mm: fix kernel BUG at huge_memory.c:1474!

Andrea's autonuma-benchmark numa01 hits kernel BUG at huge_memory.c:1474!
in change_huge_pmd called from change_protection from change_prot_numa
from task_numa_work.

That BUG, introduced in the huge zero page commit cad7f613 ("thp:
change_huge_pmd(): make sure we don't try to make a page writable")
was trying to verify that newprot never adds write permission to an
anonymous huge page; but Automatic NUMA Balancing's 4b10e7d5 ("mm:
mempolicy: Implement change_prot_numa() in terms of change_protection()")
adds a new prot_numa path into change_huge_pmd(), which makes no use of
the newprot provided, and may retain the write bit in the pmd.

Just move the BUG_ON(pmd_write(entry)) up into the !prot_numa block.
Signed-off-by: NHugh Dickins <hughd@google.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -1460,9 +1460,10 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 	if (__pmd_trans_huge_lock(pmd, vma) == 1) {
 		pmd_t entry;
 		entry = pmdp_get_and_clear(mm, addr, pmd);
-		if (!prot_numa)
+		if (!prot_numa) {
 			entry = pmd_modify(entry, newprot);
-		else {
+			BUG_ON(pmd_write(entry));
+		} else {
 			struct page *page = pmd_page(*pmd);
 
 			/* only check non-shared pages */
@@ -1471,7 +1472,6 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
 				entry = pmd_mknuma(entry);
 			}
 		}
-		BUG_ON(pmd_write(entry));
 		set_pmd_at(mm, addr, pmd, entry);
 		spin_unlock(&vma->vm_mm->page_table_lock);
 		ret = 1;