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

Huang, Ying
2022-10-27T05:13:30Z

Feng Tang <feng.tang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> writes:

> 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;

I think we should avoid (void *) conversion here.

> +#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);
> + }

If node_isset(mtc->nid, cpuset_nmask) == true, we should keep the
original 2 steps allocation and apply nodes_and() on node mask.

> +#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);

pgdat will not change in the loop, so we can move this out of the loop?

> + memcg = folio_memcg(folio);
> + if (memcg)
> + cpuset_get_allowed_mem_nodes(memcg->css.cgroup,
> + &nmask1);
> +
> + if (!nodes_intersects(nmask, nmask1))
> + skip_this_demotion = true;
> + }

If nodes_intersects() == true, we will call
cpuset_get_allowed_mem_nodes() twice. Better to pass the intersecting
mask to demote_folio_list()?

> +#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);

Best Regards,
Huang, Ying