[PATCH v2 0/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[PATCH v2 0/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[Ridong Chen]

From: Ridong Chen <chenridong@xiaomi.com>

This series fixes memory.min/low being silently bypassed for MGLRU
during non-kswapd global reclaim (global direct reclaim and root-level
memory.reclaim).

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.

---

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 | 11 ++++++++++
 mm/memcontrol.c            | 45 ++++++++++++++++++++++++++++++++++++++
 mm/page_counter.c          | 19 +++++++++++-----
 mm/vmscan.c                |  8 ++++++-
 4 files changed, 76 insertions(+), 7 deletions(-)

-- 
2.34.1

[PATCH v2 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
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 mm/page_counter.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index 661e0f2a5127..e8bd512069c5 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 +399,18 @@ static unsigned long effective_protection(unsigned long usage,
 	if (parent_effective > siblings_protected &&
 	    parent_usage > siblings_protected &&
 	    usage > protected) {
-		unsigned long unclaimed;
+		unsigned long unclaimed = parent_effective - siblings_protected;
+		unsigned long unprotected = usage - protected;
+		unsigned long parent_unprotected = parent_usage - siblings_protected;
 
-		unclaimed = parent_effective - siblings_protected;
-		unclaimed *= usage - protected;
-		unclaimed /= parent_usage - siblings_protected;
+		/*
+		 * 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(unprotected, parent_unprotected);
 
-		ep += unclaimed;
+		ep += mul_u64_u64_div_u64(unclaimed, unprotected, parent_unprotected);
 	}
 
 	return ep;
-- 
2.34.1

Re: [PATCH v2 1/2] mm/page_counter: avoid integer overflow in effective_protection()

[Barry Song]

On Fri, Aug 28, 2026 at 7:09 PM Ridong Chen <ridong.chen@linux.dev> wrote:
>
> 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
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
> ---

LGTM,
Reviewed-by: Barry Song <baohua@kernel.org>

[PATCH v2 2/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[Ridong Chen]

From: Ridong Chen <chenridong@xiaomi.com>

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: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 include/linux/memcontrol.h | 11 ++++++++++
 mm/memcontrol.c            | 45 ++++++++++++++++++++++++++++++++++++++
 mm/vmscan.c                |  8 ++++++-
 3 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 7d1c0ce189a8..8066b798a759 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -605,6 +605,10 @@ static inline void mem_cgroup_protection(struct mem_cgroup *root,
 
 void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 				     struct mem_cgroup *memcg);
+#ifdef CONFIG_LRU_GEN
+void mem_cgroup_protection_path(struct mem_cgroup *root,
+				struct mem_cgroup *memcg);
+#endif
 
 static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
 					  struct mem_cgroup *memcg)
@@ -1133,6 +1137,13 @@ static inline void mem_cgroup_calculate_protection(struct mem_cgroup *root,
 {
 }
 
+#ifdef CONFIG_LRU_GEN
+static inline void mem_cgroup_protection_path(struct mem_cgroup *root,
+					      struct mem_cgroup *memcg)
+{
+}
+#endif
+
 static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
 					  struct mem_cgroup *memcg)
 {
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1271d390b617..095050d4296a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5198,6 +5198,51 @@ 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_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();
+}
+#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 f11491ee9ed5..e0ba68ede745 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -5102,7 +5102,13 @@ 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, lru_gen_age_node() has already called
+	 * mem_cgroup_calculate_protection()
+	 */
+	if (!current_is_kswapd())
+		mem_cgroup_protection_path(NULL, memcg);
+
 	if (mem_cgroup_below_min(NULL, memcg))
 		return MEMCG_LRU_YOUNG;
 
-- 
2.34.1

Re: [PATCH v2 2/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[Barry Song]

On Fri, Aug 28, 2026 at 7:10 PM Ridong Chen <ridong.chen@linux.dev> wrote:
>
> From: Ridong Chen <chenridong@xiaomi.com>
>
> memory.min/low is silently bypassed for MGLRU during global proactive
> reclaim (writing to the root memory.reclaim) and global direct reclaim.

I guess nobody is silently bypassing anything. It's just that the
effective min is stale data. If kswapd has run at least once, should
the protection have been updated already?
I guess we need to update the changelog a bit?

