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

[Minchan Kim]

On 05/10/2012 03:53 PM, KOSAKI Motohiro wrote:

> (5/10/12 12:58 AM), Minchan Kim wrote:
>> On 05/10/2012 10:39 AM, Inki Dae wrote:
>>
>>> Hi Jerome,
>>>
>>>> -----Original Message-----
>>>> From: Jerome Glisse [mailto:j.glisse at gmail.com]
>>>> Sent: Wednesday, May 09, 2012 11:46 PM
>>>> To: Inki Dae
>>>> Cc: airlied at linux.ie; dri-devel at lists.freedesktop.org;
>>>> kyungmin.park at samsung.com; sw0312.kim at samsung.com; linux-mm at kvack.org
>>>> Subject: Re: [PATCH 2/2 v3] drm/exynos: added userptr feature.
>>>>
>>>> On Wed, May 9, 2012 at 2:17 AM, Inki Dae<inki.dae at samsung.com>  wrote:
>>>>> this feature is used to import user space region allocated by malloc()
>>>> or
>>>>> mmaped into a gem. and to guarantee the pages to user space not to be
>>>>> swapped out, the VMAs within the user space would be locked and then
>>>> unlocked
>>>>> when the pages are released.
>>>>>
>>>>> but this lock might result in significant degradation of system
>>>> performance
>>>>> because the pages couldn't be swapped out so we limit user-desired
>>>> userptr
>>>>> size to pre-defined.
>>>>>
>>>>> Signed-off-by: Inki Dae<inki.dae at samsung.com>
>>>>> Signed-off-by: Kyungmin Park<kyungmin.park at samsung.com>
>>>>
>>>>
>>>> Again i would like feedback from mm people (adding cc). I am not sure
>>>
>>> Thank you, I missed adding mm as cc.
>>>
>>>> locking the vma is the right anwser as i said in my previous mail,
>>>> userspace can munlock it in your back, maybe VM_RESERVED is better.
>>>
>>> I know that with VM_RESERVED flag, also we can avoid the pages from
>>> being
>>> swapped out. but these pages should be unlocked anytime we want
>>> because we
>>> could allocate all pages on system and lock them, which in turn, it may
>>> result in significant deterioration of system performance.(maybe other
>>> processes requesting free memory would be blocked) so I used
>>> VM_LOCKED flags
>>> instead. but I'm not sure this way is best also.
>>>
>>>> Anyway even not considering that you don't check at all that process
>>>> don't go over the limit of locked page see mm/mlock.c RLIMIT_MEMLOCK
>>>
>>> Thank you for your advices.
>>>
>>>> for how it's done. Also you mlock complete vma but the userptr you get
>>>> might be inside say 16M vma and you only care about 1M of userptr, if
>>>> you mark the whole vma as locked than anytime a new page is fault in
>>>> the vma else where than in the buffer you are interested then it got
>>>> allocated for ever until the gem buffer is destroy, i am not sure of
>>>> what happen to the vma on next malloc if it grows or not (i would
>>>> think it won't grow at it would have different flags than new
>>>> anonymous memory).
>>
>>
>> I don't know history in detail because you didn't have sent full
>> patches to linux-mm and
>> I didn't read the below code, either.
>> Just read your description and reply of Jerome. Apparently, there is
>> something I missed.
>>
>> Your goal is to avoid swap out some user pages which is used in kernel
>> at the same time. Right?
>> Let's use get_user_pages. Is there any issue you can't use it?
> 
> Maybe because get_user_pages() is fork unsafe? dunno.


If there is such problem, I think user program should handle it by MADV_DONTFORK 
and make to allow write by only parent process.

-- 
Kind regards,
Minchan Kim