Messages in this thread | | | Date | Wed, 12 Oct 2005 00:36:37 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 2.6.14-rc2-mm2] core remove PageReserved |
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Hugh Dickins wrote: > 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? >
Considering we already run on 64-bit systems with terabytes of memory and people don't seem to be breaking down your door for a fix (that I know of), I'd say no :)
> >>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. >
Right. As a security issue it is nothing new, though probably it will be eaiser for big 64-bit systems to _unintentionally_ wrap the ZERO_PAGE refcount.
-- SUSE Labs, Novell Inc.
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