[LMB][2/2] Restructure allocation loops to avoid unsigned underflow

[David Miller]

From: Paul Mackerras <paulus at samba.org>
Date: Sat, 12 Apr 2008 15:20:59 +1000

> There is a potential bug in __lmb_alloc_base where we subtract `size'
> from the base address of a reserved region without checking whether
> the subtraction could wrap around and produce a very large unsigned
> value.  In fact it probably isn't possible to hit the bug in practice
> since it would only occur in the situation where we can't satisfy the
> allocation request and there is a reserved region starting at 0.
> 
> This fixes the potential bug by breaking out of the loop when we get
> to the point where the base of the reserved region is less than the
> size requested.  This also restructures the loop to be a bit easier to
> follow. 
> 
> The same logic got copied into lmb_alloc_nid_unreserved, so this makes
> a similar change there.  Here the bug is more likely to be hit because
> the outer loop  (in lmb_alloc_nid) goes through the memory regions in
> increasing order rather than decreasing order as __lmb_alloc_base
> does, and we are therefore more likely to hit the case where we are
> testing against a reserved region with a base address of 0.
> 
> Signed-off-by: Paul Mackerras <paulus at samba.org>

This looks great, thanks for doing this work Paul.

I'll try to find some cycles to validate these changes alongside
the pending sparc64 NUMA changes I have.