mm: convert PG_balloon to PG_offline

PG_balloon was introduced to implement page migration/compaction for
pages inflated in virtio-balloon. Nowadays, it is only a marker that a
page is part of virtio-balloon and therefore logically offline.

We also want to make use of this flag in other balloon drivers - for
inflated pages or when onlining a section but keeping some pages offline
(e.g. used right now by XEN and Hyper-V via set_online_page_callback()).

We are going to expose this flag to dump tools like makedumpfile. But
instead of exposing PG_balloon, let's generalize the concept of marking
pages as logically offline, so it can be reused for other purposes later
on.

Rename PG_balloon to PG_offline. This is an indicator that the page is
logically offline, the content stale and that it should not be touched
(e.g. a hypervisor would have to allocate backing storage in order for
the guest to dump an unused page). We can then e.g. exclude such pages
from dumps.

We replace and reuse KPF_BALLOON (23), as this shouldn't really harm
(and for now the semantics stay the same). In following patches, we
will make use of this bit also in other balloon drivers. While at it,
document PGTABLE.

[[e-mail]: fix comment text, per David]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Acked-by: Konstantin Khlebnikov <[e-mail]>
Acked-by: Michael S. Tsirkin <[e-mail]>
Acked-by: Pankaj gupta <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Christian Hansen <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Pavel Tatashin <[e-mail]>
Cc: Alexander Duyck <[e-mail]>
Cc: Naoya Horiguchi <[e-mail]>
Cc: Miles Chen <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Kazuhito Hagio <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Boris Ostrovsky <[e-mail]>
Cc: Dave Young <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Haiyang Zhang <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: Julien Freche <[e-mail]>
Cc: Kairui Song <[e-mail]>
Cc: "K. Y. Srinivasan" <[e-mail]>
Cc: Len Brown <[e-mail]>
Cc: Lianbo Jiang <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Nadav Amit <[e-mail]>
Cc: Omar Sandoval <[e-mail]>
Cc: Pavel Machek <[e-mail]>
Cc: Rafael J. Wysocki <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: Stefano Stabellini <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Cc: Vitaly Kuznetsov <[e-mail]>
Cc: Xavier Deguillard <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/admin-guide/mm/pagemap.rst

···
     20. NOPAGE
     21. KSM
     22. THP
-    23. BALLOON
+    23. OFFLINE
     24. ZERO_PAGE
     25. IDLE
+    26. PGTABLE
 
  * ``/proc/kpagecgroup``.  This file contains a 64-bit inode number of the
    memory cgroup each page is charged to, indexed by PFN. Only available when
···
     identical memory pages dynamically shared between one or more processes
 22 - THP
     contiguous pages which construct transparent hugepages
-23 - BALLOON
-    balloon compaction page
+23 - OFFLINE
+    page is logically offline
 24 - ZERO_PAGE
     zero page for pfn_zero or huge_zero page
 25 - IDLE
···
     Note that this flag may be stale in case the page was accessed via
     a PTE. To make sure the flag is up-to-date one has to read
     ``/sys/kernel/mm/page_idle/bitmap`` first.
+26 - PGTABLE
+    page is in use as a page table
 
 IO related page flags
 ---------------------

fs/proc/page.c

···
 	else if (page_count(page) == 0 && is_free_buddy_page(page))
 		u |= 1 << KPF_BUDDY;
 
-	if (PageBalloon(page))
-		u |= 1 << KPF_BALLOON;
+	if (PageOffline(page))
+		u |= 1 << KPF_OFFLINE;
 	if (PageTable(page))
 		u |= 1 << KPF_PGTABLE;
 

include/linux/balloon_compaction.h

···
 static inline void balloon_page_insert(struct balloon_dev_info *balloon,
 				       struct page *page)
 {
-	__SetPageBalloon(page);
+	__SetPageOffline(page);
 	__SetPageMovable(page, balloon->inode->i_mapping);
 	set_page_private(page, (unsigned long)balloon);
 	list_add(&page->lru, &balloon->pages);
···
  */
 static inline void balloon_page_delete(struct page *page)
 {
-	__ClearPageBalloon(page);
+	__ClearPageOffline(page);
 	__ClearPageMovable(page);
 	set_page_private(page, 0);
 	/*
···
 static inline void balloon_page_insert(struct balloon_dev_info *balloon,
 				       struct page *page)
 {
-	__SetPageBalloon(page);
+	__SetPageOffline(page);
 	list_add(&page->lru, &balloon->pages);
 }
 
 static inline void balloon_page_delete(struct page *page)
 {
-	__ClearPageBalloon(page);
+	__ClearPageOffline(page);
 	list_del(&page->lru);
 }
 

include/linux/page-flags.h

···
 /* Reserve		0x0000007f to catch underflows of page_mapcount */
 #define PAGE_MAPCOUNT_RESERVE	-128
 #define PG_buddy	0x00000080
-#define PG_balloon	0x00000100
+#define PG_offline	0x00000100
 #define PG_kmemcg	0x00000200
 #define PG_table	0x00000400
 
···
 PAGE_TYPE_OPS(Buddy, buddy)
 
 /*
- * PageBalloon() is true for pages that are on the balloon page list
- * (see mm/balloon_compaction.c).
+ * PageOffline() indicates that the page is logically offline although the
+ * containing section is online. (e.g. inflated in a balloon driver or
+ * not onlined when onlining the section).
+ * The content of these pages is effectively stale. Such pages should not
+ * be touched (read/write/dump/save) except by their owner.
  */
-PAGE_TYPE_OPS(Balloon, balloon)
+PAGE_TYPE_OPS(Offline, offline)
 
 /*
  * If kmemcg is enabled, the buddy allocator will set PageKmemcg() on

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

···
 
 #define KPF_KSM			21
 #define KPF_THP			22
-#define KPF_BALLOON		23
+#define KPF_OFFLINE		23
 #define KPF_ZERO_PAGE		24
 #define KPF_IDLE		25
 #define KPF_PGTABLE		26

tools/vm/page-types.c

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