On Wed, Aug 24, 2011 at 12:13:21AM +0200, Andrea Arcangeli wrote:

Hi Andrew,

On Tue, Aug 23, 2011 at 02:14:45PM -0700, Andrew Morton wrote:

quoted

quoted

+	if ((old_addr & ~HPAGE_PMD_MASK) ||
+	    (new_addr & ~HPAGE_PMD_MASK) ||
+	    (old_addr + HPAGE_PMD_SIZE) > old_end ||

Can (old_addr + HPAGE_PMD_SIZE) wrap past zero?

Good question. old_addr is hpage aligned so to overflow it'd need to
be exactly at address 0-HPAGE_PMD_SIZE. Can any userland map an
address there? I doubt and surely not x86* or sparc (currently THP is
only enabled on x86 anyway so answer is it can't wrap past zero). But
probably we should add a wrap check for other archs in the future
unless we have a real guarantee from all archs to avoid the check. I
only can guarantee about x86*.

I suggest to incorporate this into
thp-mremap-support-and-tlb-optimization-fix.patch (it's only for the
future but it's zero cost so it's certainly worth it...). Untested...

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c

@@ -1066,7 +1066,7 @@ int move_huge_pmd(struct vm_area_struct 
 
 	if ((old_addr & ~HPAGE_PMD_MASK) ||
 	    (new_addr & ~HPAGE_PMD_MASK) ||
-	    (old_addr + HPAGE_PMD_SIZE) > old_end ||
+	    old_end - old_addr < HPAGE_PMD_SIZE ||
 	    (new_vma->vm_flags & VM_NOHUGEPAGE))
 		goto out;
 

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