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Wed, 17 Dec 2025 12:55:46 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id qROsDlKoQmklMwAAD6G6ig (envelope-from ); Wed, 17 Dec 2025 12:55:46 +0000 Message-ID: <01d780a3-16ef-4f87-b57d-00d50afa4bed@suse.cz> Date: Wed, 17 Dec 2025 13:55:45 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier Content-Language: en-US To: Akinobu Mita Cc: linux-cxl@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, hannes@cmpxchg.org, david@kernel.org, mhocko@kernel.org, zhengqi.arch@bytedance.com, shakeel.butt@linux.dev, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, rppt@kernel.org, surenb@google.com References: <20251208094028.214949-1-akinobu.mita@gmail.com> <20251208094028.214949-3-akinobu.mita@gmail.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Flag: NO X-Spam-Score: -2.80 X-Spam-Level: X-Spamd-Result: default: False [-2.80 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-0.996]; MIME_GOOD(-0.10)[text/plain]; TAGGED_RCPT(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; FREEMAIL_TO(0.00)[gmail.com]; RCPT_COUNT_TWELVE(0.00)[17]; MID_RHS_MATCH_FROM(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; URIBL_BLOCKED(0.00)[imap1.dmz-prg2.suse.org:helo,suse.cz:mid]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.cz:mid,imap1.dmz-prg2.suse.org:helo] On 12/8/25 10:40, Akinobu Mita wrote: > On systems with multiple memory-tiers consisting of DRAM and CXL memory, > the OOM killer is not invoked properly. > > Here's the command to reproduce: > > $ sudo swapoff -a > $ stress-ng --oomable -v --memrate 20 --memrate-bytes 10G \ > --memrate-rd-mbs 1 --memrate-wr-mbs 1 > > The memory usage is the number of workers specified with the --memrate > option multiplied by the buffer size specified with the --memrate-bytes > option, so please adjust it so that it exceeds the total size of the > installed DRAM and CXL memory. > > If swap is disabled, you can usually expect the OOM killer to terminate > the stress-ng process when memory usage approaches the installed memory > size. > > However, if multiple memory-tiers exist (multiple > /sys/devices/virtual/memory_tiering/memory_tier directories exist), > and /sys/kernel/mm/numa/demotion_enabled is true and > /sys/kernel/mm/lru_gen/min_ttl_ms is 0, the OOM killer will not be invoked Does this mean only mglru has the problem, or !mglru too? Also is min_ttl_ms = 0 a sensible setting? What happens without it? If !mglru doesn't have this problem, how does the fix affect it? > and the system will become inoperable. > > This issue can be reproduced using NUMA emulation even on systems with > only DRAM. You can create two-fake memory-tiers by booting a single-node > system with "numa=fake=2 numa_emulation.adistance=576,704" kernel > parameters. > > The reason for this issue is that if the target node for allocation has > an underlying memory tier, it is always assumed that it can be reclaimed > via demotion. > > So this change avoids this issue by not attempting to demote if the > demoting node has less free memory than the minimum watermark. > > Signed-off-by: Akinobu Mita > --- > mm/vmscan.c | 15 ++++++++++++++- > 1 file changed, 14 insertions(+), 1 deletion(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index fddd168a9737..f4748f258294 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -356,7 +356,20 @@ static bool can_demote(int nid, struct scan_control *sc, > return false; > > /* If demotion node isn't in the cgroup's mems_allowed, fall back */ > - return mem_cgroup_node_allowed(memcg, demotion_nid); > + if (mem_cgroup_node_allowed(memcg, demotion_nid)) { > + int z; > + struct zone *zone; > + struct pglist_data *pgdat = NODE_DATA(demotion_nid); > + unsigned int highest_zoneidx = sc ? sc->reclaim_idx : MAX_NR_ZONES - 1; > + int order = sc ? sc->order : 0; > + > + for_each_managed_zone_pgdat(zone, pgdat, z, highest_zoneidx) { > + if (zone_watermark_ok(zone, order, min_wmark_pages(zone), > + highest_zoneidx, 0)) > + return true; > + } > + } > + return false; > } > > static inline bool can_reclaim_anon_pages(struct mem_cgroup *memcg,