mm: mark pages in use for page tables

Define a new PageTable bit in the page_type and use it to mark pages in
use as page tables. This can be helpful when debugging crashdumps or
analysing memory fragmentation. Add a KPF flag to report these pages to
userspace and update page-types.c to interpret that flag.

Note that only pages currently accounted as NR_PAGETABLES are tracked as
PageTable; this does not include pgd/p4d/pud/pmd pages. Those will be the
subject of a later patch.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Matthew Wilcox <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Jérôme Glisse <[e-mail]>
Cc: Lai Jiangshan <[e-mail]>
Cc: Martin Schwidefsky <[e-mail]>
Cc: Pekka Enberg <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Andrey Ryabinin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/page.c

···
 
 	if (PageBalloon(page))
 		u |= 1 << KPF_BALLOON;
+	if (PageTable(page))
+		u |= 1 << KPF_PGTABLE;
 
 	if (page_is_idle(page))
 		u |= 1 << KPF_IDLE;

include/linux/mm.h

···
 {
 	if (!ptlock_init(page))
 		return false;
+	__SetPageTable(page);
 	inc_zone_page_state(page, NR_PAGETABLE);
 	return true;
 }
···
 static inline void pgtable_page_dtor(struct page *page)
 {
 	pte_lock_deinit(page);
+	__ClearPageTable(page);
 	dec_zone_page_state(page, NR_PAGETABLE);
 }
 

include/linux/page-flags.h

···
 #define PG_buddy	0x00000080
 #define PG_balloon	0x00000100
 #define PG_kmemcg	0x00000200
+#define PG_table	0x00000400
 
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
···
  * pages allocated with __GFP_ACCOUNT. It gets cleared on page free.
  */
 PAGE_TYPE_OPS(Kmemcg, kmemcg)
+
+/*
+ * Marks pages in use as page tables.
+ */
+PAGE_TYPE_OPS(Table, table)
 
 extern bool is_free_buddy_page(struct page *page);
 

include/uapi/linux/kernel-page-flags.h

···
 #define KPF_BALLOON		23
 #define KPF_ZERO_PAGE		24
 #define KPF_IDLE		25
-
+#define KPF_PGTABLE		26
 
 #endif /* _UAPILINUX_KERNEL_PAGE_FLAGS_H */

tools/vm/page-types.c

···
 	[KPF_KSM]		= "x:ksm",
 	[KPF_THP]		= "t:thp",
 	[KPF_BALLOON]		= "o:balloon",
+	[KPF_PGTABLE]		= "g:pgtable",
 	[KPF_ZERO_PAGE]		= "z:zero_page",
 	[KPF_IDLE]              = "i:idle_page",