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Tue, 30 Sep 2025 07:43:28 +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 wCUgOyCK22gKVAAAD6G6ig (envelope-from ); Tue, 30 Sep 2025 07:43:28 +0000 Message-ID: <877fa6d5-e2c7-41d8-88f7-6ee6ac395fc2@suse.cz> Date: Tue, 30 Sep 2025 09:43:28 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: re-enable kswapd when memory pressure subsides or demotion is toggled Content-Language: en-US To: Chanwon Park , akpm@linux-foundation.org, surenb@google.com, mhocko@suse.com, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, david@redhat.com, zhengqi.arch@bytedance.com, shakeel.butt@linux.dev, lorenzo.stoakes@oracle.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 61EA38000C X-Stat-Signature: 6q8b9eon1nq78ahq6eywqxm9rnrzw7pg X-Rspam-User: X-HE-Tag: 1759218212-859166 X-HE-Meta: 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 RPI1wQB1 WMh5dYoutYUxt98VEn2uVFFAh1g/NoQ2dK74JLSiEQeXFXLscRYow40A8adQrzl2Ylca0fNVj9WIi7eU9X1ErQjrJ0ssp6xHMwzTo7Zn/HIthB3BrwJrheb2XQblKvQeNDTD1/XA8QUzRn+3VNXth5ugFglhgWKWx2+lnydDFcMUEHxDZXaJzxW13dZ4nPORwr51XM2axMYrFhSWBFSGG0aTk9XY434xVddWj+LAbYTkrw6fNf0ETjH2ZyKcxddtAHDYEMgTLy5Jral8OK4mJu3ze/ULWTgO+jyYcUkiG6OhHeqZUh8SJxWDxbQ== 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: On 9/8/25 12:04, Chanwon Park wrote: > If kswapd fails to reclaim pages from a node MAX_RECLAIM_RETRIES in a > row, kswapd on that node gets disabled. That is, the system won't wakeup > kswapd for that node until page reclamation is observed at least once. > That reclamation is mostly done by direct reclaim, which in turn enables > kswapd back. > > However, on systems with CXL memory nodes, workloads with high anon page > usage can disable kswapd indefinitely, without triggering direct > reclaim. This can be reproduced with following steps: > > numa node 0 (32GB memory, 48 CPUs) > numa node 2~5 (512GB CXL memory, 128GB each) > (numa node 1 is disabled) > swap space 8GB > > 1) Set /sys/kernel/mm/demotion_enabled to 0. > 2) Set /proc/sys/kernel/numa_balancing to 0. > 3) Run a process that allocates and random accesses 500GB of anon > pages. > 4) Let the process exit normally. > > During 3), free memory on node 0 gets lower than low watermark, and > kswapd runs and depletes swap space. Then, kswapd fails consecutively > and gets disabled. Allocation afterwards happens on CXL memory, so node > 0 never gains more memory pressure to trigger direct reclaim. > > After 4), kswapd on node 0 remains disabled, and tasks running on that > node are unable to swap. If you turn on NUMA_BALANCING_MEMORY_TIERING > and demotion now, it won't work properly since kswapd is disabled. > > To mitigate this problem, reset kswapd_failures to 0 on following > conditions: > > a) ZONE_BELOW_HIGH bit of a zone in hopeless node with a fallback > memory node gets cleared. > b) demotion_enabled is changed from false to true. > > Rationale for a): > ZONE_BELOW_HIGH bit being cleared might be a sign that the node may > be reclaimable afterwards. This won't help much if the memory-hungry > process keeps running without freeing anything, but at least the node > will go back to reclaimable state when the process exits. > > Rationale for b): > When demotion_enabled is false, kswapd can only reclaim anon pages by > swapping them out to swap space. If demotion_enabled is turned on, > kswapd can demote anon pages to another node for reclaiming. So, the > original failure count for determining reclaimability is no longer > valid. > > Since kswapd_failures resets may be missed by ++ operation, it is > changed from int to atomic_t. > > Signed-off-by: Chanwon Park Acked-by: Vlastimil Babka