_socks()" being false and "skb_pfmemalloc(skb)" being
> true.
> 
> This is a problem because this would cause a pfmemalloc skb to be
> delivered to "taps" and protocols that don't support pfmemalloc skbs.

That would imply that the tap was communicating with a swap device to
allocate a pfmemalloc skb which shouldn't happen. Furthermore, it would
require the swap device to be deactivated while pfmemalloc skbs still
existed. Have you encountered this problem?

-- 
Mel Gorman
SUSE Labs