ksm: identify PageKsm pages

KSM will need to identify its kernel merged pages unambiguously, and
/proc/kpageflags will probably like to do so too.

Since KSM will only be substituting anonymous pages, statistics are best
preserved by making a PageKsm page a special PageAnon page: one with no
anon_vma.

But KSM then needs its own page_add_ksm_rmap() - keep it in ksm.h near
PageKsm; and do_wp_page() must COW them, unlike singly mapped PageAnons.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Chris Wright <chrisw@redhat.com>
Signed-off-by: Izik Eidus <ieidus@redhat.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index eb2a448981ee..a485c14ecd5d 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -10,6 +10,7 @@
 #include <linux/bitops.h>
 #include <linux/mm.h>
 #include <linux/sched.h>
+#include <linux/vmstat.h>
 
 #ifdef CONFIG_KSM
 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
@@ -29,6 +30,27 @@ static inline void ksm_exit(struct mm_struct *mm)
 	if (test_bit(MMF_VM_MERGEABLE, &mm->flags))
 		__ksm_exit(mm);
 }
+
+/*
+ * A KSM page is one of those write-protected "shared pages" or "merged pages"
+ * which KSM maps into multiple mms, wherever identical anonymous page content
+ * is found in VM_MERGEABLE vmas.  It's a PageAnon page, with NULL anon_vma.
+ */
+static inline int PageKsm(struct page *page)
+{
+	return ((unsigned long)page->mapping == PAGE_MAPPING_ANON);
+}
+
+/*
+ * But we have to avoid the checking which page_add_anon_rmap() performs.
+ */
+static inline void page_add_ksm_rmap(struct page *page)
+{
+	if (atomic_inc_and_test(&page->_mapcount)) {
+		page->mapping = (void *) PAGE_MAPPING_ANON;
+		__inc_zone_page_state(page, NR_ANON_PAGES);
+	}
+}
 #else  /* !CONFIG_KSM */
 
 static inline int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
@@ -45,6 +67,13 @@ static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm)
 static inline void ksm_exit(struct mm_struct *mm)
 {
 }
+
+static inline int PageKsm(struct page *page)
+{
+	return 0;
+}
+
+/* No stub required for page_add_ksm_rmap(page) */
 #endif /* !CONFIG_KSM */
 
 #endif