mm/page_table_check: fix crash on ZONE_DEVICE

Not all pages may apply to pgtable check.  One example is ZONE_DEVICE
pages: they map PFNs directly, and they don't allocate page_ext at all
even if there's struct page around.  One may reference
devm_memremap_pages().

When both ZONE_DEVICE and page-table-check enabled, then try to map some
dax memories, one can trigger kernel bug constantly now when the kernel
was trying to inject some pfn maps on the dax device:

 kernel BUG at mm/page_table_check.c:55!

While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page
fault resolutions, skip all the checks if page_ext doesn't even exist in
pgtable checker, which applies to ZONE_DEVICE but maybe more.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: df4e817 ("mm: page table check")
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: Pasha Tatashin <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Reviewed-by: Alistair Popple <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/page_table_check.c

@@ -73,6 +73,9 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
 
+	if (!page_ext)
+		return;
+
 	BUG_ON(PageSlab(page));
 	anon = PageAnon(page);
 
@@ -110,6 +113,9 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
 	page = pfn_to_page(pfn);
 	page_ext = page_ext_get(page);
 
+	if (!page_ext)
+		return;
+
 	BUG_ON(PageSlab(page));
 	anon = PageAnon(page);
 
@@ -140,7 +146,10 @@ void __page_table_check_zero(struct page *page, unsigned int order)
 	BUG_ON(PageSlab(page));
 
 	page_ext = page_ext_get(page);
-	BUG_ON(!page_ext);
+
+	if (!page_ext)
+		return;
+
 	for (i = 0; i < (1ul << order); i++) {
 		struct page_table_check *ptc = get_page_table_check(page_ext);