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

["Yin, Fengwei" <fengwei.yin-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

Hi Feng,

On 10/26/2022 3:43 PM, 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-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
> 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;
> +	}
> +
> +	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);
> +	}
> +
> +	rcu_read_unlock();
> +}
> +
>  /**
>   * cpuset_nodemask_valid_mems_allowed - check nodemask vs. current mems_allowed
>   * @nodemask: the nodemask to be checked
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 18f6497994ec..c205d98283bc 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1537,9 +1537,21 @@ static struct page *alloc_demote_page(struct page *page, unsigned long private)
>  {
>  	struct page *target_page;
>  	nodemask_t *allowed_mask;
> -	struct migration_target_control *mtc;
> +	struct migration_target_control *mtc = (void *)private;
>  
> -	mtc = (struct migration_target_control *)private;
> +#if IS_ENABLED(CONFIG_MEMCG) && IS_ENABLED(CONFIG_CPUSETS)
> +	struct mem_cgroup *memcg;
> +	nodemask_t cpuset_nmask;
> +
> +	memcg = page_memcg(page);
> +	cpuset_get_allowed_mem_nodes(memcg->css.cgroup, &cpuset_nmask);
> +
> +	if (!node_isset(mtc->nid, cpuset_nmask)) {
> +		if (mtc->nmask)
> +			nodes_and(*mtc->nmask, *mtc->nmask, cpuset_nmask);
> +		return alloc_migration_target(page, (unsigned long)mtc);
> +	}
> +#endif
>  
>  	allowed_mask = mtc->nmask;
>  	/*
> @@ -1649,6 +1661,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  		enum folio_references references = FOLIOREF_RECLAIM;
>  		bool dirty, writeback;
>  		unsigned int nr_pages;
> +		bool skip_this_demotion = false;
>  
>  		cond_resched();
>  
> @@ -1658,6 +1671,22 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  		if (!folio_trylock(folio))
>  			goto keep;
>  
> +#if IS_ENABLED(CONFIG_MEMCG) && IS_ENABLED(CONFIG_CPUSETS)
> +		if (do_demote_pass) {
> +			struct mem_cgroup *memcg;
> +			nodemask_t nmask, nmask1;
> +
> +			node_get_allowed_targets(pgdat, &nmask);
> +			memcg = folio_memcg(folio);
> +			if (memcg)
Doesn't check memcg in the change to alloc_demote_page(). What's the difference here?

> +				cpuset_get_allowed_mem_nodes(memcg->css.cgroup,
> +								&nmask1);
> +
> +			if (!nodes_intersects(nmask, nmask1))
If memcg is NULL, nmask1 will have an uninitialized value. Thanks.

Regards
Yin, Fengwei

> +				skip_this_demotion = true;
> +		}
> +#endif
> +
>  		VM_BUG_ON_FOLIO(folio_test_active(folio), folio);
>  
>  		nr_pages = folio_nr_pages(folio);
> @@ -1799,7 +1828,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  		 * Before reclaiming the folio, try to relocate
>  		 * its contents to another node.
>  		 */
> -		if (do_demote_pass &&
> +		if (do_demote_pass && !skip_this_demotion &&
>  		    (thp_migration_supported() || !folio_test_large(folio))) {
>  			list_add(&folio->lru, &demote_folios);
>  			folio_unlock(folio);