[PATCH v2 0/6] mm/memory_hotplug: Consider all zones when removing memory

[David Hildenbrand <david@redhat.com>]

Working on virtio-mem, I was able to trigger a kernel BUG (with debug
options enabled) when removing memory that was never onlined. I was able
to reproduce with DIMMs. As far as I can see the same can also happen
without debug configs enabled, if we're unlucky and the uninitialized
memmap contains selected garbage .

The root problem is that we should not try to derive the zone of memory we
are removing from the first PFN. The individual memory blocks of a DIMM
could be spanned by different ZONEs, multiple ZONES (after being offline and
re-onlined) or no ZONE at all (never onlined).

Let's process all applicable zones when removing memory so we're on the
safe side. In the long term, we want to resize the zones when offlining
memory (and before removing ZONE_DEVICE memory), however, that will require
more thought (and most probably a new SECTION_ACTIVE / pfn_active()
thingy). More details about that in patch #3.

Along with the fix, some related cleanups.

v1 -> v2:
- Include "mm: Introduce for_each_zone_nid()"
- "mm/memory_hotplug: Pass nid instead of zone to __remove_pages()"
-- Pass the nid instead of the zone and use it to reduce the number of
   zones to process

--- snip ---

I gave this a quick test with a DIMM on x86-64:

Start with a NUMA-less node 1. Hotplug a DIMM (512MB) to Node 1.
1st memory block is not onlined. 2nd and 4th is onlined MOVABLE.
3rd is onlined NORMAL.

:/# echo "online_movable" > /sys/devices/system/memory/memory41/state
[...]
:/# echo "online_movable" > /sys/devices/system/memory/memory43/state
:/# echo "online_kernel" > /sys/devices/system/memory/memory42/state
:/# cat /sys/devices/system/memory/memory40/state
offline

:/# cat /proc/zoneinfo
Node 1, zone   Normal
 [...]
        spanned  32768
        present  32768
        managed  32768
 [...]
Node 1, zone  Movable
 [...]
        spanned  98304
        present  65536
        managed  65536
 [...]

Trigger hotunplug. If it succeeds (block 42 can be offlined):

:/# cat /proc/zoneinfo

Node 1, zone   Normal
  pages free     0
        min      0
        low      0
        high     0
        spanned  0
        present  0
        managed  0
        protection: (0, 0, 0, 0, 0)
Node 1, zone  Movable
  pages free     0
        min      0
        low      0
        high     0
        spanned  0
        present  0
        managed  0
        protection: (0, 0, 0, 0, 0)

So all zones were properly fixed up and we don't access the memmap of the
first, never-onlined memory block (garbage). I am no longer able to trigger
the BUG. I did a similar test with an already populated node.

David Hildenbrand (6):
  mm/memory_hotplug: Exit early in __remove_pages() on BUGs
  mm: Exit early in set_zone_contiguous() if already contiguous
  mm/memory_hotplug: Process all zones when removing memory
  mm/memory_hotplug: Cleanup __remove_pages()
  mm: Introduce for_each_zone_nid()
  mm/memory_hotplug: Pass nid instead of zone to __remove_pages()

 arch/arm64/mm/mmu.c            |  4 +--
 arch/ia64/mm/init.c            |  4 +--
 arch/powerpc/mm/mem.c          |  3 +-
 arch/s390/mm/init.c            |  4 +--
 arch/sh/mm/init.c              |  4 +--
 arch/x86/mm/init_32.c          |  4 +--
 arch/x86/mm/init_64.c          |  4 +--
 include/linux/memory_hotplug.h |  2 +-
 include/linux/mmzone.h         |  5 ++++
 mm/memory_hotplug.c            | 51 +++++++++++++++++++---------------
 mm/memremap.c                  |  3 +-
 mm/page_alloc.c                |  3 ++
 12 files changed, 46 insertions(+), 45 deletions(-)

-- 
2.21.0