Re: [PATCH v2 1/2] mm/vmscan: respect mems_effective in demote_folio_list()

On 2025/12/22 7:36, Bing Jiao wrote:

Commit 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim") introduces the cpuset.mems_effective check and applies it to can_demote(). However, it does not apply this check in demote_folio_list().

This omission leads to situations where pages are demoted to nodes that are explicitly excluded from the task's cpuset.mems. The impact is two-fold:

1. Resource Isolation: This bug breaks resource isolation provided by cpuset.mems. It allows pages to be demoted to nodes that are dedicated to other tasks or are intended for hot-unplugging.

2. Performance Issue: In multi-tier systems, users use cpuset.mems to bind tasks to different performed-far tiers (e.g., avoiding the slowest tiers for latency-sensitive data). This bug can cause unexpected latency spikes if pages are demoted to the farthest nodes.

To address the bug, implement a new function mem_cgroup_filter_mems_allowed() to filter out nodes that are not set in mems_effective, and update demote_folio_list() to utilize this filtering logic. This ensures that demotions target respect task's memory placement constraints.

Fixes: 7d709f49babc ("vmscan,cgroup: apply mems_effective to reclaim") Signed-off-by: Bing Jiao bingjiao@google.com

---

include/linux/cpuset.h | 6 ++++++ include/linux/memcontrol.h | 7 +++++++ kernel/cgroup/cpuset.c | 18 ++++++++++++++++++ mm/memcontrol.c | 6 ++++++ mm/vmscan.c | 13 ++++++++++--- 5 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h index a98d3330385c..0e94548e2d24 100644 --- a/include/linux/cpuset.h +++ b/include/linux/cpuset.h @@ -175,6 +175,7 @@ static inline void set_mems_allowed(nodemask_t nodemask) } extern bool cpuset_node_allowed(struct cgroup *cgroup, int nid); +extern void cpuset_node_filter_allowed(struct cgroup *cgroup, nodemask_t *mask); #else /* !CONFIG_CPUSETS */ static inline bool cpusets_enabled(void) { return false; } @@ -305,6 +306,11 @@ static inline bool cpuset_node_allowed(struct cgroup *cgroup, int nid) { return true; }

+static inline void cpuset_node_filter_allowed(struct cgroup *cgroup,

• 			      nodemask_t *mask)
  

+{ +} #endif /* !CONFIG_CPUSETS */ #endif /* _LINUX_CPUSET_H */ diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index fd400082313a..7cfd71c57caa 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -1742,6 +1742,8 @@ static inline void count_objcg_events(struct obj_cgroup *objcg, bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid); +void mem_cgroup_filter_mems_allowed(struct mem_cgroup *memcg, nodemask_t *mask);

void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg); static inline bool memcg_is_dying(struct mem_cgroup *memcg) @@ -1816,6 +1818,11 @@ static inline bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid) return true; } +static inline bool mem_cgroup_filter_mems_allowed(struct mem_cgroup *memcg,

• 				  nodemask_t *mask)
  

+{ +}

static inline void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg) { } diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 6e6eb09b8db6..2925bd6bca91 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -4452,6 +4452,24 @@ bool cpuset_node_allowed(struct cgroup *cgroup, int nid) return allowed; } +void cpuset_node_filter_allowed(struct cgroup *cgroup, nodemask_t *mask) +{

• struct cgroup_subsys_state *css;
• struct cpuset *cs;
• if (!cpuset_v2())

• return;
  

• css = cgroup_get_e_css(cgroup, &cpuset_cgrp_subsys);
• if (!css)

• return;
  

• /* Follows the same assumption in cpuset_node_allowed() */
• cs = container_of(css, struct cpuset, css);
• nodes_and(*mask, *mask, cs->effective_mems);
• css_put(css);

+}

The functions cpuset_node_filter_allowed and cpuset_node_allowed are similar. We should create a helper function to obtain cs->effective_mems, which can then be used by both cpuset_node_filter_allowed and cpuset_node_allowed.

For example:

nodemask_t *mask cpuset_get_mem_allowed(struct cgroup *cgroup) { }

bool cpuset_node_allowed(struct cgroup *cgroup, int nid) { e_mask = cpuset_node_allowed(cgroup); return allowed = node_isset(nid, mask); }

void cpuset_node_filter_allowed(struct cgroup *cgroup, nodemask_t mask) { e_mask = cpuset_node_allowed(cgroup); nodes_and(mask, *mask, e_mask); }

Previously, I did not think we should distinguish between cgroup v1 and v2 here. This should be a common function; at least based on its name, it should not be solely for v2.

/**

• cpuset_spread_node() - On which node to begin search for a page
• @rotor: round robin rotor

diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 75fc22a33b28..f414653867de 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5602,6 +5602,12 @@ bool mem_cgroup_node_allowed(struct mem_cgroup *memcg, int nid) return memcg ? cpuset_node_allowed(memcg->css.cgroup, nid) : true; } +void mem_cgroup_filter_mems_allowed(struct mem_cgroup *memcg, nodemask_t *mask) +{

• if (memcg)

• cpuset_node_filter_allowed(memcg->css.cgroup, mask);
  

+}

void mem_cgroup_show_protected_memory(struct mem_cgroup *memcg) { if (mem_cgroup_disabled() || !cgroup_subsys_on_dfl(memory_cgrp_subsys)) diff --git a/mm/vmscan.c b/mm/vmscan.c index 453d654727c1..4d23c491e914 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1018,7 +1018,8 @@ static struct folio *alloc_demote_folio(struct folio *src,

• Folios which are not demoted are left on @demote_folios.

*/ static unsigned int demote_folio_list(struct list_head *demote_folios,

• 		     struct pglist_data *pgdat)
  

• 		      struct pglist_data *pgdat,
  

• 		      struct mem_cgroup *memcg)
  

{ int target_nid = next_demotion_node(pgdat->node_id); unsigned int nr_succeeded; @@ -1032,7 +1033,6 @@ static unsigned int demote_folio_list(struct list_head *demote_folios, */ .gfp_mask = (GFP_HIGHUSER_MOVABLE & ~__GFP_RECLAIM) | __GFP_NOMEMALLOC | GFP_NOWAIT,

• .nid = target_nid,
  

  .nmask = &allowed_mask, .reason = MR_DEMOTION, };

@@ -1044,6 +1044,13 @@ static unsigned int demote_folio_list(struct list_head *demote_folios, return 0; node_get_allowed_targets(pgdat, &allowed_mask);

• /* Filter the given nmask based on cpuset.mems.allowed */
• mem_cgroup_filter_mems_allowed(memcg, &allowed_mask);
• if (nodes_empty(allowed_mask))

• return 0;
  

• if (!node_isset(target_nid, allowed_mask))

• target_nid = node_random(&allowed_mask);
  

• mtc.nid = target_nid;

/* Demotion ignores all cpuset and mempolicy settings */ migrate_pages(demote_folios, alloc_demote_folio, NULL, @@ -1565,7 +1572,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, /* 'folio_list' is always empty here */ /* Migrate folios selected for demotion */

• nr_demoted = demote_folio_list(&demote_folios, pgdat);

• nr_demoted = demote_folio_list(&demote_folios, pgdat, memcg); nr_reclaimed += nr_demoted; stat->nr_demoted += nr_demoted; /* Folios that could not be demoted are still in @demote_folios */