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

[Xishi Qiu <qiuxishi@huawei.com>]

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.

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