Re: [RFC] a question about mlockall() and mprotect()

[Xishi Qiu]

On 2017/9/26 19:00, Michal Hocko wrote:

> On Tue 26-09-17 11:45:16, Vlastimil Babka wrote:
>> On 09/26/2017 11:22 AM, Xishi Qiu wrote:
>>> On 2017/9/26 17:13, Xishi Qiu wrote:
>>>>> This is still very fuzzy. What are you actually trying to achieve?
>>>>
>>>> I don't expect page fault any more after mlock.
>>>>
>>>
>>> Our apps is some thing like RT, and page-fault maybe cause a lot of time,
>>> e.g. lock, mem reclaim ..., so I use mlock and don't want page fault
>>> any more.
>>
>> Why does your app then have restricted mprotect when calling mlockall()
>> and only later adjusts the mprotect?
> 
> Ahh, OK I see what is goging on. So you have PROT_NONE vma at the time
> mlockall and then later mprotect it something else and want to fault all
> that memory at the mprotect time?
> 
> So basically to do
> ---
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 6d3e2f082290..b665b5d1c544 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -369,7 +369,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
>  	 * Private VM_LOCKED VMA becoming writable: trigger COW to avoid major
>  	 * fault on access.
>  	 */
> -	if ((oldflags & (VM_WRITE | VM_SHARED | VM_LOCKED)) == VM_LOCKED &&
> +	if ((oldflags & (VM_WRITE | VM_LOCKED)) == VM_LOCKED &&
>  			(newflags & VM_WRITE)) {
>  		populate_vma_page_range(vma, start, end, NULL);
>  	}
> 

Hi Michal,

My kernel is v3.10, and I missed this code, thank you reminding me.

Thanks,
Xishi Qiu