[PATCH v4 4/4] mm: Mark pages in use for page tables

[Matthew Wilcox <willy@infradead.org>]

From: Matthew Wilcox <mawilcox@microsoft.com>

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.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
---
 arch/tile/mm/pgtable.c                 | 3 +++
 fs/proc/page.c                         | 2 ++
 include/linux/mm.h                     | 2 ++
 include/linux/page-flags.h             | 6 ++++++
 include/uapi/linux/kernel-page-flags.h | 1 +
 tools/vm/page-types.c                  | 1 +
 6 files changed, 15 insertions(+)

diff --git a/arch/tile/mm/pgtable.c b/arch/tile/mm/pgtable.c
index ec5576fd3a86..6dff12db335d 100644
--- a/arch/tile/mm/pgtable.c
+++ b/arch/tile/mm/pgtable.c
@@ -206,6 +206,7 @@ struct page *pgtable_alloc_one(struct mm_struct *mm, unsigned long address,
 	 */
 	for (i = 1; i < order; ++i) {
 		init_page_count(p+i);
+		__SetPageTable(p+i);
 		inc_zone_page_state(p+i, NR_PAGETABLE);
 	}
 
@@ -226,6 +227,7 @@ void pgtable_free(struct mm_struct *mm, struct page *p, int order)
 
 	for (i = 1; i < order; ++i) {
 		__free_page(p+i);
+		__ClearPageTable(p+i);
 		dec_zone_page_state(p+i, NR_PAGETABLE);
 	}
 }
@@ -240,6 +242,7 @@ void __pgtable_free_tlb(struct mmu_gather *tlb, struct page *pte,
 
 	for (i = 1; i < order; ++i) {
 		tlb_remove_page(tlb, pte + i);
+		__ClearPageTable(pte + i);
 		dec_zone_page_state(pte + i, NR_PAGETABLE);
 	}
 }
diff --git a/fs/proc/page.c b/fs/proc/page.c
index c9757af919a3..80275e7a963b 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -156,6 +156,8 @@ u64 stable_page_flags(struct page *page)
 		u |= 1 << KPF_BALLOON;
 	if (PageVmalloc(page))
 		u |= 1 << KPF_VMALLOC;
+	if (PageTable(page))
+		u |= 1 << KPF_PGTABLE;
 
 	if (page_is_idle(page))
 		u |= 1 << KPF_IDLE;
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ad06d42adb1a..7a15042d6828 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1829,6 +1829,7 @@ static inline bool pgtable_page_ctor(struct page *page)
 {
 	if (!ptlock_init(page))
 		return false;
+	__SetPageTable(page);
 	inc_zone_page_state(page, NR_PAGETABLE);
 	return true;
 }
@@ -1836,6 +1837,7 @@ static inline bool pgtable_page_ctor(struct page *page)
 static inline void pgtable_page_dtor(struct page *page)
 {
 	pte_lock_deinit(page);
+	__ClearPageTable(page);
 	dec_zone_page_state(page, NR_PAGETABLE);
 }
 
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 8142ab716e90..ac6bab90849c 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -644,6 +644,7 @@ PAGEFLAG_FALSE(DoubleMap)
 #define PG_balloon	0x00000100
 #define PG_kmemcg	0x00000200
 #define PG_vmalloc	0x00000400
+#define PG_table	0x00000800
 
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
@@ -687,6 +688,11 @@ PAGE_TYPE_OPS(Kmemcg, kmemcg)
  */
 PAGE_TYPE_OPS(Vmalloc, vmalloc)
 
+/*
+ * Marks pages in use as page tables.
+ */
+PAGE_TYPE_OPS(Table, table)
+
 extern bool is_free_buddy_page(struct page *page);
 
 __PAGEFLAG(Isolated, isolated, PF_ANY);
diff --git a/include/uapi/linux/kernel-page-flags.h b/include/uapi/linux/kernel-page-flags.h
index 5f1735ff05b3..3c51d8bf8b7b 100644
--- a/include/uapi/linux/kernel-page-flags.h
+++ b/include/uapi/linux/kernel-page-flags.h
@@ -36,5 +36,6 @@
 #define KPF_ZERO_PAGE		24
 #define KPF_IDLE		25
 #define KPF_VMALLOC		26
+#define KPF_PGTABLE		27
 
 #endif /* _UAPILINUX_KERNEL_PAGE_FLAGS_H */
diff --git a/tools/vm/page-types.c b/tools/vm/page-types.c
index 116f59eff5e2..bbb992694f05 100644
--- a/tools/vm/page-types.c
+++ b/tools/vm/page-types.c
@@ -132,6 +132,7 @@ static const char * const page_flag_names[] = {
 	[KPF_THP]		= "t:thp",
 	[KPF_BALLOON]		= "o:balloon",
 	[KPF_VMALLOC]		= "V:vmalloc",
+	[KPF_PGTABLE]		= "g:pgtable",
 	[KPF_ZERO_PAGE]		= "z:zero_page",
 	[KPF_IDLE]              = "i:idle_page",
 
-- 
2.16.1