linux-kernel - Re: fadvise DONTNEED implementation (or lack thereof)

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Message-ID: <AANLkTi=4DbR8bX3VX=Wymo6Tk7yyR_=BEmJx+WY_0iRs@mail.gmail.com>
Date:	Thu, 18 Nov 2010 12:46:44 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ben Gamari <bgamari.foss@...il.com>,
	linux-kernel@...r.kernel.org, rsync@...ts.samba.org,
	linux-mm@...ck.org, Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: fadvise DONTNEED implementation (or lack thereof)

On Thu, Nov 18, 2010 at 12:24 PM, Rik van Riel <riel@...hat.com> wrote:
> On 11/17/2010 09:47 PM, Minchan Kim wrote:
>>
>> On Thu, Nov 18, 2010 at 1:22 AM, Rik van Riel<riel@...hat.com>  wrote:
>>>
>>> On 11/17/2010 05:16 AM, Minchan Kim wrote:
>>>
>>>> Absolutely. But how about rsync's two touch?
>>>> It can evict working set.
>>>>
>>>> I need the time for investigation.
>>>> Thanks for the comment.
>>>
>>> Maybe we could exempt MADV_SEQUENTIAL and FADV_SEQUENTIAL
>>> touches from promoting the page to the active list?
>>>
>>
>> The problem is non-mapped file page.
>> non-mapped file page promotion happens by only mark_page_accessed.
>> But it doesn't enough information to prevent promotion(ex, vma or file)
>
> I believe we have enough information in filemap.c and can just
> pass that as a parameter to mark_page_accessed.

FADV_SEQUENTIAL is per file/vma semantic and It is used by many place.
I think changing all those places isn't simple and I don't want to add
new structure to propagate the information to mark_page_accessed.

>
>> Here is another idea.
>> Current problem is following as.
>> User can use fadivse with FADV_DONTNEED.
>> But problem is that it can't affect when it meet dirty pages.
>> So user have to sync dirty page before calling fadvise with FADV_DONTNEED.
>> It would lose performance.
>>
>> Let's add some semantic of FADV_DONTNEED.
>> It invalidates only pages which are not dirty.
>> If it meets dirty page, let's move the page into inactive's tail or head.
>> If we move the page into tail, shrinker can move it into head again
>> for deferred write if it isn't written the backed device.
>
> That sounds like a good idea.

I will implement it.
Thanks, Rik.

>
> --
> All rights reversed
>



-- 
Kind regards,
Minchan Kim
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