linux-kernel - Re: [RFC][PATCH] mm: fix page

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Date:	Thu, 08 Mar 2007 13:25:37 +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, hugh@...itas.com, torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH] mm: fix page_mkclean() vs non-linear vmas

> > > And it doesn't sound too hard to solve: when current algorithm doesn't
> > > seem to be making progress, then it will have to be done the hard way,
> > > searching for for all nonlinear ptes of a page to unmap.
> > 
> > Ah, you see, but that is when you've already lost.
> > 
> > The DoS is about the computational complexity of the reclaim, not if it
> > will ever come out of it with free pages.
> 
> If we really want to, we could limit it to mlock for !root. This is
> a reasonable way to solve the problem, and UML could fall back on
> vma emulated version if they didn't want to use mlock memory...
> 
> Or we could limit the size/number of nonlinear vmas that could be
> created.
> 
> But just quietly, I think there are probably a lot of other ways to
> perform a local DoS anyway ;) 

I aggree, requiring apps to mlock would probably just make things
slightly worse for about 100% of users, without any gain.  There could
be a

  /proc/sys/vm/turn_off_nonlinear_for_paranoid_sysadmin

knob that would unconditionally emulate nonlinear vmas.

Miklos
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