Re: [RFC PATCH 7/8] x86: mpx: pass atomic parameter to do_munmap()

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

On 3/20/18 3:35 PM, Thomas Gleixner wrote:
> On Wed, 21 Mar 2018, Yang Shi wrote:
>
> Please CC everyone involved on the full patch set next time. I had to dig
> the rest out from my lkml archive to get the context.

Sorry for the inconvenience. Will pay attention to it next time.

>
>> Pass "true" to do_munmap() to not do unlock/relock to mmap_sem when
>> manipulating mpx map.
>> This is API change only.
> This is wrong. You cannot change the function in one patch and then clean
> up the users. That breaks bisectability.
>
> Depending on the number of callers this wants to be a single patch changing
> both the function and the callers or you need to create a new function
> which has the extra argument and switch all users over to it and then
> remove the old function.
>
>> @@ -780,7 +780,7 @@ static int unmap_entire_bt(struct mm_struct *mm,
>>   	 * avoid recursion, do_munmap() will check whether it comes
>>   	 * from one bounds table through VM_MPX flag.
>>   	 */
>> -	return do_munmap(mm, bt_addr, mpx_bt_size_bytes(mm), NULL);
>> +	return do_munmap(mm, bt_addr, mpx_bt_size_bytes(mm), NULL, true);
> But looking at the full context this is the wrong approach.
>
> First of all the name of that parameter 'atomic' is completely
> misleading. It suggests that this happens in fully atomic context, which is
> not the case.
>
> Secondly, conditional locking is frowned upon in general and rightfully so.
>
> So the right thing to do is to leave do_munmap() alone and add a new
> function do_munmap_huge() or whatever sensible name you come up with. Then
> convert the places which are considered to be safe one by one with a proper
> changelog which explains WHY this is safe.
>
> That way you avoid the chasing game of all existing do_munmap() callers and
> just use the new 'free in chunks' approach where it is appropriate and
> safe. No suprises, no bisectability issues....
>
> While at it please add proper kernel doc documentation to both do_munmap()
> and the new function which explains the intricacies.

Thanks a lot for the suggestion. Absolutely agree. Will fix the problems 
in newer version.

Yang

>
> Thanks,
>
> 	tglx