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

[Xishi Qiu]

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?

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?

Thanks,
Xishi Qiu