From patchwork Tue Jan 28 09:35:42 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Hildenbrand X-Patchwork-Id: 11353875 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 1B83B112B for ; Tue, 28 Jan 2020 09:35:58 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id CF9672467B for ; Tue, 28 Jan 2020 09:35:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="GoSERuOn" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CF9672467B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 0B7426B0003; Tue, 28 Jan 2020 04:35:57 -0500 (EST) Delivered-To: linux-mm-outgoing@kvack.org Received: by kanga.kvack.org (Postfix, from userid 40) id 0411A6B0006; Tue, 28 Jan 2020 04:35:56 -0500 (EST) X-Original-To: int-list-linux-mm@kvack.org X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E4A0B6B0007; Tue, 28 Jan 2020 04:35:56 -0500 (EST) X-Original-To: linux-mm@kvack.org X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0038.hostedemail.com [216.40.44.38]) by kanga.kvack.org (Postfix) with ESMTP id C7A8E6B0003 for ; Tue, 28 Jan 2020 04:35:56 -0500 (EST) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with SMTP id 7F6B1988F for ; Tue, 28 Jan 2020 09:35:56 +0000 (UTC) X-FDA: 76426536312.15.roll24_16b0249800a0d X-Spam-Summary: 1,0,0,,d41d8cd98f00b204,david@redhat.com,:linux-kernel@vger.kernel.org::david@redhat.com:mhocko@kernel.org:mhocko@suse.com:dan.j.williams@intel.com:gregkh@linuxfoundation.org:rafael@kernel.org:akpm@linux-foundation.org:powerpc-utils-devel@googlegroups.com:util-linux@vger.kernel.org:pbadari@us.ibm.com:nfont@linux.vnet.ibm.com:rcj@linux.vnet.ibm.com:heiko.carstens@de.ibm.com:kzak@redhat.com,RULES_HIT:30045:30051:30054:30064:30069:30070:30075,0,RBL:207.211.31.120:@redhat.com:.lbl8.mailshell.net-62.18.0.100 66.10.201.10,CacheIP:none,Bayesian:0.5,0.5,0.5,Netcheck:none,DomainCache:0,MSF:not bulk,SPF:ft,MSBL:0,DNSBL:neutral,Custom_rules:0:0:0,LFtime:25,LUA_SUMMARY:none X-HE-Tag: roll24_16b0249800a0d X-Filterd-Recvd-Size: 7891 Received: from us-smtp-1.mimecast.com (us-smtp-delivery-1.mimecast.com [207.211.31.120]) by imf15.hostedemail.com (Postfix) with ESMTP for ; Tue, 28 Jan 2020 09:35:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1580204155; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=2unjbYVQFq3ilMGQwALjLOUy/DfYMkS2idTcgLWVsuY=; b=GoSERuOnnn6zktQSzs9J2voTGZao5ok449JKOLIdr9yrXLpwe0sX1AG7NLd4zhV2Vx2sov RXh4cmmPe7oG++/fQCwU8drf2y31Ivr36T7HAT6YOMMh0OqfMSO2m0ot0TjHCPEOvo4H+D /3l+gxa6PqGXJzkDqf05ZCC2dLEsRLY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-231-u0stzb-rOxypxIhNZYKoZQ-1; Tue, 28 Jan 2020 04:35:50 -0500 X-MC-Unique: u0stzb-rOxypxIhNZYKoZQ-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id ADCB31005513; Tue, 28 Jan 2020 09:35:48 +0000 (UTC) Received: from t480s.redhat.com (ovpn-116-207.ams2.redhat.com [10.36.116.207]) by smtp.corp.redhat.com (Postfix) with ESMTP id 11E1A89D13; Tue, 28 Jan 2020 09:35:42 +0000 (UTC) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, David Hildenbrand , Michal Hocko , Michal Hocko , Dan Williams , Greg Kroah-Hartman , "Rafael J. Wysocki" , Andrew Morton , powerpc-utils-devel@googlegroups.com, util-linux@vger.kernel.org, Badari Pulavarty , Nathan Fontenot , Robert Jennings , Heiko Carstens , Karel Zak Subject: [PATCH v1] drivers/base/memory.c: indicate all memory blocks as removable Date: Tue, 28 Jan 2020 10:35:42 +0100 Message-Id: <20200128093542.6908-1-david@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 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: We see multiple issues with the implementation/interface to compute whether a memory block can be offlined (exposed via /sys/devices/system/memory/memoryX/removable) and would like to simplify it (remove the implementation). 1. It runs basically lockless. While this might be good for performance, we see possible races with memory offlining that will require at least some sort of locking to fix. 2. Nowadays, more false positives are possible. No arch-specific checks are performed that validate if memory offlining will not be denied right away (and such check will require locking). For example, arm64 won't allow to offline any memory block that was added during boot - which will imply a very high error rate. Other archs have other constraints. 