Re: [PATCH v3] mm/vmscan: fix demotion targets checks in reclaim/demotion

[Waiman Long <llong@redhat.com>]

On 12/23/25 4:19 PM, Bing Jiao wrote:
> Fix two bugs in demote_folio_list() and can_demote() due to incorrect
> demotion target checks in reclaim/demotion.
>
> Commit 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim")
> introduces the cpuset.mems_effective check and applies it to
> can_demote(). However:
>
>    1. It does not apply this check in demote_folio_list(), which leads
>       to situations where pages are demoted to nodes that are
>       explicitly excluded from the task's cpuset.mems.
>
>    2. It checks only the nodes in the immediate next demotion hierarchy
>       and does not check all allowed demotion targets in can_demote().
>       This can cause pages to never be demoted if the nodes in the next
>       demotion hierarchy are not set in mems_effective.
>
> These bugs break resource isolation provided by cpuset.mems.
> This is visible from userspace because pages can either fail to be
> demoted entirely or are demoted to nodes that are not allowed
> in multi-tier memory systems.
>
> To address these bugs, update cpuset_node_allowed() and
> mem_cgroup_node_allowed() to return effective_mems, allowing directly
> logic-and operation against demotion targets. Also update can_demote()
> and demote_folio_list() accordingly.
>
> Reproduct Bug 1:
>    Assume a system with 4 nodes, where nodes 0-1 are top-tier and
>    nodes 2-3 are far-tier memory. All nodes have equal capacity.
>
>    Test script:
>      echo 1 > /sys/kernel/mm/numa/demotion_enabled
>      mkdir /sys/fs/cgroup/test
>      echo +cpuset > /sys/fs/cgroup/cgroup.subtree_control
>      echo "0-2" > /sys/fs/cgroup/test/cpuset.mems
>      echo $$ > /sys/fs/cgroup/test/cgroup.procs
>      swapoff -a
>      # Expectation: Should respect node 0-2 limit.
>      # Observation: Node 3 shows significant allocation (MemFree drops)
>      stress-ng --oomable --vm 1 --vm-bytes 150% --mbind 0,1
>
> Reproduct Bug 2:
>    Assume a system with 6 nodes, where nodes 0-2 are top-tier,
>    node 3 is a far-tier node, and nodes 4-5 are the farthest-tier nodes.
>    All nodes have equal capacity.
>
>    Test script:
>      echo 1 > /sys/kernel/mm/numa/demotion_enabled
>      mkdir /sys/fs/cgroup/test
>      echo +cpuset > /sys/fs/cgroup/cgroup.subtree_control
>      echo "0-2,4-5" > /sys/fs/cgroup/test/cpuset.mems
>      echo $$ > /sys/fs/cgroup/test/cgroup.procs
>      swapoff -a
>      # Expectation: Pages are demoted to Nodes 4-5
>      # Observation: No pages are demoted before oom.
>      stress-ng --oomable --vm 1 --vm-bytes 150% --mbind 0,1,2
>
> Fixes: 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Bing Jiao <bingjiao@google.com>
> ---
>   include/linux/cpuset.h     |  6 +++---
>   include/linux/memcontrol.h |  6 +++---
>   kernel/cgroup/cpuset.c     | 16 ++++++++--------
>   mm/memcontrol.c            |  6 ++++--
>   mm/vmscan.c                | 35 +++++++++++++++++++++++------------
>   5 files changed, 41 insertions(+), 28 deletions(-)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index a98d3330385c..eb358c3aa9c0 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -174,7 +174,7 @@ static inline void set_mems_allowed(nodemask_t nodemask)
>   	task_unlock(current);
>   }
>
> -extern bool cpuset_node_allowed(struct cgroup *cgroup, int nid);
> +extern nodemask_t cpuset_node_get_allowed(struct cgroup *cgroup);
>   #else /* !CONFIG_CPUSETS */
>
>   static inline bool cpusets_enabled(void) { return false; }
> @@ -301,9 +301,9 @@ static inline bool read_mems_allowed_retry(unsigned int seq)
>   	return false;
>   }
>
> -static inline bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
> +static inline nodemask_t cpuset_node_get_allowed(struct cgroup *cgroup)
>   {
> -	return true;
> +	return node_possible_map;
>   }

The nodemask_t type can be large depending on the setting of 
CONFIG_NODES_SHIFT. Passing a large data structure on stack may not be a 
good idea. You can return a pointer to nodemask_t instead. In that case, 
you will have a add a "const" qualifier to the return type to make sure 
that the node mask won't get accidentally modified. Alternatively, you 
can pass a nodemask_t pointer as an output parameter and copy out the 
nodemask_t data.

The name "cpuset_node_get_allowed" doesn't fit the cpuset naming 
convention. There is a "cpuset_mems_allowed(struct task_struct *)" to 
return "mems_allowed" of a task. This new helper is for returning the 
mems_allowed defined in the cpuset. Perhaps we could just use 
"cpuset_nodes_allowed(struct cgroup *)".

Cheers,
Longman