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

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Date:   Tue, 10 Oct 2017 09:22:20 +0800
From:   Xishi Qiu <qiuxishi@...wei.com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Vlastimil Babka <vbabka@...e.cz>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        zhong jiang <zhongjiang@...wei.com>,
        yeyunfeng <yeyunfeng@...wei.com>, <wanghaitao12@...wei.com>,
        "Zhoukang (A)" <zhoukang7@...wei.com>
Subject: Re: [RFC] a question about mlockall() and mprotect()

On 2017/10/10 2:26, Michal Hocko wrote:

> On Wed 27-09-17 13:51:09, Xishi Qiu wrote:
>> 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.
> 
> I guess I didn't get your answer. Does the above diff resolves your
> problem?

Hi Michal,

This upstream patch 36f881883c57941bb32d25cea6524f9612ab5a2c has already
resolve my problem, thank you for your attention.

Thanks,
Xishi Qiu