Re: [PATCH] mm/list_lru: drain before clearing xarray entry on reparent

[Muchun Song]

> On Jun 1, 2026, at 14:34, Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote:
> 
> memcg_reparent_list_lrus() clears the dying memcg's xarray entry with
> xas_store(&xas, NULL) before reparenting its per-node lists into the
> parent. This opens a window where a concurrent list_lru_del() arriving
> for the dying memcg sees xa_load() == NULL, walks to the parent in
> lock_list_lru_of_memcg(), takes the parent's per-node lock, and calls
> list_del_init() on an item still physically linked on the dying
> memcg's list.
> 
> If another in-flight thread holds the dying memcg's per-node lock at
> the same moment (another list_lru_del, or a list_lru_walk_one running
> an isolate callback), both threads modify ->next/->prev pointers on the
> same physical list under different locks. Adjacent items can corrupt
> each other's links.
> 
> Fix it by reversing the order: reparent each per-node list and mark the
> child's list lru dead and then clear the xarray entry. Any concurrent
> list_lru op that finds the still-set xarray entry either takes the dying
> memcg's per-node lock (synchronizing with the drain) or sees LONG_MIN
> and walks to the parent, where the items now live.
> 
> Fixes: fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope")
> Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Reported-by: Chris Mason <clm@xxxxxx>
> ---
> mm/list_lru.c | 20 +++++++++-----------
> 1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index dd29bcf8eb5f..ae55a52307db 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -473,26 +473,24 @@ void memcg_reparent_list_lrus(struct mem_cgroup *memcg, struct mem_cgroup *paren
> 	mutex_lock(&list_lrus_mutex);
> 	list_for_each_entry(lru, &memcg_list_lrus, list) {
> 		struct list_lru_memcg *mlru;
> - 		XA_STATE(xas, &lru->xa, memcg->kmemcg_id);
> 
> - 		/*
> -		 * Lock the Xarray to ensure no on going list_lru_memcg
> -		 * allocation and further allocation will see css_is_dying().
> -		 */
> - 		xas_lock_irq(&xas);
> - 		mlru = xas_store(&xas, NULL);
> - 		xas_unlock_irq(&xas);
> + 		mlru = xa_load(&lru->xa, memcg->kmemcg_id);
> 		if (!mlru)
> 			continue;

Is it possible that concurrent threads running memcg_list_lru_alloc() could
allocate a new mlru after this check passes? This could happen because the
threads haven't noticed css_is_dying() yet. We would consequently miss the
reparent operation for this list. So xas_lock_irq is necessary to serialize
CSS_DYING setting here. Right?

Thanks.
Muchun

> 
> 		/*
> -		 * With Xarray value set to NULL, holding the lru lock below
> -		 * prevents list_lru_{add,del,isolate} from touching the lru,
> -		 * safe to reparent.
> +		 * Reparent each per-node list and mark the child dead
> +		 * (LONG_MIN) before clearing xarray entry otherwisw a
> +		 * concurrent list_lru_del() may corrupt the list if it arrives
> +		 * after xarray clear but before reparenting as
> +		 * lock_list_lru_of_memcg will acquire parent's lock while the
> +		 * item is still on child's list.
> 		 */
> 		for_each_node(i)
> 			memcg_reparent_list_lru_one(lru, i, &mlru->node[i], parent);
> 
> + 		xa_erase(&lru->xa, memcg->kmemcg_id);
> +
> 		/*
> 		 * Here all list_lrus corresponding to the cgroup are guaranteed
> 		 * to remain empty, we can safely free this lru, any further
> -- 
> 2.52.0
>