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

Mel Gorman
2008-08-05T16:28:00Z

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