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

[InKi Dae]

2012/5/12 KOSAKI Motohiro <kosaki.motohiro at gmail.com>:
> On Fri, May 11, 2012 at 7:29 PM, Jerome Glisse <j.glisse at gmail.com> wrote:
>> On Fri, May 11, 2012 at 6:59 PM, KOSAKI Motohiro
>> <kosaki.motohiro at gmail.com> wrote:
>>>> My point is this ioctl will be restricted to one user (Xserver if i
>>>> understand) and only this user, there is no fork in it so no need to
>>>> worry about fork, just setting the vma as locked will be enough.
>>>>
>>>> But i don't want people reading this driver suddenly think that what
>>>> it's doing is ok, it's not, it's hack and can never make to work
>>>> properly on a general case, that's why it needs a big comment stating,
>>>> stressing that. I just wanted to make sure Inki and Kyungmin
>>>> understood that this kind of ioctl should be restricted to carefully
>>>> selected user and that there is no way to make it general or reliable
>>>> outside that.
>>>
>>>
>>> first off, I'm not drm guy and then I don't intend to insist you. but if
>>> application don't use fork, get_user_pages() has no downside. I guess we
>>> don't need VM_LOCKED hack.
>>>
>>> but again, up to drm folks.
>>
>> You need the VM_LOCKED hack to mare sure that the xorg vma still point
>> to the same page, afaict with get_user_pages pages can be migrated out
>> of the anonymous vma so the vma might point to new page, while old
>> page are still in use by the gpu and not recycle until their refcount
>> drop to 0.
>
> afaik, get_user_pages() prevent page migration. (see
> migrate_page_move_mapping). but mlock doesn't.


I'd like to make sure some points before preparing next patch to
userptr feature.

in case that userptr ioctl can be accessed by user.
1. all the pages from get_user_pages can't be migrated by CMA and also
it doesn't need VM_LOCKED or VM_RESERVED flag.

2. if VM_DONTCOPY is set to vma->flags then all the pages to this vma
are safe from being COW.

3. userptr ioctl  has limited size and the limited size can be changed
by only root user. this is for preventing from dropping system
performance by malicious software.

with above actions taken, are there something we didn't care? if so,
we will preparing next path for the userptr ioctl to be accessed by
only root user. this means that this feature is used by only X Server
but isn't used by any users. so we are going to wait something
resolved fully. of course, as Jerome said, we wil add big comments
describing this feature enough to next patch.

Thanks,
Inki Dae