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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 > --- > 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