Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

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

On 3/27/18 2:29 AM, Michal Hocko wrote:
> On Tue 27-03-18 02:20:39, Yang Shi wrote:
> [...]
> The patch looks reasonable to me. Maybe it would be better to be more
> explicit about the purpose of the patch. As others noticed, this alone
> wouldn't solve the mmap_sem contention issues. I _think_ that if you
> were more explicit about the mmap_sem abuse it would trigger less
> questions.

Yes, sure.

>
> I have just one more question. Now that you are touching this area,
> would you be willing to remove the following ugliness?
>
>> diff --git a/kernel/sys.c b/kernel/sys.c
>> index f2289de..17bddd2 100644
>> --- a/kernel/sys.c
>> +++ b/kernel/sys.c
>> @@ -1959,7 +1959,7 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data
>>   			return error;
>>   	}
>>   
>> -	down_write(&mm->mmap_sem);
>> +	down_read(&mm->mmap_sem);
> Why do we need to hold mmap_sem here and call find_vma, when only
> PR_SET_MM_ENV_END: is consuming it? I guess we can replace it wit the
> new lock and take the mmap_sem only for PR_SET_MM_ENV_END.

Actually, I didn't think of why. It looks prctl_set_mm() checks if vma 
does exist when it tries to set stack_start, argv_* and env_*, btw not 
only env_end.

Cyrill may be able to give us some hint since C/R is the main user of 
this API.

Yang

>
> Thanks!