On Wed, 23 Mar 2005, David S. Miller wrote:
> On Wed, 23 Mar 2005 11:16:27 -0800
> "Luck, Tony" <[email protected]> wrote:
>
> > Can we legislate that "end==0" isn't possible.
>
> It is possible with some out-of-tree patches.
>
> I could certainly support it on sparc64, the only
> glitch is that this is where the virtually mapped
> linear page tables sit right now :-)
Though my knowledge of out-of-tree patches is very limited,
I believe "end == 0" is not possible on any arch - when "end"
originates from vma->vm_end (or vm_struct->addr + size). There
are plenty of "BUG_ON(addr >= end)"s dotted around to support that,
and other places that would be confused by vm_start > vm_end.
(And when Linus first proposed the sysenter page at 0xfffff000,
I protested, and he brought it down to 0xffffe000: I think we'll
do well ever to keep that last virtual page invalid.)
But certainly "ceiling == 0" is possible and common, and "rounded-up
end" may well be 0 with out-of-tree patches. When I did those
free_pgtables tests, it seemed simpler to treat "end" in the same
way as "ceiling", implicitly allowing it the 0 case. Perhaps
that's not so in Nick's version, I've yet to think through it.
Hugh
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