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Sat, 28 Feb 2026 13:28:45 -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 05:28:37 +0800 Message-Id: <20260228212837.59661-1-21cnbao@gmail.com> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: <20260228161008.707-1-lenohou@gmail.com> References: <20260228161008.707-1-lenohou@gmail.com> 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 12:10 AM Leno Hou wrote: > > When the Multi-Gen LRU (MGLRU) state is toggled dynamically, a race > condition exists between the state switching and the memory reclaim > path. This can lead to unexpected cgroup OOM kills, even when plenty of > reclaimable memory is available. > > *** Problem Description *** > > The issue arises from a "reclaim vacuum" during the transition: > > 1. When disabling MGLRU, lru_gen_change_state() sets lrugen->enabled to > false before the pages are drained from MGLRU lists back to > traditional LRU lists. > 2. Concurrent reclaimers in shrink_lruvec() see lrugen->enabled as false > and skip the MGLRU path. > 3. However, these pages might not have reached the traditional LRU lists > yet, or the changes are not yet visible to all CPUs due to a lack of > synchronization. > 4. get_scan_count() subsequently finds traditional LRU lists empty, > concludes there is no reclaimable memory, and triggers an OOM kill. > > A similar race can occur during enablement, where the reclaimer sees > the new state but the MGLRU lists haven't been populated via > fill_evictable() yet. > > *** Solution *** > > Introduce a 'draining' state to bridge the gap during transitions: > > - Use smp_store_release() and smp_load_acquire() to ensure the visibility > of 'enabled' and 'draining' flags across CPUs. > - Modify shrink_lruvec() to allow a "joint reclaim" period. If an lruvec > is in the 'draining' state, the reclaimer will attempt to scan MGLRU > lists first, and then fall through to traditional LRU lists instead > of returning early. This ensures that folios are visible to at least > one reclaim path at any given time. > > *** Reproduction *** > > The issue was consistently reproduced on v6.1.157 and v6.18.3 using > a high-pressure memory cgroup (v1) environment. > > Reproduction steps: > 1. Create a 16GB memcg and populate it with 10GB file cache (5GB active) > and 8GB active anonymous memory. > 2. Toggle MGLRU state while performing new memory allocations to force > direct reclaim. > > Reproduction script: > --- > #!/bin/bash > # Fixed reproduction for memcg OOM during MGLRU toggle > set -euo pipefail > > MGLRU_FILE="/sys/kernel/mm/lru_gen/enabled" > CGROUP_PATH="/sys/fs/cgroup/memory/memcg_oom_test" > > # Switch MGLRU dynamically in the background > switch_mglru() { > local orig_val=$(cat "$MGLRU_FILE") > if [[ "$orig_val" != "0x0000" ]]; then > echo n > "$MGLRU_FILE" & > else > echo y > "$MGLRU_FILE" & > fi > } > > # Setup 16G memcg > mkdir -p "$CGROUP_PATH" > echo $((16 * 1024 * 1024 * 1024)) > "$CGROUP_PATH/memory.limit_in_bytes" > echo $$ > "$CGROUP_PATH/cgroup.procs" > > # 1. Build memory pressure (File + Anon) > dd if=/dev/urandom of=/tmp/test_file bs=1M count=10240 > dd if=/tmp/test_file of=/dev/null bs=1M # Warm up cache > > stress-ng --vm 1 --vm-bytes 8G --vm-keep -t 600 & > sleep 5 > > # 2. Trigger switch and concurrent allocation > switch_mglru > stress-ng --vm 1 --vm-bytes 2G --vm-populate --timeout 5s || echo "OOM Triggered" > > # Check OOM counter > grep oom_kill "$CGROUP_PATH/memory.oom_control" > --- > > Signed-off-by: Leno Hou > > --- > To: linux-mm@kvack.org > To: linux-kernel@vger.kernel.org > Cc: Andrew Morton > Cc: Axel Rasmussen > Cc: Yuanchu Xie > Cc: Wei Xu > Cc: Barry Song <21cnbao@gmail.com> > Cc: Jialing Wang > Cc: Yafang Shao > Cc: Yu Zhao > --- > include/linux/mmzone.h | 2 ++ > mm/vmscan.c | 14 +++++++++++--- > 2 files changed, 13 insertions(+), 3 deletions(-) > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 7fb7331c5725..0648ce91dbc6 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 draining to LRU */ > + bool draining; > /* 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 06071995dacc..629a00681163 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -5222,7 +5222,8 @@ static void lru_gen_change_state(bool enabled) > VM_WARN_ON_ONCE(!seq_is_valid(lruvec)); > VM_WARN_ON_ONCE(!state_is_valid(lruvec)); > > - lruvec->lrugen.enabled = enabled; > + smp_store_release(&lruvec->lrugen.enabled, enabled); > + smp_store_release(&lruvec->lrugen.draining, true); > > while (!(enabled ? fill_evictable(lruvec) : drain_evictable(lruvec))) { > spin_unlock_irq(&lruvec->lru_lock); > @@ -5230,6 +5231,8 @@ static void lru_gen_change_state(bool enabled) > spin_lock_irq(&lruvec->lru_lock); > } > > + smp_store_release(&lruvec->lrugen.draining, false); > + > spin_unlock_irq(&lruvec->lru_lock); > } > > @@ -5813,10 +5816,15 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc) > unsigned long nr_to_reclaim = sc->nr_to_reclaim; > bool proportional_reclaim; > struct blk_plug plug; > + bool lrugen_enabled = smp_load_acquire(&lruvec->lrugen.enabled); > + bool lru_draining = smp_load_acquire(&lruvec->lrugen.draining); > > - if (lru_gen_enabled() && !root_reclaim(sc)) { > + if (lrugen_enabled || lru_draining && !root_reclaim(sc)) { > lru_gen_shrink_lruvec(lruvec, sc); > - return; Is it possible to simply wait for draining to finish instead of performing an lru_gen/lru shrink while lru_gen is being disabled or enabled? Performing a shrink in an intermediate state may still involve a lot of uncertainty, depending on how far the shrink has progressed and how much remains in each side’s LRU？ 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); while (!(enabled ? fill_evictable(lruvec) : drain_evictable(lruvec))) { spin_unlock_irq(&lruvec->lru_lock); @@ -5203,6 +5204,8 @@ static void lru_gen_change_state(bool enabled) spin_lock_irq(&lruvec->lru_lock); } + smp_store_release(&lruvec->lrugen.switching, false); + spin_unlock_irq(&lruvec->lru_lock); } @@ -5780,6 +5783,10 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc) bool proportional_reclaim; struct blk_plug plug; +#ifdef CONFIG_LRU_GEN + while (smp_load_acquire(&lruvec->lrugen.switching)) + schedule_timeout_uninterruptible(HZ/100); +#endif if (lru_gen_enabled() && !root_reclaim(sc)) { lru_gen_shrink_lruvec(lruvec, sc); return; -- > + > + if (!lru_draining) > + return; > + > } > > get_scan_count(lruvec, sc, nr); > -- > 2.52.0 > Thanks Barry