Re: [PATCH 2/2 v3] drm/exynos: added userptr feature.

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On Thu, May 10, 2012 at 10:51 PM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxx> wrote:
> (5/10/12 8:50 PM), Minchan Kim wrote:
>> On 05/11/2012 02:53 AM, KOSAKI Motohiro wrote:
>>>>>> let's assume that one application want to allocate user space memory
>>>>>> region using malloc() and then write something on the region. as you
>>>>>> may know, user space buffer doen't have real physical pages once
>>>>>> malloc() call so if user tries to access the region then page fault
>>>>>> handler would be triggered
>>>>> Understood.
>>>>>> and then in turn next process like swap in to fill physical frame
>>>>>> number
>>>>> into entry of the page faulted.
>>>>> Sorry, I can't understand your point due to my poor English.
>>>>> Could you rewrite it easiliy? :)
>>>> Simply saying, handle_mm_fault would be called to update pte after
>>>> finding
>>>> vma and checking access right. and as you know, there are many cases to
>>>> process page fault such as COW or demand paging.
>>> Hmm. If I understand correctly, you guys misunderstand mlock. it doesn't
>>> page pinning
>>> nor prevent pfn change. It only guarantee to don't make swap out. e.g.
>> Symantic point of view, you're right but the implementation makes sure
>> page pinning.
>>> memory campaction
>>> feature may automatically change page physical address.
>> I tried it last year but decided drop by realtime issue.
>> so I think mlock is a kind of page pinning. If elsewhere I don't realized
>> is doing, that place should be fixed.
>> Or my above patch should go ahead.
> Thanks pointing out. I didn't realized your patch didn't merged. I think it
> should go ahead. think autonuma case,
> if mlock disable autonuma migration, that's bug.  I don't think we can
> 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.
> And, think if application explictly use migrate_pages(2) or admins uses
> cpusets. driver code can't assume such scenario
> doesn't occur, yes?

I am ok with patch being merge as is if you add restriction for the
ioctl to be root only and a big comment stating that user ptr thing is
just abusing the kernel API and that it should not be replicated by
other driver except if fully understanding that all hell might break
loose with it.

If you know it's only the ddx that will use it and that their wont be
fork that better to not worry about but again state it in the comment
about the ioctl.

I really wish there was some magical VM_DRIVER_MAPPED flags that would
add the proper restriction to other memory code while keeping fork
behavior consistant (ie cow). But such things would need massive
chirurgy of the linux mm code.


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