> 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: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
> ---
>  include/linux/memcontrol.h | 11 ++++++++++
>  mm/memcontrol.c            | 45 ++++++++++++++++++++++++++++++++++++++
>  mm/vmscan.c                |  8 ++++++-
>  3 files changed, 63 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 7d1c0ce189a8..8066b798a759 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -605,6 +605,10 @@ static inline void mem_cgroup_protection(struct mem_cgroup *root,
>
>  void mem_cgroup_calculate_protection(struct mem_cgroup *root,
>                                      struct mem_cgroup *memcg);
> +#ifdef CONFIG_LRU_GEN
> +void mem_cgroup_protection_path(struct mem_cgroup *root,
> +                               struct mem_cgroup *memcg);
> +#endif
>
>  static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
>                                           struct mem_cgroup *memcg)
> @@ -1133,6 +1137,13 @@ static inline void mem_cgroup_calculate_protection(struct mem_cgroup *root,
>  {
>  }
>
> +#ifdef CONFIG_LRU_GEN
> +static inline void mem_cgroup_protection_path(struct mem_cgroup *root,
> +                                             struct mem_cgroup *memcg)
> +{
> +}
> +#endif
> +

I wonder if we could follow the zswap pattern?

#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);
void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size);
bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg);
#else
static inline bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
{
        return true;
}
...
#endif

>  static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
>                                           struct mem_cgroup *memcg)
>  {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 1271d390b617..095050d4296a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5198,6 +5198,51 @@ 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_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)

Can we rename it to `mem_cgroup_calculate_protection_path()`?

BTW, I see that the only caller is in vmscan and it passes NULL as
`root`. Do we need to keep the `root` argument if the new helper is
only used for global reclaim?

Best Regards
Barry

Re: [PATCH v2 2/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[Ridong Chen]

On 8/30/2026 3:53 PM, Barry Song wrote:
> On Fri, Aug 28, 2026 at 7:10 PM Ridong Chen <ridong.chen@linux.dev> wrote:
>>
>> From: Ridong Chen <chenridong@xiaomi.com>
>>
>> memory.min/low is silently bypassed for MGLRU during global proactive
>> reclaim (writing to the root memory.reclaim) and global direct reclaim.
> 
> I guess nobody is silently bypassing anything. It's just that the
> effective min is stale data. If kswapd has run at least once, should
> the protection have been updated already?
> I guess we need to update the changelog a bit?
> 

The children's emin/elow are derived from the parent's min/low settings and 
children_min_usage, both of which can change over time. As a result, emin/elow 
may become stale, even if kswapd has already run once.

>> 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: stable@vger.kernel.org
>> Assisted-by: Claude:claude-opus-4-8
>> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
>> ---
>>   include/linux/memcontrol.h | 11 ++++++++++
>>   mm/memcontrol.c            | 45 ++++++++++++++++++++++++++++++++++++++
>>   mm/vmscan.c                |  8 ++++++-
>>   3 files changed, 63 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 7d1c0ce189a8..8066b798a759 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -605,6 +605,10 @@ static inline void mem_cgroup_protection(struct mem_cgroup *root,
>>
>>   void mem_cgroup_calculate_protection(struct mem_cgroup *root,
>>                                       struct mem_cgroup *memcg);
>> +#ifdef CONFIG_LRU_GEN
>> +void mem_cgroup_protection_path(struct mem_cgroup *root,
>> +                               struct mem_cgroup *memcg);
>> +#endif
>>
>>   static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
>>                                            struct mem_cgroup *memcg)
>> @@ -1133,6 +1137,13 @@ static inline void mem_cgroup_calculate_protection(struct mem_cgroup *root,
>>   {
>>   }
>>
>> +#ifdef CONFIG_LRU_GEN
>> +static inline void mem_cgroup_protection_path(struct mem_cgroup *root,
>> +                                             struct mem_cgroup *memcg)
>> +{
>> +}
>> +#endif
>> +
> 
> I wonder if we could follow the zswap pattern?
> 
> #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);
> void obj_cgroup_uncharge_zswap(struct obj_cgroup *objcg, size_t size);
> bool mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg);
> #else
> static inline bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
> {
>          return true;
> }

That is fine for me, I will update it.

> ...
> #endif
> 
>>   static inline bool mem_cgroup_unprotected(struct mem_cgroup *target,
>>                                            struct mem_cgroup *memcg)
>>   {
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 1271d390b617..095050d4296a 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5198,6 +5198,51 @@ 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_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)
> 
> Can we rename it to `mem_cgroup_calculate_protection_path()`?
> 
> BTW, I see that the only caller is in vmscan and it passes NULL as
> `root`. Do we need to keep the `root` argument if the new helper is
> only used for global reclaim?
> 
I'd suggest keeping it as is. This function updates protection along the path 
from root to memcg, and could be reused later. Note that 
mem_cgroup_calculate_protection() assumes the caller has already performed the 
top-down walk, each level's calculation depends on its parent being updated first.

