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Date:   Wed, 4 Sep 2019 08:15:01 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>, davem@davemloft.net,
        netdev@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] net/skbuff: silence warnings under memory pressure
Message-ID: <20190904061501.GB3838@dhcp22.suse.cz>
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Cc printk maintainers

On Tue 03-09-19 17:42:28, Qian Cai wrote:
> > > I suppose what happens is those skb_build() allocations are from softirq,
> > > and
> > > once one of them failed, it calls printk() which generates more interrupts.
> > > Hence, the infinite loop.
> > 
> > Please elaborate more.
> > 
> 
> If you look at the original report, the failed allocation dump_stack() is,
> 
> ?<IRQ>
> ?warn_alloc.cold.43+0x8a/0x148
> ?__alloc_pages_nodemask+0x1a5c/0x1bb0
> ?alloc_pages_current+0x9c/0x110
> ?allocate_slab+0x34a/0x11f0
> ?new_slab+0x46/0x70
> ?___slab_alloc+0x604/0x950
> ?__slab_alloc+0x12/0x20
> ?kmem_cache_alloc+0x32a/0x400
> ?__build_skb+0x23/0x60
> ?build_skb+0x1a/0xb0
> ?igb_clean_rx_irq+0xafc/0x1010 [igb]
> ?igb_poll+0x4bb/0xe30 [igb]
> ?net_rx_action+0x244/0x7a0
> ?__do_softirq+0x1a0/0x60a
> ?irq_exit+0xb5/0xd0
> ?do_IRQ+0x81/0x170
> ?common_interrupt+0xf/0xf
> ?</IRQ>
> 
> Since it has no __GFP_NOWARN to begin with, it will call,
> 
> printk
>   vprintk_default
>     vprintk_emit
>       wake_up_klogd
>         irq_work_queue
>           __irq_work_queue_local
>             arch_irq_work_raise
>               apic->send_IPI_self(IRQ_WORK_VECTOR)
>                 smp_irq_work_interrupt
>                   exiting_irq
>                     irq_exit
> 
> and end up processing pending?net_rx_action softirqs again which are plenty due
> to connected via ssh etc.
-- 
Michal Hocko
SUSE Labs