[PATCH v1 5/7] mm: introduce page_offline_(begin|end|freeze|unfreeze) to synchronize setting PageOffline()

David Hildenbrand
2021-04-29T12:26:44Z

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|unfreeze) for
synchronizing.

page_offline_freeze()/page_offline_unfreeze() 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.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/page-flags.h | 5 +++++
mm/util.c | 38 ++++++++++++++++++++++++++++++++++++++
2 files changed, 43 insertions(+)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b8c56672a588..e3d00c72f459 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -767,6 +767,11 @@ PAGE_TYPE_OPS(Buddy, buddy)
*/
PAGE_TYPE_OPS(Offline, offline)

+extern void page_offline_freeze(void);
+extern void page_offline_unfreeze(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 54870226cea6..95395d4e4209 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1013,3 +1013,41 @@ void mem_dump_obj(void *object)
}
pr_cont(" non-slab/vmalloc memory.\n");
}
+
+/*
+ * 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_unfreeze() 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_unfreeze(void)
+{
+ up_read(&page_offline_rwsem);
+}
+
+void page_offline_begin(void)
+{
+ down_write(&page_offline_rwsem);
+}
+
+void page_offline_end(void)
+{
+ up_write(&page_offline_rwsem);
+}
--
2.30.2