linux-kernel - Re: [PATCH v3 01/17] mm: support madvise(MADV

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Date:	Fri, 13 Nov 2015 03:13:14 -0500
From:	Daniel Micay <danielmicay@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jason Evans <je@...com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Shaohua Li <shli@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	yalin wang <yalin.wang2010@...il.com>
Subject: Re: [PATCH v3 01/17] mm: support madvise(MADV_FREE)

On 13/11/15 02:03 AM, Minchan Kim wrote:
> On Fri, Nov 13, 2015 at 01:45:52AM -0500, Daniel Micay wrote:
>>> And now I am thinking if we use access bit, we could implment MADV_FREE_UNDO
>>> easily when we need it. Maybe, that's what you want. Right?
>>
>> Yes, but why the access bit instead of the dirty bit for that? It could
>> always be made more strict (i.e. access bit) in the future, while going
>> the other way won't be possible. So I think the dirty bit is really the
>> more conservative choice since if it turns out to be a mistake it can be
>> fixed without a backwards incompatible change.
> 
> Absolutely true. That's why I insist on dirty bit until now although
> I didn't tell the reason. But I thought you wanted to change for using
> access bit for the future, too. It seems MADV_FREE start to bloat
> over and over again before knowing real problems and usecases.
> It's almost same situation with volatile ranges so I really want to
> stop at proper point which maintainer should decide, I hope.
> Without it, we will make the feature a lot heavy by just brain storming
> and then causes lots of churn in MM code without real bebenfit
> It would be very painful for us.

Well, I don't think you need more than a good API and an implementation
with no known bugs, kernel security concerns or backwards compatibility
issues. Configuration and API extensions are something for later (i.e.
land a baseline, then submit stuff like sysctl tunables). Just my take
on it though...


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