linux-kernel - Re: [RFC v3] Support volatile range for anon vma

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Date:	Wed, 12 Dec 2012 08:21:01 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Arun Sharma <asharma@...com>, sanjay@...gle.com,
	Paul Turner <pjt@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	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@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC v3] Support volatile range for anon vma

Hi John,

On Tue, Dec 11, 2012 at 10:45:27AM -0800, John Stultz wrote:
> On 12/10/2012 06:34 PM, Minchan Kim wrote:
> >This still is [RFC v3] because just passed my simple test
> >with TCMalloc tweaking.
> >
> >I hope more inputs from user-space allocator people and test patch
> >with their allocator because it might need design change of arena
> >management design for getting real vaule.
> >
> >Changelog from v2
> >
> >  * Removing madvise(addr, length, MADV_NOVOLATILE).
> >  * add vmstat about the number of discarded volatile pages
> >  * discard volatile pages without promotion in reclaim path
> >
> >This is based on v3.6.
> >
> >- What's the madvise(addr, length, MADV_VOLATILE)?
> >
> >   It's a hint that user deliver to kernel so kernel can *discard*
> >   pages in a range anytime.
> >
> >- What happens if user access page(ie, virtual address) discarded
> >   by kernel?
> >
> >   The user can see zero-fill-on-demand pages as if madvise(DONTNEED).
> >
> >- What happens if user access page(ie, virtual address) doesn't
> >   discarded by kernel?
> >
> >   The user can see old data without page fault.
> >
> >- What's different with madvise(DONTNEED)?
> >
> >   System call semantic
> >
> >   DONTNEED makes sure user always can see zero-fill pages after
> >   he calls madvise while VOLATILE can see zero-fill pages or
> >   old data.
> I still need to really read and understand the patch, but at a high
> level I'm not sure how this works. So does the VOLATILE flag get
> cleared on any access, even if the pages have not been discarded?

No. It is cleared when user try to access discareded pages so
This patch is utter crap. I missed that point.
Thanks for pointing out, John.

Hmm, in the end, we need NOVOLATILE.

> What happens if an application wants to store non-volatile data in
> an area that was once marked volatile. If there was never memory
> pressure, it seems the volatility would persist with no way of
> removing it.

Yes. that's why this patch is crap and I'm insane. :(

> 
> Either way, I feel that with this revision, specifically dropping
> the NOVOLATILE call and the SIGBUS optimization the Mozilla folks
> suggested, your implementation has drifted quite far from the
> concept I'm pushing. While I hope we can still align the underlying
> mm implementation, I might ask that you use a different term for the
> semantics you propose, so we don't add too much confusion to the
> discussion.
> 
> Maybe you could call it DONTNEED_DEFERRED or something?
> 
> In the meantime, I'll be reading your patch in detail and seeing how
> we might be able to combine our differing approaches.

You don't need it. Ignore this patch.
I will rework.

Thanks.

> 
> thanks
> -john
> 
> --
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-- 
Kind regards,
Minchan Kim
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