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

Thu May 10 01:11:24 PDT 2012

On 05/10/2012 04:59 PM, InKi Dae wrote:

> 2012/5/10, Minchan Kim <minchan at kernel.org>:
>> On 05/10/2012 03:57 PM, Inki Dae wrote:
>>
>>>
>>>
>>>> -----Original Message-----
>>>> From: Minchan Kim [mailto:minchan at kernel.org]
>>>> Sent: Thursday, May 10, 2012 1:58 PM
>>>> To: Inki Dae
>>>> Cc: 'Jerome Glisse'; 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 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?
>>>> It increases page count so reclaimer can't swap out page.
>>>> Isn't it enough?
>>>> Marking whole VMA into MLCOKED is overkill.
>>>>
>>>
>>> As I mentioned, we are already using get_user_pages. as you said, this
>>> function increases page count but just only things to the user address
>>> space
>>> cpu already accessed. other would be allocated by page fault hander once
>>> get_user_pages call. if so... ok, after that refcount(page->_count) of
>>> the
>>
>>
>> Not true. Look __get_user_pages.
>> It handles case you mentioned by handle_mm_fault.
>> Do I miss something?
>>
> 
> 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? :)

Thanks.

> of course,if user never access the buffer and requested userptr then

> handle_mm_fault would be called by __get_user_pages. please give me
> any comments if there is my missing point.
> 
> Thanks,
> Inki Dae
> 
> 
>>> pages user already accessed would have 2 and just 1 for other all pages.
>>> so
>>> we may have to consider only pages never accessed by cpu to be locked to
>>> avoid from swapped out.
>>>
>>> Thanks,
>>> Inki Dae
>>>
>>>> --
>>>> Kind regards,
>>>> Minchan Kim
>>>
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>>
>>
>>
>> --
>> Kind regards,
>> Minchan Kim
>>
>> --
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>>
> 
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> 

-- 
Kind regards,
Minchan Kim