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

[Hugh Dickins <hugh@veritas.com>]

On Tue, 11 Oct 2005, Nick Piggin wrote:
> Alan Cox wrote:
> > 
> > 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.

It needs 16GB of page table space to get the mapcount to go negative,
or if we refine the atomic_add_negative test, 32GB of page table space
to wrap (I'm assuming an 8-byte PAE page table entries for each unit
of mapcount, since we're well beyond the 4GB non-PAE limit).

Do we actually need to worry about i386 above 32GB in the x86_64 era?

> 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.

Yes, and I'm imagining some scheme of sharing _count and _mapcount in
a single atomic64, since we don't want to expand struct page for this.
Implement that shortly, unless we find a way to eliminate _mapcount
instead.

But Alan's overflow issue is not new, it's not brought on refcounting
the ZERO_PAGE: sys_remap_file_pages already allows a file page to be
mapped multiple times in single process, without the constriction of
needing lots of vm_area_struct space.  ZERO_PAGE is just more obvious.

Hugh