[PATCH v1 5/7] mm: introduce page_offline_(begin|end|freeze|unfreeze) to synchronize setting PageOffline()
From: David Hildenbrand <hidden>
Date: 2021-04-29 12:26:41
Also in:
linux-fsdevel, linux-hyperv, linux-mm, lkml
Subsystem:
memory management, memory management - core, the rest · Maintainers:
Andrew Morton, David Hildenbrand, Linus Torvalds
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 <redacted> --- 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
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization