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[109.81.26.156]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4831088d318sm51779665e9.10.2026.02.04.01.25.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Feb 2026 01:25:04 -0800 (PST) Date: Wed, 4 Feb 2026 10:25:03 +0100 From: Michal Hocko To: Akinobu Mita Cc: Joshua Hahn , 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, zhengqi.arch@bytedance.com, shakeel.butt@linux.dev, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, ziy@nvidia.com, matthew.brost@intel.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, bingjiao@google.com, jonathan.cameron@huawei.com, pratyush.brahma@oss.qualcomm.com Subject: Re: [PATCH v4 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier Message-ID: References: <20260127220003.3993576-1-joshua.hahnjy@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-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed 04-02-26 11:07:03, Akinobu Mita wrote: > 2026年2月2日(月) 22:11 Michal Hocko : > > > > On Thu 29-01-26 09:40:17, Akinobu Mita wrote: > > > 2026年1月28日(水) 7:00 Joshua Hahn : > > > > > > > > > > > Therefore, it appears that the behavior of get_swappiness() is important > > > > > > > in this issue. > > > > > > > > > > > > This is quite mysterious. > > > > > > > > > > > > Especially because get_swappiness() is an MGLRU exclusive function, I find > > > > > > it quite strange that the issue you mention above occurs regardless of whether > > > > > > MGLRU is enabled or disabled. With MGLRU disabled, did you see the same hangs > > > > > > as before? Were these hangs similarly fixed by modifying the callsite in > > > > > > get_swappiness? > > > > > > > > > > Good point. > > > > > When MGLRU is disabled, changing only the behavior of can_demote() > > > > > called by get_swappiness() did not solve the problem. > > > > > > > > > > Instead, the problem was avoided by changing only the behavior of > > > > > can_demote() called by can_reclaim_anon_page(), without changing the > > > > > behavior of can_demote() called from other places. > > > > > > > > > > > On a separate note, I feel a bit uncomfortable for making this the default > > > > > > setting, regardless of whether there is swap space or not. Just as it is > > > > > > easy to create a degenerate scenario where all memory is unreclaimable > > > > > > and the system starts going into (wasteful) reclaim on the lower tiers, > > > > > > it is equally easy to create a scenario where all memory is very easily > > > > > > reclaimable (say, clean pagecache) and we OOM without making any attempt to > > > > > > free up memory on the lower tiers. > > > > > > > > > > > > Reality is likely somewhere in between. And from my perspective, as long as > > > > > > we have some amount of easily reclaimable memory, I don't think immediately > > > > > > OOMing will be helpful for the system (and even if none of the memory is > > > > > > easily reclaimable, we should still try doing something before killing). > > > > > > > > > > > > > > > The reason for this issue is that memory allocations do not directly > > > > > > > > > trigger the oom-killer, assuming that if the target node has an underlying > > > > > > > > > memory tier, it can always be reclaimed by demotion. > > > > > > > > > > > > This patch enforces that the opposite of this assumption is true; that even > > > > > > if a target node has an underlying memory tier, it can never be reclaimed by > > > > > > demotion. > > > > > > > > > > > > Certainly for systems with swap and some compression methods (z{ram, swap}), > > > > > > this new enforcement could be harmful to the system. What do you think? > > > > > > > > > > Thank you for the detailed explanation. > > > > > > > > > > I understand the concern regarding the current patch, which only > > > > > checks the free memory of the demotion target node. > > > > > I will explore a solution. > > > > > > > > Hello Akinobu, I hope you had a great weekend! > > > > > > > > I noticed something that I thought was worth flagging. It seems like the > > > > primary addition of this patch, which is to check for zone_watermark_ok > > > > across the zones, is already a part of should_reclaim_retry(): > > > > > > > > /* > > > > * Keep reclaiming pages while there is a chance this will lead > > > > * somewhere. If none of the target zones can satisfy our allocation > > > > * request even if all reclaimable pages are considered then we are > > > > * screwed and have to go OOM. > > > > */ > > > > for_each_zone_zonelist_nodemask(zone, z, ac->zonelist, > > > > ac->highest_zoneidx, ac->nodemask) { > > > > > > > > [...snip...] > > > > > > > > /* > > > > * Would the allocation succeed if we reclaimed all > > > > * reclaimable pages? > > > > */ > > > > wmark = __zone_watermark_ok(zone, order, min_wmark, > > > > ac->highest_zoneidx, alloc_flags, available); > > > > > > > > if (wmark) { > > > > ret = true; > > > > break; > > > > } > > > > } > > > > > > > > ... which is called in __alloc_pages_slowpath. I wonder why we don't already > > > > hit this. It seems to do the same thing your patch is doing? > > > > > > I checked the number of calls and the time spent for several functions > > > called by __alloc_pages_slowpath(), and found that time is spent in > > > __alloc_pages_direct_reclaim() before reaching the first should_reclaim_retry(). > > > > > > After a few minutes have passed and the debug code that automatically > > > resets numa_demotion_enabled to false is executed, it appears that > > > __alloc_pages_direct_reclaim() immediately exits. > > > > First of all is this MGLRU or traditional reclaim? Or both? > > The behavior is almost the same whether MGLRU is enabled or not. > However, one difference is that __alloc_pages_direct_reclaim() may be > called multiple times when __alloc_pages_slowpath() is called, and > should_reclaim_retry() also returns true several times. > > This is probably because the watermark check in should_reclaim_retry() > considers not only NR_FREE_PAGES but also NR_ZONE_INACTIVE_ANON and > NR_ZONE_ACTIVE_ANON as potential free memory. (zone_reclaimable_pages()) Yes, seems like the same problem as with get_scan_count. > The following is the increment of stats in /proc/vmstat from the start > of the reproduction test until the problem occurred and > numa_demotion_enabled was automatically reset by the debug code and > OOM occurred a few minutes later: > > workingset_nodes 578 > workingset_refault_anon 5054381 > workingset_refault_file 41502 > workingset_activate_anon 3003283 > workingset_activate_file 33232 > workingset_restore_anon 2556549 > workingset_restore_file 27139 > workingset_nodereclaim 3472 > pgdemote_kswapd 121684 > pgdemote_direct 23977 > pgdemote_khugepaged 0 > pgdemote_proactive 0 > pgsteal_kswapd 3480404 > pgsteal_direct 2602011 > pgsteal_khugepaged 74 > pgsteal_proactive 0 > pgscan_kswapd 93334262 > pgscan_direct 227649302 > pgscan_khugepaged 1232161 > pgscan_proactive 0 > pgscan_direct_throttle 18 > pgscan_anon 320480379 > pgscan_file 1735346 > pgsteal_anon 5828270 > pgsteal_file 254219 You can clearly see that the order of magnitute of pages scanned is completely disproportional to pages actually reclaimed. So there is a lot of scanning without any progress at all. > > Then another thing I've noticed only now. There seems to be a layering > > discrepancy (for traditional LRU reclaim) when get_scan_count which > > controls the to-be-reclaimed lrus always relies on can_reclaim_anon_pages > > while down the reclaim path shrink_folio_list tries to be more clever > > and avoid demotion if it turns out to be inefficient. > > > > I wouldn't be surprised if get_scan_count predominantly (or even > > exclusively) scanned anon LRUs only while increasing the reclaim > > priority (so essentially just checked all anon pages on the LRU list) > > before concluding that it makes no sense. This can take quite some time > > and in the worst case you could be recycling couple of page cache pages > > remaining on the list to make small but sufficient progress to loop > > around. > > > > So I think the first step is to make the demotion behavior consistent. > > If demotion fails then it would probably makes sense to set sc->no_demotion > > so that get_scan_count can learn from the reclaim feedback that > > anonymous pages are not a good reclaim target in this situation. But the > > whole reclaim path needs a careful review I am afraid. > > If migrate_pages() in demote_folio_list() detects that it cannot > migrate any folios and all calls to alloc_demote_folio() also fail > (this is made possible by adding a few fields to migration_target_control), > it sets sc->no_demotion to true, which also resolves the issue. > > migrate_pages(demote_folios, alloc_demote_folio, NULL, > (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION, > &nr_succeeded); > if (!nr_succeeded && mtc.nr_alloc_tried > 0 && > (mtc.nr_alloc_tried == mtc.nr_alloc_failed)) { > sc->no_demotion = 1; > } This seems to low level place to make such a decision. Keep in mind that shrink_list operates on SWAP_CLUSTER_MAX batches so the backoff could be pre-mature. shrink_lruvec seems like a better place to make such a decision but this really requires a deeper evaluation. Anyway, it is good that we have a better understanding what is going on. Thanks for confirming the theory. -- Michal Hocko SUSE Labs