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

[Ridong Chen]

On 8/28/2026 1:21 AM, Johannes Weiner wrote:
> 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...

Yep, that's it.

> > --- 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.

Will add.

--
Best regards
Ridong