Re: [PATCH v3 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier

[Andrew Morton]

On Thu,  8 Jan 2026 19:15:35 +0900 Akinobu Mita <akinobu.mita@xxxxxxxxx> 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:
> 
> $ sudo swapoff -a
> $ 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, the OOM killer will not be
> invoked and the system will become inoperable, regardless of whether MGLRU
> is enabled or not.
> 
> This issue can be reproduced using NUMA emulation even on systems with
> only DRAM.  You can create two-fake memory-tiers by booting a single-node
> system with "numa=fake=2 numa_emulation.adistance=576,704" kernel
> parameters.
> 
> The reason for this issue is that memory allocations do not directly
> trigger the oom-killer, assuming that if the target node has an underlying
> memory tier, it can always be reclaimed by demotion.
> 
> So this change avoids this issue by not attempting to demote if the
> underlying node has less free memory than the minimum watermark, and the
> oom-killer will be triggered directly from memory allocations.
> 

Thanks.

An oom-killer fix which doesn't touch mm/oom-kill.c Hopefully
David/Shakeel/Michal can take a look.

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -358,7 +358,21 @@ static bool can_demote(int nid, struct scan_control *sc,
>  
>  	/* Filter out nodes that are not in cgroup's mems_allowed. */
>  	mem_cgroup_node_filter_allowed(memcg, &allowed_mask);
> -	return !nodes_empty(allowed_mask);
> +	if (nodes_empty(allowed_mask))
> +		return false;
> +
> +	for_each_node_mask(nid, allowed_mask) {
> +		int z;
> +		struct zone *zone;
> +		struct pglist_data *pgdat = NODE_DATA(nid);
> +
> +		for_each_managed_zone_pgdat(zone, pgdat, z, MAX_NR_ZONES - 1) {
> +			if (zone_watermark_ok(zone, 0, min_wmark_pages(zone),
> +						ZONE_MOVABLE, 0))
> +				return true;
> +		}
> +	}
> +	return false;
>  }

It would be nice to have a code comment in here to explain to readers
why we're doing this.