From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 34DEA282F0E; Wed, 22 Jul 2026 12:30:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784723408; cv=none; b=CGuo1H3qAue2hsLK5sWVc+QQ3cXUcOt2/+vwiSGvok1pd8ZoUg+Go7ma9YTXj2319WHedGYindfi6o49zoRYrITBbmvC3IW2CeRHlT/yz9a5VevzQifbqNxIralFR0Um0toUrn4L0vVPOUM7oQw74WwRIvDKAq84QjikjgP7CoU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784723408; c=relaxed/simple; bh=v8f3Y6Va4MYAMte3Ym0aeTlEoMhGlP40kL/cMrbSs0Q=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=AcjGqbsVKODCyqwTiCeRlSoIB883GsZKmftfo124zh7pbgLJ3++npwRS6xp4qT3FjZ/llruCqYn/JlXSWHpchfkGNDgfG7C/13L8eTR9zvBa7BZgIHjmPZSvCyedlOZGLElb/1lTxaqq3kYvM0d8FILtaww7pm7aSIqnH/XtMbE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GBTHz7JI; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="GBTHz7JI" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 713261F00A3A; Wed, 22 Jul 2026 12:30:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784723406; bh=hrOPOUTvi7d4zFr0ZQTGTnbL7Djm33SC2gEDM33Natk=; h=Date:Subject:From:To:Cc:References:In-Reply-To; b=GBTHz7JIktkPshg/tdO4IZLMEYgC4lBVN+oR275OWfoy/V5zPsN3Xh8oClpeby6h3 nGEx4OrQsRalXgfgRXARR7ArxWouv8ZdMroO+Vuky0e+RuG2THbaNoNgWGDDq2xtGA Q5h9cQ+LajqztSLJDtZnrwb+X0iYDUIMyk5d7pgNI7EBbwtrlps9yLdFb8IwsE3cNh 0/rOfHClCEmplfhT+jypne0Xxx0Pdm8KqfYKYDsEqTyfnActQjev2Q7QFTi6xPkizp TpiaUcnGsbMiquG/ctvf55hSCAxNkBqV3t3d4PX8xOcYaIVch9tNpaeRY7z2yelfCU fvaz9qi8FjkIQ== Message-ID: Date: Wed, 22 Jul 2026 14:29:57 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/2] mm/memory_hotplug: bound offline retry loops with a configurable limit From: "David Hildenbrand (Arm)" To: Aboorva Devarajan , Andrew Morton , Oscar Salvador Cc: Michal Hocko , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Jonathan Corbet , Shuah Khan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, "Ritesh Harjani (IBM)" References: <20260722074811.378283-1-aboorvad@linux.ibm.com> <3d18e400-5e7f-468a-b96e-d603d9535ac9@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <3d18e400-5e7f-468a-b96e-d603d9535ac9@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/22/26 14:08, David Hildenbrand (Arm) wrote: > On 7/22/26 09:48, Aboorva Devarajan wrote: >> Memory offlining can loop forever when a memory block holds a page that >> can never be migrated or freed. This series adds an opt-in retry limit >> that lets offline_pages() bail out with -EBUSY instead of retrying >> forever. The default (0) preserves today's behaviour exactly. >> >> Patch 1 is the fix; patch 2 is an in-tree selftest that reproduces the >> hang from an unsignalable kworker and verifies the limit recovers it. >> >> 1. The problem >> ============== >> >> offline_pages() repeats two nested steps until the whole range is >> isolated: >> >> 1. Inner loop: find and migrate movable pages out of the range >> (scan_movable_pages() + do_migrate_range()). >> 2. Outer loop: re-check isolation (test_pages_isolated()); if not >> fully isolated, go back to step 1. >> >> Neither loop has a termination condition. When a page can never be >> migrated, scan_movable_pages() keeps returning the same pfn, >> do_migrate_range() keeps failing on it, and control never even reaches >> the outer isolation re-check - the inner loop spins forever. The admin >> guide acknowledges this: >> >> "Further, memory offlining might retry for a long time (or even >> forever), until aborted by the user." >> >> The single escape in the loop is signal_pending(current): >> >> offline_pages(pfn, end_pfn): >> do { /* outer: isolation */ >> do { /* inner: migration */ >> if (signal_pending(current)) /* <-- ONLY escape */ >> goto failed_removal; >> pfn = scan_movable_pages(pfn, end_pfn); >> do_migrate_range(pfn, end_pfn); /* may never succeed */ >> } while (pfn < end_pfn); >> } while (test_pages_isolated(...)); >> >> >> 2. Why the kernel itself should be able to bail >> =============================================== >> >> We keep hitting this during memory hot-remove operations: a single >> stuck page can block the whole operation. This was also reported in >> [4] earlier; one example, where offlining kept failing to migrate a >> busy block-device page-cache page (aops:def_blk_aops) in a normal zone: >> >> [10880.889199] page dumped because: migration failure >> [10880.889232] migrating pfn 2a87b failed ret:1 >> [10880.889235] page: refcount:3 mapcount:0 mapping:00000000718ec5a6 index:0x857 pfn:0x2a87b >> [10880.889241] aops:def_blk_aops ino:800003 dentry name(?):"" >> [10880.889245] flags: 0x33ffffe00004104(referenced|active|private|node=3|zone=0|lastcpupid=0x1fffff) >> ... >> [10880.889291] migrating pfn 2a87b failed ret:1 > > Hi, > > the text reads AI generated. If you did use AI, please disclose it properly. > > Quick feedback: > > 1) Using passes is not really what we want in many cases (e.g., ZONE_MOVABLE > where we might need a couple of seconds/minutes to complete offlining with a lot > of concurrent activity). An actual timeout is also problematic (see below). > > 2) Having a toggle for all offline_pages() users is questionable. In particular, > user-triggered offlining or offlinig triggered on ZONE_MOVABLE etc should not > obey such timeouts. > > I proposed a more restricted approach only for offline_and_remove_memory() > previously [1] > > [1] https://lore.kernel.org/all/20230627112220.229240-1-david@redhat.com/ > > Michal back then commented "I really hate having timeouts back. They just proven > to be hard to get right and it is essentially a policy implemented in the > kernel. They simply do not belong to the kernel space IMHO." > > And I agree. > > I assume you run into such issues with DIMMs in VMs? virtio-mem handles that > much nicer nowadays, by essentially doing the hard part that should fail easily > through alloc_contig_range(). > > offline_pages() is just designed to retry forever. > One thing we can definitely do is to just fail faster if we find an unmovable page in !ZONE_MOVABLE. Usually that happens when we race offlining (has_unmovable_pages()) with page allocation, and failing on unmovable pages is actually perfectly fine. But for ZONE_MOVABLE we should keep retrying, because some pages might only look temporarily unmovable. So that would be the low hanging fruit: on !ZONE_MOVABLE, fail faster. -- Cheers, David