Re: oom-killer not invoked on systems with multiple memory-tiers

[Shakeel Butt]

Hi Akinobu,

On Wed, Oct 22, 2025 at 10:57:35PM +0900, Akinobu Mita wrote:
> On systems with multiple memory-tiers consisting of DRAM and CXL memory,
> the OOM killer is not invoked properly.
> 
> Here's the command to reproduce:
> 
> $ stress-ng --oomable -v --memrate 20 --memrate-bytes 10G \
>     --memrate-rd-mbs 1 --memrate-wr-mbs 1
> 
> The memory usage is the number of workers specified with the --memrate
> option multiplied by the buffer size specified with the --memrate-bytes
> option, so please adjust it so that it exceeds the total size of the
> installed DRAM and CXL memory.
> 
> If swap is disabled, you can usually expect the OOM killer to terminate
> the stress-ng process when memory usage approaches the installed memory size.
> 
> However, if multiple memory-tiers exist (multiple
> /sys/devices/virtual/memory_tiering/memory_tier<N> directories exist),
> and /sys/kernel/mm/numa/demotion_enabled is true and
> /sys/kernel/mm/lru_gen/min_ttl_ms is 0, the OOM killer will not be invoked
> and the system will become inoperable.
> 
> If /sys/kernel/mm/numa/demotion_enabled is false, or if demotion_enabled
> is true but /sys/kernel/mm/lru_gen/min_ttl_ms is set to a non-zero value
> such as 1000, the OOM killer will be invoked properly.
> 
> This issue can be reproduced using NUMA emulation even on systems with
> only DRAM. However, to configure multiple memory-tiers using fake nodes,
> you must apply the attached patch.
> 
> You can create two-fake memory-tiers by booting a single-node system with
> the following boot options:
> 
> numa=fake=2
> numa_emulation.default_dram=1,0
> numa_emulation.read_latency=100,1000
> numa_emulation.write_latency=100,1000
> numa_emulation.read_bandwidth=100000,10000
> numa_emulation.write_bandwidth=100000,10000
> 

Thanks for the report. Can you try to repro this with traditional LRU
i.e. not MGLRU? I just want to see if this is MGLRU only issue or more
general.