linux-kernel - Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

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Date:	Wed, 07 Mar 2007 14:53:07 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	npiggin@...e.de
CC:	a.p.zijlstra@...llo.nl, miklos@...redi.hu,
	akpm@...ux-foundation.org, mingo@...e.hu, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, benh@...nel.crashing.org,
	jdike@...toit.com
Subject: Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

> On Wed, Mar 07, 2007 at 02:19:22PM +0100, Peter Zijlstra wrote:
> > On Wed, 2007-03-07 at 14:08 +0100, Nick Piggin wrote:
> > 
> > > > > The thing is, I don't think anybody who uses these things cares
> > > > > about any of the 'problems' you want to fix, do they? We are
> > > > > interested in dirty pages only for the correctness issue, rather
> > > > > than performance. Same as reclaim.
> > > > 
> > > > If so, we can just stick to the dead slow but correct 'scan the full
> > > > vma' page_mkclean() and nobody would ever trigger it.
> > > 
> > > Not if we restricted it to root and mlocked tmpfs. But then why
> > > wouldn't you just do it with the much more efficient msync walk,
> > > so that if root does want to do writeout via these things, it does
> > > not blow up?
> > 
> > This is all used on ram based filesystems right, they all have
> > BDI_CAP_NO_WRITEBACK afaik, so page_mkclean will never get called
> > anyway. Mlock doesn't avoid getting page_mkclean called.
> > 
> > Those who use this on a 'real' filesystem will get hit in the face by a
> > linear scanning page_mkclean(), but AFAIK nobody does this anyway.
> 
> But somebody might do it. I just don't know why you'd want to make
> this _worse_ when the msync option would work?
> 
> > Restricting it to root for such filesystems is unwanted, that'd severely
> > handicap both UML and Oracle as I understand it (are there other users
> > of this feature around?)
> 
> Why? I think they all use tmpfs backings, don't they?
> 
> > msync() might never get called and then we're back with the old
> > behaviour where we can surprise the VM with a ton of dirty pages.
> 
> But we're root. With your patch, root *can't* do nonlinear writeback
> well. Ever. With msync, at least you give them enough rope.

Restricting to root doesn't buy you much, nobody wants to be root.
Restricting to mlock is similarly pointless.  UML _will_ want to get
swapped out if there's no activity.

Restricting to tmpfs makes sense, but it's probably not what UML
wants.

Conclusion: there's no good solution for UML in kernel-space.

Miklos
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