linux-kernel - Re: [RFC] [PATCH 0/5 V2] Huge page backed user-space stacks

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Message-ID: <20080805162800.GJ20243@csn.ul.ie>
Date:	Tue, 5 Aug 2008 17:28:00 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, ebmunson@...ibm.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, libhugetlbfs-devel@...ts.sourceforge.net,
	abh@...y.com
Subject: Re: [RFC] [PATCH 0/5 V2] Huge page backed user-space stacks

On (05/08/08 09:12), Dave Hansen didst pronounce:
> On Tue, 2008-08-05 at 12:11 +0100, Mel Gorman wrote:
> > See, that's great until you start dealing with MAP_SHARED|MAP_ANONYMOUS.
> > To get that right between children, you end up something very fs-like
> > when the child needs to fault in a page that is already populated by the
> > parent. I strongly suspect we end up back at hugetlbfs backing it :/
> 
> Yeah, but the case I'm worried about is plain anonymous.  We already
> have the fs to back SHARED|ANONYMOUS, and they're not really
> anonymous. :)
> 
> This patch *really* needs anonymous pages, and it kinda shoehorns them
> in with the filesystem.  Stacks aren't shared at all, so this is a
> perfect example of where we can forget the fs, right?
> 

Ok sure, you could do direct inserts for MAP_PRIVATE as conceptually it
suits this patch.  However, I don't see what you gain. By reusing hugetlbfs,
we get things like proper reservations which we can do for MAP_PRIVATE these
days. Again, we could call that sort of thing directly if the reservation
layer was split out separate from hugetlbfs but I still don't see the gain
for all that churn.

What am I missing?

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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