For mem_cgroup_calculate_protection_path(), it can be called in any context 
without such a precondition.

-- 
Best regards
Ridong

Re: [PATCH v2 2/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[Barry Song]

On Sun, Aug 30, 2026 at 6:13 PM Ridong Chen <ridong.chen@linux.dev> wrote:
>
>
>
> On 8/30/2026 3:53 PM, Barry Song wrote:
> > On Fri, Aug 28, 2026 at 7:10 PM Ridong Chen <ridong.chen@linux.dev> wrote:
> >>
> >> From: Ridong Chen <chenridong@xiaomi.com>
> >>
> >> memory.min/low is silently bypassed for MGLRU during global proactive
> >> reclaim (writing to the root memory.reclaim) and global direct reclaim.
> >
> > I guess nobody is silently bypassing anything. It's just that the
> > effective min is stale data. If kswapd has run at least once, should
> > the protection have been updated already?
> > I guess we need to update the changelog a bit?
> >
>
> The children's emin/elow are derived from the parent's min/low settings and
> children_min_usage, both of which can change over time. As a result, emin/elow
> may become stale, even if kswapd has already run once.

right, let's just say this in changelog, we are *not* bypassing we are
just checking
against stable data. The current changelog seems to be misleading.

[...]
> >> +void mem_cgroup_protection_path(struct mem_cgroup *root,
> >> +                               struct mem_cgroup *memcg)
> >
> > Can we rename it to `mem_cgroup_calculate_protection_path()`?
> >
> > BTW, I see that the only caller is in vmscan and it passes NULL as
> > `root`. Do we need to keep the `root` argument if the new helper is
> > only used for global reclaim?
> >
> I'd suggest keeping it as is. This function updates protection along the path
> from root to memcg, and could be reused later. Note that
> mem_cgroup_calculate_protection() assumes the caller has already performed the
> top-down walk, each level's calculation depends on its parent being updated first.
>
> For mem_cgroup_calculate_protection_path(), it can be called in any context
> without such a precondition.

I am fine with this -  keeping the root there. but I guess rename is worth it.

Best Regards
Barry

Re: [PATCH v2 2/2] mm/mglru: fix ineffective memory protection for non-kswapd reclaim

[Ridong Chen]

On 8/30/2026 6:40 PM, Barry Song wrote:
> On Sun, Aug 30, 2026 at 6:13 PM Ridong Chen <ridong.chen@linux.dev> wrote:
>>
>>
>>
>> On 8/30/2026 3:53 PM, Barry Song wrote:
>>> On Fri, Aug 28, 2026 at 7:10 PM Ridong Chen <ridong.chen@linux.dev> wrote:
>>>>
>>>> From: Ridong Chen <chenridong@xiaomi.com>
>>>>
>>>> memory.min/low is silently bypassed for MGLRU during global proactive
>>>> reclaim (writing to the root memory.reclaim) and global direct reclaim.
>>>
>>> I guess nobody is silently bypassing anything. It's just that the
>>> effective min is stale data. If kswapd has run at least once, should
>>> the protection have been updated already?
>>> I guess we need to update the changelog a bit?
>>>
>>
>> The children's emin/elow are derived from the parent's min/low settings and
>> children_min_usage, both of which can change over time. As a result, emin/elow
>> may become stale, even if kswapd has already run once.
> 
> right, let's just say this in changelog, we are *not* bypassing we are
> just checking
> against stable data. The current changelog seems to be misleading.
> 

Thanks, Will update.

> [...]
>>>> +void mem_cgroup_protection_path(struct mem_cgroup *root,
>>>> +                               struct mem_cgroup *memcg)
>>>
>>> Can we rename it to `mem_cgroup_calculate_protection_path()`?
>>>
>>> BTW, I see that the only caller is in vmscan and it passes NULL as
>>> `root`. Do we need to keep the `root` argument if the new helper is
>>> only used for global reclaim?
>>>
>> I'd suggest keeping it as is. This function updates protection along the path
>> from root to memcg, and could be reused later. Note that
>> mem_cgroup_calculate_protection() assumes the caller has already performed the
>> top-down walk, each level's calculation depends on its parent being updated first.
>>
>> For mem_cgroup_calculate_protection_path(), it can be called in any context
>> without such a precondition.
> 
> I am fine with this -  keeping the root there. but I guess rename is worth it.
> 

Yeah, I will rename in the next version.

-- 
Best regards
Ridong