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

[Yang Shi <yang.shi@linux.alibaba.com>]

On 2/20/18 2:38 PM, Alexey Dobriyan wrote:
> 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

Yes, it is. Since I just ran into the above splat, so I just did the 
minimum change.

> 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)

Sorry, I don't get your point here. Could you please elaborate?

> 2) access_remote_vm() et al will do the same ->mmap_sem, and

Yes, it does. But, __access_remote_vm() is called by access_process_vm() 
too, which is used by much more places, i.e. ptrace, so I was not sure 
if it is preferred to convert to killable version. So, I leave it untouched.

> 3) /proc/*/environ and get_cmdline() do the same.

They look suitable to use killable version.

BTW, I just realized the code should go to out_free_page instead of 
out_mmput. Will rectify in newer version once we decide the extra places 
to use killable version.

Thanks,
Yang