[PATCH] fix NUMA interleaving for huge pages

Since vma->vm_pgoff is in units of smallpages, VMAs for huge pages have the
lower HPAGE_SHIFT - PAGE_SHIFT bits always cleared, which results in badd
offsets to the interleave functions. Take this difference from small pages
into account when calculating the offset. This does add a 0-bit shift into
the small-page path (via alloc_page_vma()), but I think that is negligible.
Also add a BUG_ON to prevent the offset from growing due to a negative
right-shift, which probably shouldn't be allowed anyways.

Tested on an 8-memory node ppc64 NUMA box and got the interleaving I
expected.

Signed-off-by: Nishanth Aravamudan <[e-mail]>
Signed-off-by: Adam Litke <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Acked-by: Christoph Lameter <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/mempolicy.c

···
 	if (vma) {
 		unsigned long off;
 
-		off = vma->vm_pgoff;
+		/*
+		 * for small pages, there is no difference between
+		 * shift and PAGE_SHIFT, so the bit-shift is safe.
+		 * for huge pages, since vm_pgoff is in units of small
+		 * pages, we need to shift off the always 0 bits to get
+		 * a useful offset.
+		 */
+		BUG_ON(shift < PAGE_SHIFT);
+		off = vma->vm_pgoff >> (shift - PAGE_SHIFT);
 		off += (addr - vma->vm_start) >> shift;
 		return offset_il_node(pol, vma, off);
 	} else