The patch titled
Subject: mm: introduce page_offline_(begin|end|freeze|thaw) to synchronize setting PageOffline()
has been added to the -mm tree. Its filename is
mm-introduce-page_offline_beginendfreezethaw-to-synchronize-setting-pageoffline.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-introduce-page_offline_beginendfreezethaw-to-synchronize-setting-pageoffline.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-introduce-page_offline_beginendfreezethaw-to-synchronize-setting-pageoffline.patch
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
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/20210526093041.8800-5-david@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
Cc: Aili Yao <yaoaili@xxxxxxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
Cc: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
Cc: Jason Wang <jasowang@xxxxxxxxxx>
Cc: Jiri Bohac <jbohac@xxxxxxx>
Cc: "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
Cc: Roman Gushchin <guro@xxxxxx>
Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>
Cc: Steven Price <steven.price@xxxxxxx>
Cc: Wei Liu <wei.liu@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/page-flags.h | 10 ++++++++
mm/util.c | 40 +++++++++++++++++++++++++++++++++++
2 files changed, 50 insertions(+)
--- 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);
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
perf-map_executable-does-not-indicate-vm_mayexec.patch
binfmt-remove-in-tree-usage-of-map_executable.patch
mm-ignore-map_executable-in-ksys_mmap_pgoff.patch
fs-proc-kcore-drop-kcore_remap-and-kcore_other.patch
fs-proc-kcore-pfn_is_ram-check-only-applies-to-kcore_ram.patch
fs-proc-kcore-dont-read-offline-sections-logically-offline-pages-and-hwpoisoned-pages.patch
mm-introduce-page_offline_beginendfreezethaw-to-synchronize-setting-pageoffline.patch
virtio-mem-use-page_offline_startend-when-setting-pageoffline.patch
fs-proc-kcore-use-page_offline_freezethaw.patch
mm-make-variable-names-for-populate_vma_page_range-consistent.patch
mm-madvise-introduce-madv_populate_readwrite-to-prefault-page-tables.patch
maintainers-add-tools-testing-selftests-vm-to-memory-management.patch
selftests-vm-add-protection_keys_32-protection_keys_64-to-gitignore.patch
selftests-vm-add-test-for-madv_populate_readwrite.patch
memory-hotplugrst-complete-admin-guide-overhaul.patch
