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

[Minchan Kim <minchan@kernel.org>]

On 05/10/2012 04:56 PM, Minchan Kim wrote:

> On 05/10/2012 04:31 PM, Kyungmin Park wrote:
> 
>> On 5/10/12, Minchan Kim <minchan@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@gmail.com]
>>>>>>> Sent: Wednesday, May 09, 2012 11:46 PM
>>>>>>> To: Inki Dae
>>>>>>> Cc: airlied@linux.ie; dri-devel@lists.freedesktop.org;
>>>>>>> kyungmin.park@samsung.com; sw0312.kim@samsung.com; linux-mm@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@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@samsung.com>
>>>>>>>> Signed-off-by: Kyungmin Park<kyungmin.park@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?
> 
> 
> Where can I read original discussion history?
> I am not expert of DRAM so I can answer only mm stuff and it's why Jerome ccing mm-list.

                     ^^^ silly typo
                     DRM

-- 
Kind regards,
Minchan Kim

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