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

[Xishi Qiu <qiuxishi@huawei.com>]

On 2017/9/26 17:13, Xishi Qiu wrote:

> On 2017/9/26 17:02, Michal Hocko wrote:
> 
>> On Tue 26-09-17 16:39:56, Xishi Qiu wrote:
>>> On 2017/9/26 16:17, Michal Hocko wrote:
>>>
>>>> On Tue 26-09-17 15:56:55, Xishi Qiu wrote:
>>>>> When we call mlockall(), we will add VM_LOCKED to the vma,
>>>>> if the vma prot is ---p,
>>>>
>>>> not sure what you mean here. apply_mlockall_flags will set the flag on
>>>> all vmas except for special mappings (mlock_fixup). This phase will
>>>> cause that memory reclaim will not free already mapped pages in those
>>>> vmas (see page_check_references and the lazy mlock pages move to
>>>> unevictable LRUs).
>>>>
>>>>> then mm_populate -> get_user_pages will not alloc memory.
>>>>
>>>> mm_populate all the vmas with pages. Well there are certainly some
>>>> constrains - e.g. memory cgroup hard limit might be hit and so the
>>>> faulting might fail.
>>>>
>>>>> I find it said "ignore errors" in mm_populate()
>>>>> static inline void mm_populate(unsigned long addr, unsigned long len)
>>>>> {
>>>>> 	/* Ignore errors */
>>>>> 	(void) __mm_populate(addr, len, 1);
>>>>> }
>>>>
>>>> But we do not report the failure because any failure past
>>>> apply_mlockall_flags would be tricky to handle. We have already dropped
>>>> the mmap_sem lock so some other address space operations could have
>>>> interfered.
>>>>  
>>>>> And later we call mprotect() to change the prot, then it is
>>>>> still not alloc memory for the mlocked vma.
>>>>>
>>>>> My question is that, shall we alloc memory if the prot changed,
>>>>> and who(kernel, glibc, user) should alloc the memory?
>>>>
>>>> I do not understand your question but if you are asking how to get pages
>>>> to map your vmas then touching that area will fault the memory in.
>>>
>>> Hi Michal,
>>>
>>> syscall mlockall() will first apply the VM_LOCKED to the vma, then
>>> call mm_populate() to map the vmas.
>>>
>>> mm_populate
>>> 	populate_vma_page_range
>>> 		__get_user_pages
>>> 			check_vma_flags
>>> And the above path maybe return -EFAULT in some case, right?
>>>
>>> If we call mprotect() to change the prot of vma, just let
>>> check_vma_flags() return 0, then we will get the mlocked pages
>>> in following page-fault, right?
>>
>> Any future page fault to the existing vma will result in the mlocked
>> page. That is what VM_LOCKED guarantess.
>>
>>> My question is that, shall we map the vmas immediately when
>>> the prot changed? If we should map it immediately, who(kernel, glibc, user)
>>> do this step?
>>
>> 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.

Thanks,
Xishi Qiu

> 
> .
>