On Fri, May 14, 2021 at 07:22:44PM +0200, David Hildenbrand wrote:
Let's avoid reading:
1) Offline memory sections: the content of offline memory sections is stale
as the memory is effectively unused by the kernel. On s390x with standby
memory, offline memory sections (belonging to offline storage
increments) are not accessible. With virtio-mem and the hyper-v balloon,
we can have unavailable memory chunks that should not be accessed inside
offline memory sections. Last but not least, offline memory sections
might contain hwpoisoned pages which we can no longer identify
because the memmap is stale.
2) PG_offline pages: logically offline pages that are documented as
"The content of these pages is effectively stale. Such pages should not
be touched (read/write/dump/save) except by their owner.".
Examples include pages inflated in a balloon or unavailble memory
ranges inside hotplugged memory sections with virtio-mem or the hyper-v
balloon.
3) PG_hwpoison pages: Reading pages marked as hwpoisoned can be fatal.
As documented: "Accessing is not safe since it may cause another machine
check. Don't touch!"
Introduce is_page_hwpoison(), adding a comment that it is inherently
racy but best we can really do.
Reading /proc/kcore now performs similar checks as when reading
/proc/vmcore for kdump via makedumpfile: problematic pages are exclude.
It's also similar to hibernation code, however, we don't skip hwpoisoned
pages when processing pages in kernel/power/snapshot.c:saveable_page() yet.
Note 1: we can race against memory offlining code, especially
memory going offline and getting unplugged: however, we will properly tear
down the identity mapping and handle faults gracefully when accessing
this memory from kcore code.
Note 2: we can race against drivers setting PageOffline() and turning
memory inaccessible in the hypervisor. We'll handle this in a follow-up
patch.
Reviewed-by: Mike Rapoport <redacted>
Signed-off-by: David Hildenbrand <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
--
Oscar Salvador
SUSE L3