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Date:   Wed, 21 Mar 2018 14:14:49 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Yang Shi <yang.shi@linux.alibaba.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section
Message-ID: <20180321131449.GN23100@dhcp22.suse.cz>
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On Wed 21-03-18 05:31:19, Yang Shi wrote:
> When running some mmap/munmap scalability tests with large memory (i.e.
> > 300GB), the below hung task issue may happen occasionally.
> 
> INFO: task ps:14018 blocked for more than 120 seconds.
>        Tainted: G            E 4.9.79-009.ali3000.alios7.x86_64 #1
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this
> message.
>  ps              D    0 14018      1 0x00000004
>   ffff885582f84000 ffff885e8682f000 ffff880972943000 ffff885ebf499bc0
>   ffff8828ee120000 ffffc900349bfca8 ffffffff817154d0 0000000000000040
>   00ffffff812f872a ffff885ebf499bc0 024000d000948300 ffff880972943000
>  Call Trace:
>   [<ffffffff817154d0>] ? __schedule+0x250/0x730
>   [<ffffffff817159e6>] schedule+0x36/0x80
>   [<ffffffff81718560>] rwsem_down_read_failed+0xf0/0x150
>   [<ffffffff81390a28>] call_rwsem_down_read_failed+0x18/0x30
>   [<ffffffff81717db0>] down_read+0x20/0x40
>   [<ffffffff812b9439>] proc_pid_cmdline_read+0xd9/0x4e0

Slightly off-topic:
Btw. this sucks as well. Do we really need to take mmap_sem here? Do any
of
	arg_start = mm->arg_start;
	arg_end = mm->arg_end;
	env_start = mm->env_start;
	env_end = mm->env_end;

change after exec or while the pid is already visible in proc? If yes
maybe we can use a dedicated lock.
-- 
Michal Hocko
SUSE Labs