Date: Fri, 13 Nov 2015 15:38:02 +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)
Message-ID: <20151113063802.GF5235@bbox>
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On Fri, Nov 13, 2015 at 01:16:54AM -0500, Daniel Micay wrote:
> On 13/11/15 01:15 AM, Minchan Kim wrote:
> > On Thu, Nov 12, 2015 at 12:21:30AM -0500, Daniel Micay wrote:
> >>> I also think that the kernel should commit to either zeroing the page
> >>> or leaving it unchanged in response to MADV_FREE (even if the decision
> >>> of which to do is made later on).  I think that your patch series does
> >>> this, but only after a few of the patches are applied (the swap entry
> >>> freeing), and I think that it should be a real guaranteed part of the
> >>> semantics and maybe have a test case.
> >>
> >> This would be a good thing to test because it would be required to add
> >> MADV_FREE_UNDO down the road. It would mean the same semantics as the
> >> MEM_RESET and MEM_RESET_UNDO features on Windows, and there's probably
> >> value in that for the sake of migrating existing software too.
> > 
> > So, do you mean that we could implement MADV_FREE_UNDO with "read"
> > opearation("just access bit marking) easily in future?
> > 
> > If so, it would be good reason to change MADV_FREE from dirty bit to
> > access bit. Okay, I will look at that.
> 
> I just meant testing that the data is either zero or the old data if
> it's read before it's written to. Not having it stay around once there
> is a read. Not sure if that's what Andy meant.

Either zero of old data is gauranteed.
Now:

        MADV_FREE(range)
        A = read from the range
        ...
        ...
        B = read from the range


        A and B could have different value. But value should be old or zero.

But Andy want more strict ABI so he suggested access bit instead of dirty bit.

        MADV_FREE(range)
        A = read from the range
        ...
        ...
        B = read from the range

        A and B cannot have different value.

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?
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