mm/mglru: fix ineffective memory protection for non-kswapd reclaim

From: Ridong Chen <[email protected]>

memory.min/low is silently bypassed for MGLRU during global proactive reclaim (writing to the root memory.reclaim) and global direct reclaim. It can be reproduced as follows:

  # echo 7 > /sys/kernel/mm/lru_gen/enabled
  # cd /sys/fs/cgroup
  # mkdir -p a/b
  # echo 100M > a/memory.min
  # echo +memory > a/cgroup.subtree_control
  # echo 100M > a/b/memory.min
  # echo $$ > a/b/cgroup.procs
  # dd if=/dev/zero of=/tmp/testfile bs=1M count=200
  # cat a/b/memory.current
  222650368
  # echo 500M > memory.reclaim
  -bash: echo: write error: Resource temporarily unavailable
  # cat a/b/memory.current
  6070272

memory.min is 100M, yet reclaim drops a/b down to 6M, breaking the protection. The traditional LRU path is not affected because shrink_node() calls mem_cgroup_calculate_protection() for each memcg it visits during a top-down tree walk.

Commit 30d77b7eef01 ("mm/mglru: fix ineffective protection calculation") moved the protection computation into lru_gen_age_node(), which only runs for kswapd. Non-kswapd global reclaim reaches shrink_one() through lru_gen_shrink_node() -> shrink_many() without any protection computation, so emin/elow remain stale or zero.

Introduce mem_cgroup_protection_path() which computes emin/elow along the root-to-target path only by iterating through the cgroup ancestors array top-down. This avoids the full tree traversal that would be needed with mem_cgroup_calculate_protection(), limiting the cost to O(depth) per memcg - typically 3-5 levels.

Call it from shrink_one() for the non-kswapd path so that each memcg about to be shrunk has correct protection values.

Fixes e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")

Cc [email protected]

Signed-off-by Ridong Chen <[email protected]>

---
 include/linux/memcontrol.h |  7 +++++++
 mm/memcontrol.c            | 42 ++++++++++++++++++++++++++++++++++++++
 mm/vmscan.c                |  3 +++
 3 files changed, 52 insertions(+)

include/linux/memcontrol.h +7 -0

index 8170bb8066a2..4e0dd5d98f3a 100644
@@ -600,6 +600,8 @@ static inline void mem_cgroup_protection(struct mem_cgroup *root,
 
 void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 				     struct mem_cgroup *memcg);
+void mem_cgroup_protection_path(struct mem_cgroup *root,
+				struct mem_cgroup *memcg);
 
 static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
 					  struct mem_cgroup *memcg)

@@ -1126,6 +1128,11 @@ static inline void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 {
 }
 
+static inline void mem_cgroup_protection_path(struct mem_cgroup *root,
+					      struct mem_cgroup *memcg)
+{
+}
+
 static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
 					  struct mem_cgroup *memcg)
 {

mm/memcontrol.c +42 -0

index 69b37f63a307..e915176ac06b 100644
@@ -5134,6 +5134,48 @@ void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 	page_counter_calculate_protection(&root->memory, &memcg->memory, recursive_protection);
 }
 
+/**
+ * mem_cgroup_protection_path - compute protection along root->memcg path
+ * @root: the top ancestor of the sub-tree being checked (NULL for root_mem_cgroup)
+ * @memcg: the target memory cgroup
+ *
+ * Walk the ancestor path from @root down to @memcg and compute the effective
+ * protection at each level.  This is safe for isolated queries because it
+ * ensures parents are computed before children.
+ */
+void mem_cgroup_protection_path(struct mem_cgroup *root,
+				struct mem_cgroup *memcg)
+{
+	bool recursive_protection =
+		cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT;
+	struct cgroup *cg;
+	int root_level, i;
+
+	if (mem_cgroup_disabled())
+		return;
+
+	if (!root)
+		root = root_mem_cgroup;
+
+	if (memcg == root)
+		return;
+
+	root_level = root->css.cgroup->level;
+	cg = memcg->css.cgroup;
+
+	rcu_read_lock();
+	for (i = root_level + 1; i <= cg->level; i++) {
+		struct mem_cgroup *cur;
+
+		cur = mem_cgroup_from_css(cgroup_css(cg->ancestors[i],
+						    &memory_cgrp_subsys));
+		if (cur)
+			page_counter_calculate_protection(&root->memory,
+					&cur->memory, recursive_protection);
+	}
+	rcu_read_unlock();
+}
+
 static int charge_memcg(struct folio *folio, struct mem_cgroup *memcg,
 			gfp_t gfp)
 {

mm/vmscan.c +0 -0

index 4ca9775ceee8..70e28d583255 100644