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

[Peter Zijlstra]

On Wed, 2007-03-07 at 14:36 +0100, Nick Piggin wrote:
> 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?

Ooh, you only want to restrict remap_file_pages on mappings from bdi's
without BDI_CAP_NO_WRITEBACK. Sure, I can live with that, and I suspect
others can as well.

> > 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.

True. We could even guesstimate the nonlinear dirty pages by subtracting
the result of page_mkclean() from page_mapcount() and force an
msync(MS_ASYNC) on said mapping (or all (nonlinear) mappings of the
related file) when some threshold gets exceeded.

> > > > 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.

I think it does, but lets ask, Jeff?

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