Date:	Mon, 07 Oct 2013 17:18:40 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Minchan Kim <minchan@...nel.org>, "H. Peter Anvin" <hpa@...or.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Andrea Righi <andrea@...terlinux.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	Dhaval Giani <dhaval.giani@...il.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Rob Clark <robdclark@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 05/14] vrange: Add new vrange(2) system call

On 10/07/2013 05:13 PM, Minchan Kim wrote:
> Hello Peter,
>
> On Mon, Oct 07, 2013 at 04:59:40PM -0700, H. Peter Anvin wrote:
>> On 10/07/2013 04:54 PM, John Stultz wrote:
>>>> And wouldn't this apply to MADV_DONTNEED just as well?  Perhaps what we
>>>> should do is an enhanced madvise() call?
>>> Well, I think MADV_DONTNEED doesn't *have* do to anything at all. Its
>>> advisory after all. So it may immediately wipe out any data, but it may not.
>>>
>>> Those advisory semantics work fine w/ VRANGE_VOLATILE. However,
>>> VRANGE_NONVOLATILE is not quite advisory, its telling the system that it
>>> requires the memory at the specified range to not be volatile, and we
>>> need to correctly inform userland how much was changed and if any of the
>>> memory we did change to non-volatile was purged since being set volatile.
>>>
>>> In that way it is sort of different from madvise. Some sort of an
>>> madvise2 could be done, but then the extra purge state argument would be
>>> oddly defined for any other mode.
>>>
>>> Is your main concern here just wanting to have a zero-fill mode with
>>> volatile ranges? Or do you really want to squeeze this in to the madvise
>>> call interface?
>> The point is that MADV_DONTNEED is very similar in that sense,
>> especially if allowed to be lazy.  It makes a lot of sense to permit
>> both scrubbing modes orthogonally.
>>
>> The point you're making has to do with withdrawal of permission to flush
>> on demand, which is a result of having the lazy mode (ongoing
>> permission) and having to be able to withdraw such permission.
> I'm sorry I could not understand what you wanted to say.
> Could you elaborate a bit?
My understanding of his point is that VRANGE_VOLATILE is like a lazy
MADV_DONTNEED (with sigbus, rather then zero fill on fault), suggests
that we should find a way to have VRANGE_VOLATILE be something like
MADV_DONTNEED|MADV_LAZY|MADV_SIGBUS_FAULT, instead of adding a new
syscall.  This would provide more options, since one could instead just
do MADV_DONTNEED|MADV_LAZY if they wanted zero-fill faults.

And indeed, for the VRANGE_VOLATILE case, we could do something like
that, but the unresolved problem I see is that that we still need to
handle the VRANGE_NONVOLATILE case, and the madvise() interface doesn't
seem to accomodate the needed semantics well.

thanks
-john


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