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

[Nick Piggin]

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.

> > > What is the DoS scenario wrt reclaim? We really ought to fix that if
> > > real, those UML farms run on nothing but nonlinear reclaim I'd think.
> > 
> > I guess you can just increase the computational complexity of
> > reclaim quite easily.
> 
> Right, on first glance it doesn't look to be too bad, but I should take
> a closer look.

Well I don't think UML uses nonlinear yet anyway, does it? Can they
make do with restricting nonlinear to mlocked vmas, I wonder? Probably
not.

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