mm: numa: do not clear PMD during PTE update scan

commit 5a6dac3e upstream.

If the PMD is flushed then a parallel fault in handle_mm_fault() will
enter the pmd_none and do_huge_pmd_anonymous_page() path where it'll
attempt to insert a huge zero page.  This is wasteful so the patch
avoids clearing the PMD when setting pmd_numa.
Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Alex Thorlton <athorlton@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

mm/huge_memory.c

@@ -1474,20 +1474,22 @@ 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) {
+			entry = pmdp_get_and_clear(mm, addr, pmd);
 			entry = pmd_modify(entry, newprot);
 			BUG_ON(pmd_write(entry));
+			set_pmd_at(mm, addr, pmd, entry);
 		} else {
 			struct page *page = pmd_page(*pmd);
+			entry = *pmd;
 
 			/* only check non-shared pages */
 			if (page_mapcount(page) == 1 &&
 			    !pmd_numa(*pmd)) {
 				entry = pmd_mknuma(entry);
+				set_pmd_at(mm, addr, pmd, entry);
 			}
 		}
-		set_pmd_at(mm, addr, pmd, entry);
 		spin_unlock(&vma->vm_mm->page_table_lock);
 		ret = 1;
 	}