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

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Date:   Fri, 9 Apr 2021 01:33:24 -0700
From:   Xie He <xie.he.0141@...il.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
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 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.