Re: [PATCH 2/2] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier

[Vlastimil Babka <vbabka@suse.cz>]

On 12/8/25 10:40, 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:
> 
> $ 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 and
> /sys/kernel/mm/lru_gen/min_ttl_ms is 0, the OOM killer will not be invoked

Does this mean only mglru has the problem, or !mglru too?
Also is min_ttl_ms = 0 a sensible setting? What happens without it?
If !mglru doesn't have this problem, how does the fix affect it?

> and the system will become inoperable.
> 
> 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 if the target node for allocation has
> an underlying memory tier, it is always assumed that it can be reclaimed
> via demotion.
> 
> So this change avoids this issue by not attempting to demote if the
> demoting node has less free memory than the minimum watermark.
> 
> Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
> ---
>  mm/vmscan.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fddd168a9737..f4748f258294 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -356,7 +356,20 @@ static bool can_demote(int nid, struct scan_control *sc,
>  		return false;
>  
>  	/* If demotion node isn't in the cgroup's mems_allowed, fall back */
> -	return mem_cgroup_node_allowed(memcg, demotion_nid);
> +	if (mem_cgroup_node_allowed(memcg, demotion_nid)) {
> +		int z;
> +		struct zone *zone;
> +		struct pglist_data *pgdat = NODE_DATA(demotion_nid);
> +		unsigned int highest_zoneidx = sc ? sc->reclaim_idx : MAX_NR_ZONES - 1;
> +		int order = sc ? sc->order : 0;
> +
> +		for_each_managed_zone_pgdat(zone, pgdat, z, highest_zoneidx) {
> +			if (zone_watermark_ok(zone, order, min_wmark_pages(zone),
> +						highest_zoneidx, 0))
> +				return true;
> +		}
> +	}
> +	return false;
>  }
>  
>  static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,