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Subject: Re: Problem in pfmemalloc skb handling in net/core/dev.c
To:     Xie He <xie.he.0141@gmail.com>,
        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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 <20210409084436.GK3697@techsingularity.net>
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From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <87ab3d13-f95d-07c5-fc6a-fb33e32685e5@gmail.com>
Date:   Fri, 9 Apr 2021 12:04:11 +0200
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On 4/9/21 11:14 AM, Xie He wrote:
> On Fri, Apr 9, 2021 at 1:44 AM Mel Gorman <mgorman@techsingularity.net> wrote:
>>
>> 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?
> 
> I'm not a user of swap devices or pfmemalloc skbs. I just want to make
> sure the protocols that I'm developing (not IP or IPv6) won't get
> pfmemalloc skbs when receiving, because those protocols cannot handle
> them.
> 
> According to the code, it seems always possible to get a pfmemalloc
> skb when a network driver calls "__netdev_alloc_skb". The skb will
> then be queued in per-CPU backlog queues when the driver calls
> "netif_rx". There seems to be nothing preventing "sk_memalloc_socks()"
> from becoming "false" after the skb is allocated and before it is
> handled by "__netif_receive_skb".
> 
> Do you mean that at the time "sk_memalloc_socks()" changes from "true"
> to "false", there would be no in-flight skbs currently being received,
> and all network communications have been paused?
> 


Note that pfmemalloc skbs are normally dropped in sk_filter_trim_cap()

Simply make sure your protocol use it.