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 <20210409073046.GI3697@techsingularity.net>
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From:   Xie He <xie.he.0141@gmail.com>
Date:   Fri, 9 Apr 2021 01:33:24 -0700
Message-ID: <CAJht_EPXS3wVoNyaD6edqLPKvDTG2vg4qxiGuWBgWpFsNhB-4g@mail.gmail.com>
Subject: Re: Problem in pfmemalloc skb handling in net/core/dev.c
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Mel Gorman <mgorman@suse.de>, jslaby@suse.cz,
        Neil Brown <neilb@suse.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Mike Christie <michaelc@cs.wisc.edu>,
        Eric B Munson <emunson@mgebm.net>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Sebastian Andrzej Siewior <sebastian@breakpoint.cc>,
        Christoph Lameter <cl@linux.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Apr 9, 2021 at 12:30 AM Mel Gorman <mgorman@techsingularity.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.