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

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Message-ID: <4CE49C8B.2050005@redhat.com>
Date:	Wed, 17 Nov 2010 22:24:59 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Minchan Kim <minchan.kim@...il.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 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.

> 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.

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