3. The interface is inherently racy. E.g., if a memory block is detected to be removable (and was not a false positive at that time), there is still no guarantee that offlining will actually succeed. So any caller already has to deal with false positives. 4. It is unclear which performance benefit this interface actually provides. The introducing commit 5c755e9fd813 ("memory-hotplug: add sysfs removable attribute for hotplug memory remove") mentioned "A user-level agent must be able to identify which sections of memory are likely to be removable before attempting the potentially expensive operation." However, no actual performance comparison was included. Known users: - lsmem: Will group memory blocks based on the "removable" property. [1] - chmem: Indirect user. It has a RANGE mode where one can specify removable ranges identified via lsmem to be offlined. However, it also has a "SIZE" mode, which allows a sysadmin to skip the manual "identify removable blocks" step. [2] - powerpc-utils: Uses the "removable" attribute to skip some memory blocks right away when trying to find some to offline+remove. However, with ballooning enabled, it already skips this information completely (because it once resulted in many false negatives). Therefore, the implementation can deal with false positives properly already. [3] According to Nathan Fontenot, DLPAR on powerpc is nowadays no longer driven from userspace via the drmgr command (powerpc-utils). Nowadays it's managed in the kernel - including onlining/offlining of memory blocks - triggered by drmgr writing to /sys/kernel/dlpar. So the affected legacy userspace handling is only active on old kernels. Only very old versions of drmgr on a new kernel (unlikely) might execute slower - totally acceptable. With CONFIG_MEMORY_HOTREMOVE, always indicating "removable" should not break any user space tool. We implement a very bad heuristic now. Without CONFIG_MEMORY_HOTREMOVE we cannot offline anything, so report "not removable" as before. Original discussion can be found in [4] ("[PATCH RFC v1] mm: is_mem_section_removable() overhaul"). Other users of is_mem_section_removable() will be removed next, so that we can remove is_mem_section_removable() completely. [1] http://man7.org/linux/man-pages/man1/lsmem.1.html [2] http://man7.org/linux/man-pages/man8/chmem.8.html [3] https://github.com/ibm-power-utilities/powerpc-utils [4] https://lkml.kernel.org/r/20200117105759.27905-1-david@redhat.com Suggested-by: Michal Hocko Acked-by: Michal Hocko Cc: Dan Williams Cc: Greg Kroah-Hartman Cc: "Rafael J. Wysocki" Cc: Andrew Morton Cc: powerpc-utils-devel@googlegroups.com Cc: util-linux@vger.kernel.org Cc: Badari Pulavarty Cc: Nathan Fontenot Cc: Robert Jennings Cc: Heiko Carstens Cc: Karel Zak Signed-off-by: David Hildenbrand Reviewed-by: Nathan Fontenot --- RFC -> v1: - Use IS_ENABLED() instead of ifdefs - Add information from Nathan (thanks!) --- drivers/base/memory.c | 23 +++-------------------- 1 file changed, 3 insertions(+), 20 deletions(-) diff --git a/drivers/base/memory.c b/drivers/base/memory.c index 6503f5d0b749..9664be00a4de 100644 --- a/drivers/base/memory.c +++ b/drivers/base/memory.c @@ -105,30 +105,13 @@ static ssize_t phys_index_show(struct device *dev, } /* - * Show whether the memory block is likely to be offlineable (or is already - * offline). Once offline, the memory block could be removed. The return - * value does, however, not indicate that there is a way to remove the - * memory block. + * Legacy interface that we cannot remove. Always indicate "removable" + * with CONFIG_MEMORY_HOTREMOVE - bad heuristic. */ static ssize_t removable_show(struct device *dev, struct device_attribute *attr, char *buf) { - struct memory_block *mem = to_memory_block(dev); - unsigned long pfn; - int ret = 1, i; - - if (mem->state != MEM_ONLINE) - goto out; - - for (i = 0; i < sections_per_block; i++) { - if (!present_section_nr(mem->start_section_nr + i)) - continue; - pfn = section_nr_to_pfn(mem->start_section_nr + i); - ret &= is_mem_section_removable(pfn, PAGES_PER_SECTION); - } - -out: - return sprintf(buf, "%d\n", ret); + return sprintf(buf, "%d\n", (int)IS_ENABLED(CONFIG_MEMORY_HOTREMOVE)); } /*