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@xxxxxxxxx> 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.

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@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> ---
>  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