linux-kernel - Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm

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Message-ID: <20180415204913.GC19578@uranus.lan>
Date:   Sun, 15 Apr 2018 23:49:13 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     adobriyan@...il.com, mhocko@...nel.org, willy@...radead.org,
        mguzik@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v4 PATCH] mm: introduce arg_lock to protect arg_start|end and
 env_start|end in mm_struct

On Sun, Apr 15, 2018 at 02:24:51AM +0800, Yang Shi wrote:
> mmap_sem is on the hot path of kernel, and it very contended, but it is
> abused too. It is used to protect arg_start|end and evn_start|end when
> reading /proc/$PID/cmdline and /proc/$PID/environ, but it doesn't make
> sense since those proc files just expect to read 4 values atomically and
> not related to VM, they could be set to arbitrary values by C/R.
> 
> And, the mmap_sem contention may cause unexpected issue like below:
> 
> 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
>   [<ffffffff81253c95>] ? do_filp_open+0xa5/0x100
>   [<ffffffff81241d87>] __vfs_read+0x37/0x150
>   [<ffffffff812f824b>] ? security_file_permission+0x9b/0xc0
>   [<ffffffff81242266>] vfs_read+0x96/0x130
>   [<ffffffff812437b5>] SyS_read+0x55/0xc0
>   [<ffffffff8171a6da>] entry_SYSCALL_64_fastpath+0x1a/0xc5
> 
> Both Alexey Dobriyan and Michal Hocko suggested to use dedicated lock
> for them to mitigate the abuse of mmap_sem.
> 
> So, introduce a new spinlock in mm_struct to protect the concurrent
> access to arg_start|end, env_start|end and others, as well as replace
> write map_sem to read to protect the race condition between prctl and
> sys_brk which might break check_data_rlimit(), and makes prctl more
> friendly to other VM operations.
> 
> This patch just eliminates the abuse of mmap_sem, but it can't resolve the
> above hung task warning completely since the later access_remote_vm() call
> needs acquire mmap_sem. The mmap_sem scalability issue will be solved in the
> future.
> 
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>
> Cc: Alexey Dobriyan <adobriyan@...il.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Mateusz Guzik <mguzik@...hat.com>
> Cc: Cyrill Gorcunov <gorcunov@...il.com>
> ---

Looks ok to me, thanks!
Reviewed-by: Cyrill Gorcunov <gorcunov@...nvz.org>