[PATCH 5.4 090/222] mm/memory_hotplug: dont free usage map when removing a re-added early section

Greg Kroah-Hartman
2020-01-22T13:21:12Z

From: David Hildenbrand <david@redhat.com>

commit 8068df3b60373c390198f660574ea14c8098de57 upstream.

When we remove an early section, we don't free the usage map, as the
usage maps of other sections are placed into the same page. Once the
section is removed, it is no longer an early section (especially, the
memmap is freed). When we re-add that section, the usage map is reused,
however, it is no longer an early section. When removing that section
again, we try to kfree() a usage map that was allocated during early
boot - bad.

Let's check against PageReserved() to see if we are dealing with an
usage map that was allocated during boot. We could also check against
!(PageSlab(usage_page) || PageCompound(usage_page)), but PageReserved() is
cleaner.

Can be triggered using memtrace under ppc64/powernv:

$ mount -t debugfs none /sys/kernel/debug/
$ echo 0x20000000 > /sys/kernel/debug/powerpc/memtrace/enable
$ echo 0x20000000 > /sys/kernel/debug/powerpc/memtrace/enable
------------[ cut here ]------------
kernel BUG at mm/slub.c:3969!
Oops: Exception in kernel mode, sig: 5 [#1]
LE PAGE_SIZE=3D64K MMU=3DHash SMP NR_CPUS=3D2048 NUMA PowerNV
Modules linked in:
CPU: 0 PID: 154 Comm: sh Not tainted 5.5.0-rc2-next-20191216-00005-g0be1dba7b7c0 #61
NIP kfree+0x338/0x3b0
LR section_deactivate+0x138/0x200
Call Trace:
section_deactivate+0x138/0x200
__remove_pages+0x114/0x150
arch_remove_memory+0x3c/0x160
try_remove_memory+0x114/0x1a0
__remove_memory+0x20/0x40
memtrace_enable_set+0x254/0x850
simple_attr_write+0x138/0x160
full_proxy_write+0x8c/0x110
__vfs_write+0x38/0x70
vfs_write+0x11c/0x2a0
ksys_write+0x84/0x140
system_call+0x5c/0x68
---[ end trace 4b053cbd84e0db62 ]---

The first invocation will offline+remove memory blocks. The second
invocation will first add+online them again, in order to offline+remove
them again (usually we are lucky and the exact same memory blocks will
get "reallocated").

Tested on powernv with boot memory: The usage map will not get freed.
Tested on x86-64 with DIMMs: The usage map will get freed.

Using Dynamic Memory under a Power DLAPR can trigger it easily.

Triggering removal (I assume after previously removed+re-added) of
memory from the HMC GUI can crash the kernel with the same call trace
and is fixed by this patch.

Link: http://lkml.kernel.org/r/20191217104637.5509-1-david@redhat.com
Fixes: 326e1b8f83a4 ("mm/sparsemem: introduce a SECTION_IS_EARLY flag")
Signed-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Pingfan Liu <piliu@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/sparse.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -775,7 +775,14 @@ static void section_deactivate(unsigned
if (bitmap_empty(subsection_map, SUBSECTIONS_PER_SECTION)) {
unsigned long section_nr = pfn_to_section_nr(pfn);

- if (!section_is_early) {
+ /*
+ * When removing an early section, the usage map is kept (as the
+ * usage maps of other sections fall into the same page). It
+ * will be re-used when re-adding the section - which is then no
+ * longer an early section. If the usage map is PageReserved, it
+ * was allocated during boot.
+ */
+ if (!PageReserved(virt_to_page(ms->usage))) {
kfree(ms->usage);
ms->usage = NULL;
}