Re: [PATCH] fs: proc: use down_read_killable in proc_pid_cmdline_read()

[Alexey Dobriyan <adobriyan@gmail.com>]

On Wed, Feb 21, 2018 at 03:49:29AM +0800, Yang Shi wrote:
> When running vm-scalability with large memory (> 300GB), the below hung
> task issue happens 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
>   [<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
> 
> When manipulating a large mapping, the process may hold the mmap_sem for
> long time, so reading /proc/<pid>/cmdline may be blocked in
> uninterruptible state for long time.
> 
> down_read_trylock() sounds too aggressive, and we already have killable
> version APIs for semaphore, here use down_read_killable() to improve the
> responsiveness.

> -	down_read(&mm->mmap_sem);
> +	rv = down_read_killable(&mm->mmap_sem);
> +	if (rv)
> +		goto out_mmput;
>  	arg_start = mm->arg_start;
>  	arg_end = mm->arg_end;
>  	env_start = mm->env_start;

Fix is incomplete
1) /proc/*/cmdline only wants to read 4 values atomically,
   those 4 values are basically random values and aren't
   related to VM part at all (after C/R went in, they are
   settable to arbitrary values)
2) access_remote_vm() et al will do the same ->mmap_sem, and
3) /proc/*/environ and get_cmdline() do the same.