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

[Kyungmin Park]

On 5/10/12, Minchan Kim <minchan at kernel.org> wrote:
> 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.
Please read the original patches and discuss the root cause. Does it
harm to pass user space memory to kernel space and how to make is
possible at DRM?

Thank you,
Kyungmin Park
>
> --
> Kind regards,
> Minchan Kim
>
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