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From: Shakeel Butt To: Qi Zheng Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, muchun.song@linux.dev, david@kernel.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, harry.yoo@oracle.com, yosry.ahmed@linux.dev, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, chenridong@huaweicloud.com, mkoutny@suse.com, akpm@linux-foundation.org, hamzamahfooz@linux.microsoft.com, apais@linux.microsoft.com, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Qi Zheng Subject: Re: [PATCH v3 26/30] mm: vmscan: prepare for reparenting MGLRU folios Message-ID: References: <92e0728fed3d68855173352416cf8077670610f0.1768389889.git.zhengqi.arch@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <92e0728fed3d68855173352416cf8077670610f0.1768389889.git.zhengqi.arch@bytedance.com> X-Migadu-Flow: FLOW_OUT X-Stat-Signature: dko55w1kmpctnw4h8j9hxu7iu3ncbd36 X-Rspam-User: X-Rspamd-Queue-Id: 59B54A0003 X-Rspamd-Server: rspam08 X-HE-Tag: 1768706964-699046 X-HE-Meta: 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 iHcYklAT dPSvrz9XfoyeQT2Mq+33fGCSdB8GlGTHtb1hUuklBlABFrdO9GtNeLpFMC2WEYGVl96C05CxyHe7r6XfzbeoLM09qnwixeUVVbNj9g6/lC95BPTqDGaC4uRtVp3x/4yY8fzEvCbPS8lBjILyQ7Y7BYDpDp5dLvHnSt+DH0v391zKROJIfcUHmlTWlkKksjeUicTFMtJHjezMHfG6JokzkpOnorALq1uPFSj8962qyDF/MZNTNpG0LplWv2XEQnGNwV/At X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: I asked AI to review this patch and it find following potential issues: Author: Qi Zheng mm: vmscan: prepare for reparenting MGLRU folios This commit adds infrastructure for reparenting MGLRU folios from dying memcg to parent memcg, including functions to ensure parent has MAX_NR_GENS generations before reparenting. Link: https://lkml.kernel.org/r/92e0728fed3d68855173352416cf8077670610f0.1768389889.git.zhengqi.arch@bytedance.com > diff --git a/mm/vmscan.c b/mm/vmscan.c > index f1a45c94c2fc..284292653d14 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c [ ... ] > +static void try_to_inc_max_seq_nowalk(struct mem_cgroup *memcg, > + struct lruvec *lruvec) > +{ > + struct lru_gen_mm_list *mm_list = get_mm_list(memcg); > + struct lru_gen_mm_state *mm_state = get_mm_state(lruvec); > + int swappiness = mem_cgroup_swappiness(memcg); > + DEFINE_MAX_SEQ(lruvec); > + bool success = false; > + > + /* > + * We are not iterating the mm_list here, updating mm_state->seq is just > + * to make mm walkers work properly. > + */ > + if (mm_state) { > + spin_lock(&mm_list->lock); > + VM_WARN_ON_ONCE(mm_state->seq + 1 < max_seq); > + if (max_seq > mm_state->seq) { ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ When mm_state->seq equals max_seq (which can happen when mm walkers have completed their iteration), this condition is false, success remains false, and inc_max_seq() is never called. The caller max_lru_gen_memcg() loops with: while (get_nr_gens(lruvec, type) < MAX_NR_GENS) { try_to_inc_max_seq_nowalk(memcg, lruvec); cond_resched(); } If try_to_inc_max_seq_nowalk() fails to increment max_seq, get_nr_gens() returns the same value, and the while loop continues indefinitely. Can this cause an infinite loop during memcg offline when mm_state->seq has caught up to max_seq? Suggested fix: Remove the success variable and always call inc_max_seq(). The mm_state->seq update is only for coordination with mm walkers - it should not gate whether we increment the generation. inc_max_seq() has its own staleness check (seq < READ_ONCE(lrugen->max_seq)) so calling it unconditionally is safe: if (mm_state) { spin_lock(&mm_list->lock); VM_WARN_ON_ONCE(mm_state->seq + 1 < max_seq); if (max_seq > mm_state->seq) WRITE_ONCE(mm_state->seq, mm_state->seq + 1); spin_unlock(&mm_list->lock); } inc_max_seq(lruvec, max_seq, swappiness); > + WRITE_ONCE(mm_state->seq, mm_state->seq + 1); > + success = true; > + } > + spin_unlock(&mm_list->lock); > + } else { > + success = true; > + } > + > + if (success) > + inc_max_seq(lruvec, max_seq, swappiness); > +} [ ... ] > +static void __lru_gen_reparent_memcg(struct lruvec *child_lruvec, struct lruvec *parent_lruvec, > + int zone, int type) > +{ > + struct lru_gen_folio *child_lrugen, *parent_lrugen; > + enum lru_list lru = type * LRU_INACTIVE_FILE; > + int i; [ ... ] > + WRITE_ONCE(child_lrugen->nr_pages[gen][type][zone], 0); > + WRITE_ONCE(parent_lrugen->nr_pages[gen][type][zone], > + parent_lrugen->nr_pages[gen][type][zone] + nr_pages); > + > + update_lru_size(parent_lruvec, lru + dst_lru_active, zone, nr_pages); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This update_lru_size() call updates both __update_lru_size() counters and mem_cgroup_update_lru_size(). However, lru_gen_reparent_memcg() also calls mem_cgroup_update_lru_size() for LRU_UNEVICTABLE. The combination appears to cause incorrect accounting. Note: This issue was fixed in later commit ("mm: mglru: do not call update_lru_size() during reparenting"). > + } > +}