On Fri, 2020-06-19 at 14:59 +0200, David Hildenbrand wrote:
Especially with memory hotplug, we can have offline sections (with a garbage memmap) and overlapping zones. We have to make sure to only touch initialized memmaps (online sections managed by the buddy) and that the zone matches, to not move pages between zones.
To test if this can actually happen, I added a simple BUG_ON(page_zone(page_i) != page_zone(page_j)); right before the swap. When hotplugging a 256M DIMM to a 4G x86-64 VM and onlining the first memory block "online_movable" and the second memory block "online_kernel", it will trigger the BUG, as both zones (NORMAL and MOVABLE) overlap.
This might result in all kinds of weird situations (e.g., double allocations, list corruptions, unmovable allocations ending up in the movable zone).
Fixes: e900a918b098 ("mm: shuffle initial free memory to improve memory-side-cache utilization") Acked-by: Michal Hocko mhocko@suse.com Cc: stable@vger.kernel.org # v5.2+ Cc: Andrew Morton akpm@linux-foundation.org Cc: Johannes Weiner hannes@cmpxchg.org Cc: Michal Hocko mhocko@suse.com Cc: Minchan Kim minchan@kernel.org Cc: Huang Ying ying.huang@intel.com Cc: Wei Yang richard.weiyang@gmail.com Cc: Mel Gorman mgorman@techsingularity.net Signed-off-by: David Hildenbrand david@redhat.com
Looks good to me.
Acked-by: Dan Williams dan.j.williams@intel.com