Re: [PATCH 3 of 3] thp: mremap support and TLB optimization

[Andrea Arcangeli <aarcange@redhat.com>]

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:
> > > +	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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