[PATCH v4 0/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[PATCH v4 0/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[Ridong Chen]

From: Ridong Chen <chenridong@xiaomi.com>

For MGLRU, memory.min/low is not honored during non-kswapd global
reclaim (global direct reclaim and root-level memory.reclaim), because
these paths shrink memcgs using stale protection (emin/elow).

Patch 2 is the actual fix. Patch 1 is a prerequisite: an integer overflow
in effective_protection(), spotted by the sashiko review tool, which
patch 2's new caller would also be exposed to.

---

v4:
- Per Barry's and Johannes's review comments.

v3:
- Per Barry Song's review comments

v2:
- Fix an issue caused by non-atomic read races in patch 1 [1]

[1] https://sashiko.dev/#/patchset/20260828092432.1257917-1-ridong.chen@linux.dev?part=1

Ridong Chen (2):
  mm/page_counter: avoid integer overflow in effective_protection()
  mm/mglru: fix ineffective memory protection for non-kswapd reclaim

 include/linux/memcontrol.h | 10 +++++++++
 mm/memcontrol.c            | 45 ++++++++++++++++++++++++++++++++++++++
 mm/page_counter.c          | 21 +++++++++++++-----
 mm/vmscan.c                |  8 ++++++-
 4 files changed, 77 insertions(+), 7 deletions(-)

-- 
2.34.1

[PATCH v4 1/2] mm/page_counter: avoid integer overflow in effective_protection()

[Ridong Chen]

From: Ridong Chen <chenridong@xiaomi.com>

effective_protection() scales a parent's protection by a ratio of page
counts, e.g. for recursive protection:

	(parent_effective - siblings_protected) * (usage - protected)
		/ (parent_usage - siblings_protected)

The multiply is done at unsigned long width before dividing. On systems
with >= 16TB RAM the product can exceed 2^64 and wrap, giving a bogus
protection value and silently breaking memory.min/low enforcement.

Use mul_u64_u64_div_u64() to multiply in a 128-bit intermediate. Because
usage and parent_usage are not read atomically (a child is charged
before its parent), usage - protected can briefly exceed the divisor,
making the quotient overflow 64 bits and trap (#DE on x86). Cap it so
the ratio stays <= 1.

Reported by the sashiko review tool [1].

[1] https://sashiko.dev/#/patchset/20260826133054.88529-1-ridong.chen@linux.dev?part=1

Fixes: bc50bcc6e00b ("mm: memcontrol: clean up and document effective low/min calculations")
Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 mm/page_counter.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index 661e0f2a5127a..ea0d1646cff85 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -8,6 +8,7 @@
 #include <linux/page_counter.h>
 #include <linux/atomic.h>
 #include <linux/kernel.h>
+#include <linux/math64.h>
 #include <linux/string.h>
 #include <linux/sched.h>
 #include <linux/bug.h>
@@ -356,7 +357,8 @@ static unsigned long effective_protection(unsigned long usage,
 	 * otherwise get a smaller chunk than what they claimed.
 	 */
 	if (siblings_protected > parent_effective)
-		return protected * parent_effective / siblings_protected;
+		return mul_u64_u64_div_u64(protected, parent_effective,
+					   siblings_protected);
 
 	/*
 	 * Ok, utilized protection of all children is within what the
@@ -397,13 +399,20 @@ static unsigned long effective_protection(unsigned long usage,
 	if (parent_effective > siblings_protected &&
 	    parent_usage > siblings_protected &&
 	    usage > protected) {
-		unsigned long unclaimed;
+		unsigned long parent_unclaimed, parent_unprotected, unprotected;
 
-		unclaimed = parent_effective - siblings_protected;
-		unclaimed *= usage - protected;
-		unclaimed /= parent_usage - siblings_protected;
+		parent_unclaimed = parent_effective - siblings_protected;
+		parent_unprotected = parent_usage - siblings_protected;
 
-		ep += unclaimed;
+		/*
+		 * The usages aren't read atomically, so a child can transiently
+		 * appear to use more than its parent, making the ratio exceed 1
+		 * and the quotient overflow 64 bits (#DE on x86).  Cap it.
+		 */
+		unprotected = min(usage - protected, parent_unprotected);
+
+		ep += mul_u64_u64_div_u64(parent_unclaimed, unprotected,
+					  parent_unprotected);
 	}
 
 	return ep;
-- 
2.34.1

[PATCH v4 2/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[Ridong Chen]

From: Ridong Chen <chenridong@xiaomi.com>

For MGLRU, memory.min/low is not honored during global proactive reclaim
(writing to the root memory.reclaim) and global direct reclaim, because
these paths shrink memcgs using stale protection (emin/elow).
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 are whatever a previous kswapd run left behind
- or zero if kswapd never ran on this node. Relying on a prior kswapd
pass is not correct either: a memcg's emin/elow are derived from its
ancestors' memory.min/low settings and from children_min_usage, both of
which change over time, so emin/elow go stale even after kswapd has run
and must be recomputed at the point of reclaim.

Introduce mem_cgroup_calculate_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: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 include/linux/memcontrol.h | 10 +++++++++
 mm/memcontrol.c            | 45 ++++++++++++++++++++++++++++++++++++++
 mm/vmscan.c                |  8 ++++++-
 3 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index f227348a3f24a..a65a516adc665 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1885,6 +1885,16 @@ static inline bool memcg_is_dying(struct mem_cgroup *memcg)
 }
 #endif /* CONFIG_MEMCG */
 
+#if defined(CONFIG_MEMCG) && defined(CONFIG_LRU_GEN)
+void mem_cgroup_calculate_protection_path(struct mem_cgroup *root,
+					  struct mem_cgroup *memcg);
+#else
+static inline void mem_cgroup_calculate_protection_path(struct mem_cgroup *root,
+							struct mem_cgroup *memcg)
+{
+}
+#endif
+
 #if defined(CONFIG_MEMCG) && defined(CONFIG_ZSWAP)
 bool obj_cgroup_may_zswap(struct obj_cgroup *objcg);
 void obj_cgroup_charge_zswap(struct obj_cgroup *objcg, size_t size);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 256b68ffca70e..ae568fc688130 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5214,6 +5214,51 @@ void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 	page_counter_calculate_protection(&root->memory, &memcg->memory, recursive_protection);
 }
 
+#ifdef CONFIG_LRU_GEN
+/**
+ * mem_cgroup_calculate_protection_path - compute protection along a 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_calculate_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();
+}
+#endif /* CONFIG_LRU_GEN */
+
 static int charge_memcg(struct folio *folio, struct mem_cgroup *memcg,
 			gfp_t gfp)
 {
diff --git a/mm/vmscan.c b/mm/vmscan.c
index b4c9b8f3dfe99..8409ea4bbf379 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5111,7 +5111,13 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
 	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
 	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
 
-	/* lru_gen_age_node() called mem_cgroup_calculate_protection() */
+	/*
+	 * For kswapd, mem_cgroup_calculate_protection() has already
+	 * been called during the top-down cgroup traversal.
+	 */
+	if (!current_is_kswapd())
+		mem_cgroup_calculate_protection_path(NULL, memcg);
+
 	if (mem_cgroup_below_min(NULL, memcg))
 		return MEMCG_LRU_YOUNG;
 
-- 
2.34.1