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

From: Minchan Kim <minchan@kernel.org>
To: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Inki Dae <inki.dae@samsung.com>, 'InKi Dae' <daeinki@gmail.com>,
	'Jerome Glisse' <j.glisse@gmail.com>,
	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.
Date: Fri, 11 May 2012 09:50:06 +0900	[thread overview]
Message-ID: <4FAC623E.7090209@kernel.org> (raw)
In-Reply-To: <4FAC0091.7070606@gmail.com>

Hi KOSAKI,

On 05/11/2012 02:53 AM, KOSAKI Motohiro wrote:

>>>> 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? :)
>>>
>>
>> Simply saying, handle_mm_fault would be called to update pte after
>> finding
>> vma and checking access right. and as you know, there are many cases to
>> process page fault such as COW or demand paging.
> 
> Hmm. If I understand correctly, you guys misunderstand mlock. it doesn't
> page pinning
> nor prevent pfn change. It only guarantee to don't make swap out. e.g.

Symantic point of view, you're right but the implementation makes sure page pinning.

> memory campaction
> feature may automatically change page physical address.

I tried it last year but decided drop by realtime issue.
https://lkml.org/lkml/2011/8/29/295

so I think mlock is a kind of page pinning. If elsewhere I don't realized is doing, that place should be fixed.
Or my above patch should go ahead.

> 
> 
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-- 
Kind regards,
Minchan Kim

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