[057/192] mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()

Commit Message

Andrew Morton July 1, 2021, 1:50 a.m. UTC

From: David Hildenbrand <[e-mail]>
Subject: mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()

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.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Reviewed-by: Mike Rapoport <[e-mail]>
Reviewed-by: Oscar Salvador <[e-mail]>
Cc: Aili Yao <[e-mail]>
Cc: Alexey Dobriyan <[e-mail]>
Cc: Alex Shi <[e-mail]>
Cc: Haiyang Zhang <[e-mail]>
Cc: Jason Wang <[e-mail]>
Cc: Jiri Bohac <[e-mail]>
Cc: "K. Y. Srinivasan" <[e-mail]>
Cc: "Matthew Wilcox (Oracle)" <[e-mail]>
Cc: "Michael S. Tsirkin" <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Cc: Naoya Horiguchi <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Stephen Hemminger <[e-mail]>
Cc: Steven Price <[e-mail]>
Cc: Wei Liu <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 include/linux/page-flags.h |   10 ++++++++
 mm/util.c                  |   40 +++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

Patch

--- a/include/linux/page-flags.h~mm-introduce-page_offline_beginendfreezethaw-to-synchronize-setting-pageoffline
+++ a/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 expect there won't be
+ * 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.
  */
--- a/mm/util.c~mm-introduce-page_offline_beginendfreezethaw-to-synchronize-setting-pageoffline
+++ a/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);