Re: Allow migration of mlocked page?

[Minchan Kim <minchan@kernel.org>]

On 05/11/2012 10:14 PM, Mel Gorman wrote:

> On Fri, May 11, 2012 at 01:37:26PM +0900, Minchan Kim wrote:
>>> <SNIP>
>>> promise mlock don't change physical page.
>>> I wonder if any realtime guys page migration is free lunch. they should
>>> disable both auto migration and compaction.
>>
>> I think disable migration is overkill. We can do better than it.
> 
> The reason why we do not migrate mlock() pages is down to expectations of the
> application developer.  mlock historically was a real-time extention. For
> files, there is no guarantee of latency because obviously things like
> writing to the page can stall in balance_dirty_pages() but for anonymous
> memory, there is an expectation that access be low or zero latency. This
> would be particularly true if they used something like MAP_POPULATE.
> 
>> Quote from discussion last year from me.
>>
>> "
>> We can solve a bit that by another approach if it's really problem
>> with RT processes. The another approach is to separate mlocked pages
>> with allocation time like below pseudo patch which just show the
>> concept)
>>
>> ex)
>> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
>> index 3a93f73..8ae2e60 100644
>> --- a/include/linux/highmem.h
>> +++ b/include/linux/highmem.h
>> @@ -175,7 +175,8 @@ static inline struct page *
>>  alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
>>                                         unsigned long vaddr)
>>  {
>> -       return __alloc_zeroed_user_highpage(__GFP_MOVABLE, vma, vaddr);
>> +       gfp_t gfp_flag = vma->vm_flags & VM_LCOKED ? 0 : __GFP_MOVABLE;
>> +       return __alloc_zeroed_user_highpage(gfp_flag, vma, vaddr);
>>  }
>>
>> But it's a solution about newly allocated page on mlocked vma.
>> Old pages in the VMA is still a problem.
> 
> Yes.
> 
>> We can solve it at mlock system call through migrating the pages to
>> UNMOVABLE block.

> 

> Combining the two would be suitable because once mlock returns, any mapped
> page is locked in place and future allocations will be placed suitable. I'd
> also be ok allowing file-backed mlocked pages to be migrated on the grounds
> that no assumptions can be made about access latency anyway.
> 
>> "
>> It would be a solution to enhance compaction/CMA and we can make that compaction doesn't migrate
>> UNMOVABLE_PAGE_GROUP which make full by unevictable pages so mlocked page is still pinning page.
>> But get_user_pages in drivers still a problem. Or we can migrate unevictable pages, too so that
>> compaction/CMA would be good much but we lost pinning concept(It would break man page of mlocked
>> about real-time application stuff). Hmm.
>>
>>>
>>> And, think if application explictly use migrate_pages(2) or admins uses
>>> cpusets. driver code can't assume such scenario
>>> doesn't occur, yes?
>>
>> Yes. it seems to migrate mlocked page now.
>> Hmm,
>> Johannes, Mel.
>> Why should we be unfair on only compaction?
>>
> 
> If CMA decide they want to alter mlocked pages in this way, it's sortof
> ok. While CMA is being used, there are no expectations on the RT
> behaviour of the system - stalls are expected. In their use cases, CMA
> failing is far worse than access latency to an mlocked page being
> variable while CMA is running.
> 
> Compaction on the other hand is during the normal operation of the
> machine. There are applications that assume that if anonymous memory
> is mlocked() then access to it is close to zero latency. They are
> not RT-critical processes (or they would disable THP) but depend on
> this. Allowing compaction to migrate mlocked() pages will result in bugs
> being reported by these people.
> 
> I've received one bug this year about access latency to mlocked() regions but
> it turned out to be a file-backed region and related to when the write-fault
> is incurred. The ultimate fix was in the application but we'll get new bug
> reports if anonymous mlocked pages do not preserve the current guarantees
> on access latency.
> 


If so, what do you think about migration of mlocked pages by migrate_pages, cpuset_migrate_mm and memcg?
I think they all is done by under user's control while compaction happens regardless of user.
So do you think that's why compaction shouldn't migrate mlocked page?


-- 
Kind regards,
Minchan Kim

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