Re: [PATCH 2.6.14-rc2-mm2] core remove PageReserved

From: Nick Piggin
Date: Tue Oct 11 2005 - 08:48:53 EST

Alan Cox wrote:
On Maw, 2005-10-11 at 19:00 +1000, Nick Piggin wrote:

A last caveat: the ZERO_PAGE is now refcounted and managed with rmap
(and thus mapcounted and count towards shared rss).

32000 processes each with 2G mapped as zero pages appears to allow the
refcount to overflow ?

That's right (though I count only 8192 required with 4K page size) -
close to impossible on 32-bit architectures, though not so the 64-bit
ones, which still use 32-bits for count and mapcount.

I was a bit worried about this too, but Hugh didn't think it was a
really big a deal - I guess because the real solution for the refcount
overflow on 64-bit is to expand the refcount type.

Note also that we can exclude ZERO_PAGE from being refcounted, which
may be desirable for a scalability perspective on big NUMA machines.

The aim with this patch is to provide a patch which is as simple as
possible in order to get the mechanism right. But you raise a valid
concern and we do need to discuss these peripheral issues and sort
them out before it gets merged upstream.


SUSE Labs, Novell Inc.

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