linux-kernel - Re: [PATCH 0/4] Volatile Ranges (v14

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Date:	Tue, 3 Jun 2014 10:57:10 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	John Stultz <john.stultz@...aro.org>
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@...1.net>,
	Rik van Riel <riel@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Neil Brown <neilb@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Keith Packard <keithp@...thp.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 0/4] Volatile Ranges (v14 - madvise reborn edition!)

On Thu, May 08, 2014 at 10:12:40AM -0700, John Stultz wrote:
> On 04/29/2014 02:21 PM, John Stultz wrote:
> > Another few weeks and another volatile ranges patchset...
> >
> > After getting the sense that the a major objection to the earlier
> > patches was the introduction of a new syscall (and its somewhat
> > strange dual length/purged-bit return values), I spent some time
> > trying to rework the vma manipulations so we can be we won't fail
> > mid-way through changing volatility (basically making it atomic).
> > I think I have it working, and thus, there is no longer the
> > need for a new syscall, and we can go back to using madvise()
> > to set and unset pages as volatile.
> 
> Johannes: To get some feedback, maybe I'll needle you directly here a
> bit. :)
> 
> Does moving this interface to madvise help reduce your objections?  I
> feel like your cleaning-the-dirty-bit idea didn't work out, but I was
> hoping that by reworking the vma manipulations to be atomic, we could
> move to madvise and still avoid the new syscall that you seemed bothered
> by. But I've not really heard much from you recently so I worry your
> concerns on this were actually elsewhere, and I'm just churning the
> patch needlessly.

My objection was not the syscall.

>From a reclaim perspective, using the dirty state to denote whether a
swap-backed page needs writeback before reclaim is quite natural and I
much prefer Minchan's changes to the reclaim code over yours.

>From an interface point of view, I would prefer the simplicity of
cleaning dirty bits to invalidate pages, and a default of zero-filling
invalidated pages instead of sending SIGBUS.  This also is quite
natural when you think of anon/shmem mappings as cache pages on top of
/dev/zero (see mmap_zero() and shmem_zero_setup()).  And it translates
well to tmpfs.

At the same time, I acknowledge that there are usecases that want
SIGBUS delivery for more than just convenience in order to implement
userspace fault handling, and this is the only place where I see a
real divergence in actual functionality from Minchan's code.

That, however, truly is a separate virtual memory feature.  Would it
be possible for you to take MADV_FREE and MADV_REVIVE as a base and
implement an madvise op that switches the no-page behavior of a VMA
from zero-filling to SIGBUS delivery?
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