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 E235C3EEAF2; Wed, 22 Jul 2026 12:09:06 +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=1784722149; cv=none; b=paH6s7nYfVHcivaJbVFlyab7ZFmZcmb2vNyQ14ey7xCO99MVPKiaVhz844XlX45M3odBqSj8QaENJbxl9wdFnj4NpDvoNcr1REzEIeP46u/TYYgDMypVh3uWGsokVH8SbNdR+8gHyVUumrsdzug/22ElT384NXBghlgX0F0LSmU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784722149; c=relaxed/simple; bh=yKqgBFVepbuyl85FKJAdLutciDnB1GkWBw0Spr1i/t8=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=aL1if5I8F5hTTxAthKoE92xBv5HKex7ZZON9eogjJNt8Q0agfcQuUqzwjSD06f2s08U5eBKbf7npUacWSqsV+fL9AnV8IH14pUs+lQGIDQX0S+fMXu2Vw4qZH+NvsH3gaqZim+/Pq3XB81DTXdnpiORInR9Qe3i73qn6On/21fo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=h4BjvBWS; 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="h4BjvBWS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 37C0A1F00AC4; Wed, 22 Jul 2026 12:08:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784722146; bh=fzaCjd1V+ASyDQB1PUAamb4N4H4/SzFykypVnhK0PwI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=h4BjvBWSQi5Rs/paOo/+kJWHoJlxaxqTMYShK/HqlnqH/gD4EBG+2xyucREFSdcuU 2mZpE67IWbvDj4TicB5UhTUL2g0VhS9/UxgVa+ak3cwefPLRTReebKV4KX7S3F8FRy CDxKxkt7JTlmmAQgqdJe5AslfO/x96cTDWEb2RSwlsJAhqQQ5rGjbyLZa95FdejakJ rjO92b6OTgVOSU+Sk0xZVjPVakVu8XwzAFweyU5C2+u0tGtmGY83n2M1oC38TIn8VT ZKqESCG8leQqRevl1VuQDvmALiHzgS4p0cLdRyBVSCPRBDXDtkjhb8kdNaOcqCutvC 4iuNo+2Imb24g== Message-ID: <3d18e400-5e7f-468a-b96e-d603d9535ac9@kernel.org> Date: Wed, 22 Jul 2026 14:08:53 +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 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> From: "David Hildenbrand (Arm)" 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: <20260722074811.378283-1-aboorvad@linux.ibm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. -- Cheers, David