[PATCH v2 4/6] mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()

[David Hildenbrand <david@xxxxxxxxxx>]

A driver might set a page logically offline -- PageOffline() -- and
turn the page inaccessible in the hypervisor; after that, access to page
content can be fatal. One example is virtio-mem; while unplugged memory
-- marked as PageOffline() can currently be read in the hypervisor, this
will no longer be the case in the future; for example, when having
a virtio-mem device backed by huge pages in the hypervisor.

Some special PFN walkers -- i.e., /proc/kcore -- read content of random
pages after checking PageOffline(); however, these PFN walkers can race
with drivers that set PageOffline().

Let's introduce page_offline_(begin|end|freeze|thaw) for
synchronizing.

page_offline_freeze()/page_offline_thaw() allows for a subsystem to
synchronize with such drivers, achieving that a page cannot be set
PageOffline() while frozen.

page_offline_begin()/page_offline_end() is used by drivers that care about
such races when setting a page PageOffline().

For simplicity, use a rwsem for now; neither drivers nor users are
performance sensitive.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/page-flags.h | 10 ++++++++++
 mm/util.c                  | 40 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index daed82744f4b..ea2df9a247b3 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -769,9 +769,19 @@ PAGE_TYPE_OPS(Buddy, buddy)
  * relies on this feature is aware that re-onlining the memory block will
  * require to re-set the pages PageOffline() and not giving them to the
  * buddy via online_page_callback_t.
+ *
+ * There are drivers that mark a page PageOffline() and do not expect any
+ * further access to page content. PFN walkers that read content of random
+ * pages should check PageOffline() and synchronize with such drivers using
+ * page_offline_freeze()/page_offline_thaw().
  */
 PAGE_TYPE_OPS(Offline, offline)
 
+extern void page_offline_freeze(void);
+extern void page_offline_thaw(void);
+extern void page_offline_begin(void);
+extern void page_offline_end(void);
+
 /*
  * Marks pages in use as page tables.
  */
diff --git a/mm/util.c b/mm/util.c
index a8bf17f18a81..a034525e7ba2 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1010,3 +1010,43 @@ void mem_dump_obj(void *object)
 }
 EXPORT_SYMBOL_GPL(mem_dump_obj);
 #endif
+
+/*
+ * A driver might set a page logically offline -- PageOffline() -- and
+ * turn the page inaccessible in the hypervisor; after that, access to page
+ * content can be fatal.
+ *
+ * Some special PFN walkers -- i.e., /proc/kcore -- read content of random
+ * pages after checking PageOffline(); however, these PFN walkers can race
+ * with drivers that set PageOffline().
+ *
+ * page_offline_freeze()/page_offline_thaw() allows for a subsystem to
+ * synchronize with such drivers, achieving that a page cannot be set
+ * PageOffline() while frozen.
+ *
+ * page_offline_begin()/page_offline_end() is used by drivers that care about
+ * such races when setting a page PageOffline().
+ */
+static DECLARE_RWSEM(page_offline_rwsem);
+
+void page_offline_freeze(void)
+{
+	down_read(&page_offline_rwsem);
+}
+
+void page_offline_thaw(void)
+{
+	up_read(&page_offline_rwsem);
+}
+
+void page_offline_begin(void)
+{
+	down_write(&page_offline_rwsem);
+}
+EXPORT_SYMBOL(page_offline_begin);
+
+void page_offline_end(void)
+{
+	up_write(&page_offline_rwsem);
+}
+EXPORT_SYMBOL(page_offline_end);
-- 
2.31.1