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Sat, 28 Feb 2026 20:10:30 -0800 (PST) From: Barry Song <21cnbao@gmail.com> To: lenohou@gmail.com Cc: 21cnbao@gmail.com, akpm@linux-foundation.org, axelrasmussen@google.com, laoar.shao@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, weixugc@google.com, wjl.linux@gmail.com, yuanchu@google.com, yuzhao@google.com Subject: Re: [PATCH] mm/mglru: fix cgroup OOM during MGLRU state switching Date: Sun, 1 Mar 2026 12:10:22 +0800 Message-Id: <20260301041022.66111-1-21cnbao@gmail.com> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Sun, Mar 1, 2026 at 6:41 AM Barry Song <21cnbao@gmail.com> wrote: > > On Sun, Mar 1, 2026 at 5:28 AM Barry Song <21cnbao@gmail.com> wrote: > [...] > > > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > > index 3e51190a55e4..ba306e986050 100644 > > --- a/include/linux/mmzone.h > > +++ b/include/linux/mmzone.h > > @@ -509,6 +509,8 @@ struct lru_gen_folio { > >         atomic_long_t refaulted[NR_HIST_GENS][ANON_AND_FILE][MAX_NR_TIERS]; > >         /* whether the multi-gen LRU is enabled */ > >         bool enabled; > > +       /* whether the multi-gen LRU is switching from/to active/inactive LRU */ > > +       bool switching; > >         /* the memcg generation this lru_gen_folio belongs to */ > >         u8 gen; > >         /* the list segment this lru_gen_folio belongs to */ > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index 0fc9373e8251..60fc611067c7 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -5196,6 +5196,7 @@ static void lru_gen_change_state(bool enabled) > >                         VM_WARN_ON_ONCE(!state_is_valid(lruvec)); > > > >                         lruvec->lrugen.enabled = enabled; > > +                       smp_store_release(&lruvec->lrugen.switching, true); > > Sorry, I actually meant: > >  +                       smp_store_release(&lruvec->lrugen.switching, true); >                          lruvec->lrugen.enabled = enabled; > > But I guess we could still hit a race condition in extreme cases—switching > MGLRU on or off as frequently as possible. The only reliable way is to check > enabled during shrinking while holding the lruvec’s lock. Sorry, I was talking to myself.... Since the switching and the 'enabled' state are not inherently serialized with shrink_lruvec(), their values can change at any time, leading to race conditions. Therefore, I believe the only safe approach is: 1. Do not allow enabling or disabling MGLRU on an lruvec while    shrink_lruvec() is running. 2. Do not allow shrink_lruvec() to run while MGLRU is being enabled    or disabled on that lruvec. Something like the following: diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 3e51190a55e4..c4b07159577e 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -509,6 +509,7 @@ struct lru_gen_folio { atomic_long_t refaulted[NR_HIST_GENS][ANON_AND_FILE][MAX_NR_TIERS]; /* whether the multi-gen LRU is enabled */ bool enabled; + struct rw_semaphore switch_lock; /* the memcg generation this lru_gen_folio belongs to */ u8 gen; /* the list segment this lru_gen_folio belongs to */ diff --git a/mm/vmscan.c b/mm/vmscan.c index 0fc9373e8251..aadf1e7c31cf 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -5190,6 +5190,7 @@ static void lru_gen_change_state(bool enabled) for_each_node(nid) { struct lruvec *lruvec = get_lruvec(memcg, nid); + down_write(&lruvec->lrugen.switch_lock); spin_lock_irq(&lruvec->lru_lock); VM_WARN_ON_ONCE(!seq_is_valid(lruvec)); @@ -5204,6 +5205,7 @@ static void lru_gen_change_state(bool enabled) } spin_unlock_irq(&lruvec->lru_lock); + up_write(&lruvec->lrugen.switch_lock); } cond_resched(); @@ -5680,6 +5682,7 @@ void lru_gen_init_lruvec(struct lruvec *lruvec) lrugen->max_seq = MIN_NR_GENS + 1; lrugen->enabled = lru_gen_enabled(); + init_rwsem(&lrugen->switch_lock); for (i = 0; i <= MIN_NR_GENS + 1; i++) lrugen->timestamps[i] = jiffies; @@ -5780,10 +5783,14 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc) bool proportional_reclaim; struct blk_plug plug; - if (lru_gen_enabled() && !root_reclaim(sc)) { +#ifdef CONFIG_LRU_GEN + down_read(&lruvec->lrugen.switch_lock); + if (lruvec->lrugen.enabled && !root_reclaim(sc)) { lru_gen_shrink_lruvec(lruvec, sc); + up_read(&lruvec->lrugen.switch_lock); return; } +#endif get_scan_count(lruvec, sc, nr); @@ -5885,6 +5892,9 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc) inactive_is_low(lruvec, LRU_INACTIVE_ANON)) shrink_active_list(SWAP_CLUSTER_MAX, lruvec, sc, LRU_ACTIVE_ANON); +#ifdef CONFIG_LRU_GEN + up_read(&lruvec->lrugen.switch_lock); +#endif } /* Use reclaim/compaction for costly allocs or under memory pressure */ -- Thanks Barry