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To: Gerald Schaefer , Michal Hocko , "akpm@linux-foundation.org" , Greg KH , =?UTF-8?Q?Jan_H=c3=b6ppner?= , Heiko Carstens , "linux-mm@kvack.org" , linux-api@vger.kernel.org, Dave Hansen , "linux-kernel@vger.kernel.org" Message-ID: <21852ccb-bd06-9281-7c8e-485ec02f2883@redhat.com> Date: Thu, 10 Sep 2020 12:20:34 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi everybody, I was just exploring how /sys/devices/system/memory/memoryX/phys_device is/was used. It's one of these interfaces that most probably never should have been added but now we are stuck with it. "phys_device" was used on s390x in older versions of lsmem[2]/chmem[3], back when they were still part of s390x-tools. They were later replaced [5] by the variants in linux-utils. For example, RHEL6 and RHEL7 contain lsmem/chmem from s390-utils. RHEL8 switched to versions from util-linux on s390x [4]. "phys_device" was added with sysfs support for memory hotplug in commit 3947be1969a9 ("[PATCH] memory hotplug: sysfs and add/remove functions") in 2005. It always returned 0. s390x started returning something != 0 on some setups (if sclp.rzm is set by HW) in 2010 via commit 57b552ba0b2f("memory hotplug/s390: set phys_device"). For s390x, it allowed for identifying which memory block devices belong to the same memory increment (RZM). Only if all memory block devices comprising a single memory increment were offline, the memory could actually be removed in the hypervisor. Since commit e5d709bb5fb7 ("s390/memory hotplug: provide memory_block_size_bytes() function") in 2013 a memory block devices spans at least one memory increment - which is why the interface isn't really helpful/used anymore (except by old lsmem/chmem tools). There were once RFC patches to make use of it in ACPI, but it could be solved using different interfaces [1]. While I'd love to rip it out completely, I think it would break old lsmem/chmem completely - and I assume that's not acceptable. I was wondering what would be considered safe to do now/in the future: 1. Make it always return 0 (just as if "sclp.rzm" would be set to 0 on s390x). This will make old lsmem/chmem behave differently after switching to a new kernel, like if sclp.rzm would not be set by HW - AFAIU, it will assume all memory is in a single memory increment. Do we care? 2. Restrict it to s390x only. It always returned 0 on other architectures, I was not able to find any user. I think 2 should be safe to do (never used on other archs). I do wonder what the feelings are about 1. Thoughts? [1] https://patchwork.kernel.org/patch/2163871/ [2] https://github.com/ibm-s390-tools/s390-tools/blob/v2.1.0/zconf/lsmem [3] https://github.com/ibm-s390-tools/s390-tools/blob/v2.1.0/zconf/chmem [4] https://bugzilla.redhat.com/show_bug.cgi?id=1504134 [5] https://github.com/ibm-s390-tools/s390-tools/commit/778292e771fb00cfcbd7ff6535ee3d9fde612dc5#diff-82c32a7f4c597c50db90157ed0c581b3 -- Thanks, David / dhildenb