Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

[Waiman Long]

On 10/26/22 03:43, Feng Tang wrote:
> In page reclaim path, memory could be demoted from faster memory tier
> to slower memory tier. Currently, there is no check about cpuset's
> memory policy, that even if the target demotion node is not allowd
> by cpuset, the demotion will still happen, which breaks the cpuset
> semantics.
>
> So add cpuset policy check in the demotion path and skip demotion
> if the demotion targets are not allowed by cpuset.
>
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> ---
> Hi reviewers,
>
> For easy bisectable, I combined the cpuset change and mm change
> in one patch, if you prefer to separate them, I can turn it into
> 2 patches.
>
> Thanks,
> Feng
>
>   include/linux/cpuset.h |  6 ++++++
>   kernel/cgroup/cpuset.c | 29 +++++++++++++++++++++++++++++
>   mm/vmscan.c            | 35 ++++++++++++++++++++++++++++++++---
>   3 files changed, 67 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index d58e0476ee8e..6fcce2bd2631 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -178,6 +178,8 @@ static inline void set_mems_allowed(nodemask_t nodemask)
>   	task_unlock(current);
>   }
>   
> +extern void cpuset_get_allowed_mem_nodes(struct cgroup *cgroup,
> +						nodemask_t *nmask);
>   #else /* !CONFIG_CPUSETS */
>   
>   static inline bool cpusets_enabled(void) { return false; }
> @@ -299,6 +301,10 @@ static inline bool read_mems_allowed_retry(unsigned int seq)
>   	return false;
>   }
>   
> +static inline void cpuset_get_allowed_mem_nodes(struct cgroup *cgroup,
> +						nodemask_t *nmask)
> +{
> +}
>   #endif /* !CONFIG_CPUSETS */
>   
>   #endif /* _LINUX_CPUSET_H */
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 3ea2e836e93e..cbb118c0502f 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3750,6 +3750,35 @@ nodemask_t cpuset_mems_allowed(struct task_struct *tsk)
>   	return mask;
>   }
>   
> +/*
> + * Retrieve the allowed memory nodemask for a cgroup.
> + *
> + * Set *nmask to cpuset's effective allowed nodemask for cgroup v2,
> + * and NODE_MASK_ALL (means no constraint) for cgroup v1 where there
> + * is no guaranteed association from a cgroup to a cpuset.
> + */
> +void cpuset_get_allowed_mem_nodes(struct cgroup *cgroup, nodemask_t *nmask)
> +{
> +	struct cgroup_subsys_state *css;
> +	struct cpuset *cs;
> +
> +	if (!is_in_v2_mode()) {
> +		*nmask = NODE_MASK_ALL;
> +		return;
> +	}

You are allowing all nodes to be used for cgroup v1. Is there a reason 
why you ignore v1?

> +
> +	rcu_read_lock();
> +	css = cgroup_e_css(cgroup, &cpuset_cgrp_subsys);
> +	if (css) {
> +		css_get(css);
> +		cs = css_cs(css);
> +		*nmask = cs->effective_mems;
> +		css_put(css);
> +	}
Since you are holding an RCU read lock and copying out the whole 
nodemask, you probably don't need to do a css_get/css_put pair.
> +
> +	rcu_read_unlock();
> +}
> +
Cheers,

Longman