Re: [PATCH 1/2] mm/vmscan: remove redundant !mem_cgroup_online() checks

[Balbir Singh]

On Tue, Oct 27, 2026 at 06:06:05PM -0700, T.J. Mercier wrote:
> Since commit f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of
> dying memory cgroup for LRU folios") and commit 0e0ac326c511 ("memcg:
> move LRU size accounting on reparenting instead of copying it"),
> mem_cgroup_css_offline() reparents all LRU folios to the parent memcg
> and zeroes the child's lrugen->nr_pages and lru_zone_size before the
> memcg is offlined. Nothing can repopulate the offlined memcg's lruvec
> afterwards. A folio that is off the LRU during reparenting looks up its
> lruvec through its objcg, which now points to the parent so it is put
> back on the parent's LRU.
> 
> As a result, lruvec_lru_size() and lruvec_evictable_size() are always 0
> when !mem_cgroup_online(memcg) is true, making the offlined memcg special
> cases in get_scan_count(), lruvec_is_sizable(), and shrink_one() dead
> code. Remove them.
> 
> With the get_nr_to_scan() special case removed, an offlined memcg that
> still has stale effective protection now has
> apply_proportional_protection() applied like get_scan_count() does, so
> the MGLRU and traditional LRU behavior is now the same.
> 
> Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
> ---
>  include/linux/mmzone.h |  8 ++++----
>  mm/vmscan.c            | 28 +++++-----------------------
>  2 files changed, 9 insertions(+), 27 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 3b96d6c7123b..7f078ff0dd76 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -638,10 +638,10 @@ struct lru_gen_mm_walk {
>   * The events that trigger the above operations are:
>   * 1. The first attempt to reclaim a memcg below low, which triggers
>   *    MEMCG_LRU_TAIL;
> - * 2. The first attempt to reclaim a memcg offlined or below reclaimable size
> - *    threshold, which triggers MEMCG_LRU_TAIL;
> - * 3. The second attempt to reclaim a memcg offlined or below reclaimable size
> - *    threshold, which triggers MEMCG_LRU_YOUNG;
> + * 2. The first attempt to reclaim a memcg below reclaimable size threshold,
> + *    which triggers MEMCG_LRU_TAIL;
> + * 3. The second attempt to reclaim a memcg below reclaimable size threshold,
> + *    which triggers MEMCG_LRU_YOUNG;
>   * 4. Attempting to reclaim a memcg below min, which triggers MEMCG_LRU_YOUNG;
>   * 5. Finishing the aging on the eviction path, which triggers MEMCG_LRU_YOUNG;
>   * 6. Offlining a memcg, which triggers MEMCG_LRU_OLD.
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9959ca158ce8..636152d8c94d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2754,13 +2754,6 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  		scan = apply_proportional_protection(memcg, sc, lruvec_size);
>  		scan >>= sc->priority;
>  
> -		/*
> -		 * If the cgroup's already been deleted, make sure to
> -		 * scrape out the remaining cache.
> -		 */
> -		if (!scan && !mem_cgroup_online(memcg))
> -			scan = min(lruvec_size, SWAP_CLUSTER_MAX);
> -
>  		switch (scan_balance) {
>  		case SCAN_EQUAL:
>  			/* Scan lists relative to size */
> @@ -2769,14 +2762,8 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  			/*
>  			 * Scan types proportional to swappiness and
>  			 * their relative recent reclaim efficiency.
> -			 * Make sure we don't miss the last page on
> -			 * the offlined memory cgroups because of a
> -			 * round-off error.
>  			 */
> -			scan = mem_cgroup_online(memcg) ?
> -			       div64_u64(scan * fraction[file], denominator) :
> -			       DIV64_U64_ROUND_UP(scan * fraction[file],
> -						  denominator);
> +			scan = div64_u64(scan * fraction[file], denominator);
>  			break;
>  		case SCAN_FILE:
>  		case SCAN_ANON:
> @@ -4355,12 +4342,11 @@ static bool lruvec_is_sizable(struct lruvec *lruvec, struct scan_control *sc)
>  {
>  	unsigned long total;
>  	int swappiness = get_swappiness(lruvec, sc);
> -	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>  
>  	total = lruvec_evictable_size(lruvec, swappiness);
>  
>  	/* whether the size is big enough to be helpful */
> -	return mem_cgroup_online(memcg) ? (total >> sc->priority) : total;
> +	return total >> sc->priority;
>  }
>  
>  static bool lruvec_is_reclaimable(struct lruvec *lruvec, struct scan_control *sc,
> @@ -5199,10 +5185,6 @@ static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc,
>  
>  	evictable = lruvec_evictable_size(lruvec, swappiness);
>  
> -	/* try to scrape all its memory if this memcg was deleted */
> -	if (!mem_cgroup_online(memcg))
> -		return evictable;
> -
>  	nr_to_scan = apply_proportional_protection(memcg, sc, evictable);
>  	nr_to_scan >>= sc->priority;
>  
> @@ -5325,13 +5307,13 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
>  
>  	flush_reclaim_state(sc);
>  
> -	if (need_rotate && mem_cgroup_online(memcg))
> +	if (need_rotate)
>  		return MEMCG_LRU_YOUNG;
>  
> -	if (!need_rotate && lruvec_is_sizable(lruvec, sc))
> +	if (lruvec_is_sizable(lruvec, sc))
>  		return 0;
>  
> -	/* one retry if offlined or too small */
> +	/* one retry if too small */
>  	return READ_ONCE(lruvec->lrugen.seg) != MEMCG_LRU_TAIL ?
>  	       MEMCG_LRU_TAIL : MEMCG_LRU_YOUNG;
>  }
> -- 

In some of the places do you want to consider adding a VM_WARN_ON_ONCE
for memcg_online not being true?

Acked-by: Balbir Singh <balbirs@xxxxxxxxxx>