Date: Fri, 13 Nov 2015 16:03:56 +0900
From: Minchan Kim <minchan@kernel.org>
To: Daniel Micay <danielmicay@gmail.com>
CC: Andy Lutomirski <luto@amacapital.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Linux API <linux-api@vger.kernel.org>, Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Rik van Riel <riel@redhat.com>,
        Mel Gorman <mgorman@suse.de>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Jason Evans <je@fb.com>, "Kirill A. Shutemov" <kirill@shutemov.name>,
        Shaohua Li <shli@kernel.org>, Michal Hocko <mhocko@suse.cz>,
        yalin wang <yalin.wang2010@gmail.com>
Subject: Re: [PATCH v3 01/17] mm: support madvise(MADV_FREE)
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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.
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