RE: [PATCH 1/6] freepgt: free_pgtables use vma list

["Luck, Tony" <tony.luck@intel.com>]

>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.)

IS_ERR(ptr) and PTR_ERR(ptr) would also yield some interesting bizarre
errors if the last page (last 1000 bytes in the current implementation
of IS_ERR) were valid!

>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.

Yes ... rounding 'end' up to pmd/pud/pgd boundaries can certainly
wrap around to zero ... giving up the last page of address space
seems reasonable.  Giving up the last PGD_SIZE just to make some
math a bit easier sounds like overkill.

-Tony
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