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

[Johannes Weiner]

On Wed, Aug 26, 2026 at 09:30:54PM +0800, Ridong Chen wrote:
> From: Ridong Chen <chenridong@xxxxxxxxxx>
> 
> 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.

Right, because of the memcg-lru...

> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5198,6 +5198,48 @@ 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();
> +}

Please wrap this in a #ifdef CONFIG_LRU_GEN block.