[lkml]   [2005]   [Oct]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 2.6.14-rc2-mm2] core remove PageReserved
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

SUSE Labs, Novell Inc.

Send instant messages to your online friends

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2005-10-11 16:42    [W:0.033 / U:3.620 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site