Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section

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

On 3/21/18 2:23 PM, Michal Hocko wrote:
> On Wed 21-03-18 10:16:41, Yang Shi wrote:
>>
>> On 3/21/18 9:50 AM, Yang Shi wrote:
>>>
>>> On 3/21/18 6:14 AM, Michal Hocko wrote:
>>>> 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.
>> BTW, this is not the only place to acquire mmap_sem in
>> proc_pid_cmdline_read(), it calls access_remote_vm() which need acquire
>> mmap_sem too, so the mmap_sem scalability issue will be hit sooner or later.
> Ohh, absolutely. mmap_sem is unfortunatelly abused and it would be great
> to remove that. munmap should perform much better. How to do that safely

Yes, agree. We are on the same page.

> is a different question. I am not yet convinced that tearing down a vma
> in batches is safe. The vast majority of time is spent on tearing down

You can try my patches. I did full LTP test and running multiple kernel 
build in parallel. It survives.

> pages and that is quite easy to move out of the write lock. That would
> be an improvement already and it should be risk safe. If even that is
> not sufficient then using range locking should help a lot. There
> shouldn't be really any other address space operations within the range
> most of the time so this would be basically non-contended access.

It might depend on how the range is defined. Too big range may lead to 
surprisingly more contention, but too small range may bring in too much 
lock/unlock operations.

Thanks,
Yang