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[2003:d8:2f28:7c00:a95e:ac49:f2ad:ab84]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-447f6b297easm41646885e9.6.2025.05.20.10.55.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 20 May 2025 10:55:19 -0700 (PDT) Message-ID: Date: Tue, 20 May 2025 19:55:18 +0200 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: [RFC PATCH 1/4] mm/memory_hotplug: Add interface for runtime (de)configuration of memory To: Sumanth Korikkar Cc: linux-mm , Andrew Morton , Oscar Salvador , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , linux-s390 , LKML References: <20241202082732.3959803-1-sumanthk@linux.ibm.com> <20241202082732.3959803-2-sumanthk@linux.ibm.com> <3151b9a0-3e96-4820-b6af-9f9ec4996ee1@redhat.com> <1b9285ba-4118-4572-9392-42ec6ba6728c@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 20.05.25 15:06, Sumanth Korikkar wrote: >> Maybe "standby memory" might make it clearer. The concept is s390x specific, >> and it will likely stay s390x specific. >> >> I like the idea (frontend/tool interface), all we need is a way for these >> commands to detect ranges and turn them from standby into usable memory. >> >>> >>> The user can still determine the available memory ranges and make them >>> configurable using tools like lsmem or chmem with this approach atleast >>> on s390 with this approach. >>> >>>> My thinking was that s390x would expose the standby memory ranges somewhere >>>> arch specific in sysfs. From there, one could simply trigger the adding >>>> (maybe specifying e.g, memmap_on_memory) of selected ranges. > > Hi David, Hi! > > Sorry for the late reply. > > Potential design approach for enabling dynamic configuration and > deconfiguration of hotplug memory with support for both altmap and > non-altmap usage. > > Introduces flexibility, allowing users to specify at runtime which > memory ranges should utilize altmap, rather than relying on a static > system-wide setting that applies uniformly to all hotplugged memory. > > Introduce new interface on s390 with the following attributes: > > 1) Attribute1: > /sys/firmware/memory/block_size_bytes I assume this will be the storage increment size. > > 2) Attribute2: > /sys/firmware/memory/memoryX/config > echo 0 > /sys/firmware/memory/memoryX/config -> deconfigure memoryX > echo 1 > /sys/firmware/memory/memoryX/config -> configure memoryX And these would configure individual storage increments, essentially calling add_memory() and (if possible because we could offline the memory) remove_memory(). > > 3) Attribute3: > /sys/firmware/memory/memoryX/altmap_required > echo 0 > /sys/firmware/memory/memoryX/altmap_required -> noaltmap > echo 1 > /sys/firmware/memory/memoryX/altmap_required -> altmap > echo N > /sys/firmware/memory/memoryX/altmap_required -> variable size > altmap grouping (possible future requirements), > where N specifies the number of memory blocks that the current > memory block manages altmap. There are two possibilities here: >       * If the altmap cannot fit entirely within memoryX, it can >        extend into memoryX+1, meaning the altmap metadata will span >        across multiple memory blocks. >       * If the altmap for memory range cannot fit within memoryX, >         then config will return -EINVAL. Do we really still need this when we can configure/deconfigure? I mean, on s390x, the most important use case for memmap-on-memory was not wasting memory for offline memory blocks. But with a configuration interface like this ... the only benefit is being able to more-reliably add memory in low-memory conditions. An unlikely scenario with standby storage IMHO. Note that I dislike exposing "altmap" to the user :) Dax calls it "memmap_on_memory", and it is a device attrivute. As soon as we go down that path we have the complexity of having to group memory blocks etc, and if we can just not go down that path right now it will make things a lot simpler. (especially, as you document above, the semantics become *really* weird) As yet another point, I am not sure if someone really needs a per-memory block control of the memmap-on-memory feature. If we could simplify here, that would be great ... > > NOTE: “altmap_required” attribute must be set before setting the block as > configured via “config” attribute. (Dependancy) > > 4) Additionally add the patch to check if the memory block is configured > with altmap or not. Similar to [RFC PATCH 2/4] mm/memory_hotplug: Add > memory block altmap sysfs attribute. > > Most of the code changes will be s390 specific with this interface. > > Request your inputs on the potential interface. Thank you. > > Other questions: > 1. I’m just wondering how variable-sized altmap grouping will be > structured in the future. Is it organized by grouping the memory blocks > that require altmap, with the first memory block storing the altmap > metadata for all of them? Or is it possible for the altmap metadata to > span across multiple memory blocks? That exactly is unclear, which is why we should probably avoid doing that for now. Also, with other developments happening (memdesc), and ongoing effort to shrink "struct page", maybe we will not even need most of this in the future? > > 2. OR, will dedicated memory blocks be used exclusively for altmap > metadata, which the memory blocks requiring altmap would then consume? (To > prevent fragmentation) ? One idea I had was that you would do the add_memory() in bigger granularity. Then, the memory blocks hosting the memmap would have to get onlined first. And offlining of them would fail until all dependent ones were offlined. That would at least limit the impact. Then, the question would be, how could you "group" these memory blocks from your interface to do a single add_memory() etc. But again, maybe we can leave that part out for now ... -- Cheers, David / dhildenb