From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4E081C33CB1 for ; Fri, 17 Jan 2020 13:08:22 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 0385D2083E for ; Fri, 17 Jan 2020 13:08:21 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="eMdJiZi7" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0385D2083E 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 7E2C46B049C; Fri, 17 Jan 2020 08:08:21 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 76DAF6B049E; Fri, 17 Jan 2020 08:08:21 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 634686B049F; Fri, 17 Jan 2020 08:08:21 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0082.hostedemail.com [216.40.44.82]) by kanga.kvack.org (Postfix) with ESMTP id 47AA56B049C for ; Fri, 17 Jan 2020 08:08:21 -0500 (EST) Received: from smtpin23.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id 021F2181AEF00 for ; Fri, 17 Jan 2020 13:08:21 +0000 (UTC) X-FDA: 76387154802.23.can72_48d1e62756e0b X-HE-Tag: can72_48d1e62756e0b X-Filterd-Recvd-Size: 9490 Received: from us-smtp-delivery-1.mimecast.com (us-smtp-1.mimecast.com [207.211.31.81]) by imf16.hostedemail.com (Postfix) with ESMTP for ; Fri, 17 Jan 2020 13:08:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1579266499; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=f0fEIf10fwHy27Ekg6+K5QOo8x0RxiGO588jBAOheeA=; b=eMdJiZi7+ASSID5RCFsSq464Uk/JzJ/EVfrFqDk2uHyNt5njtUe16EacqQ0/fSEnm0Q2VU Wqhfsfv6geaV6paiU8OXaW2iHM0HmmHqmU1RYdXYmtUpWwhTlqIS7mpZf0BZeYy25jXJgb 228a6C5YSHUNGhZHB/u6gwQZk8IwD6M= 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-305-CTHBq2s9MYiAiMJGd1-F8A-1; Fri, 17 Jan 2020 08:08:15 -0500 X-MC-Unique: CTHBq2s9MYiAiMJGd1-F8A-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 59617800D41; Fri, 17 Jan 2020 13:08:12 +0000 (UTC) Received: from [10.36.117.199] (ovpn-117-199.ams2.redhat.com [10.36.117.199]) by smtp.corp.redhat.com (Postfix) with ESMTP id C218A81200; Fri, 17 Jan 2020 13:08:07 +0000 (UTC) Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul To: Michal Hocko Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Greg Kroah-Hartman , "Rafael J. Wysocki" , Andrew Morton , Leonardo Bras , Nathan Lynch , Allison Randal , Nathan Fontenot , Thomas Gleixner , Dan Williams , Stephen Rothwell , Anshuman Khandual , lantianyu1986@gmail.com, linuxppc-dev@lists.ozlabs.org References: <20200117105759.27905-1-david@redhat.com> <20200117113353.GT19428@dhcp22.suse.cz> From: David Hildenbrand Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= mQINBFXLn5EBEAC+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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABtCREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT6JAlgEEwEIAEICGwMFCQlmAYAGCwkIBwMCBhUI AgkKCwQWAgMBAh4BAheAFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl3pImkCGQEACgkQTd4Q 9wD/g1o+VA//SFvIHUAvul05u6wKv/pIR6aICPdpF9EIgEU448g+7FfDgQwcEny1pbEzAmiw zAXIQ9H0NZh96lcq+yDLtONnXk/bEYWHHUA014A1wqcYNRY8RvY1+eVHb0uu0KYQoXkzvu+s Dncuguk470XPnscL27hs8PgOP6QjG4jt75K2LfZ0eAqTOUCZTJxA8A7E9+XTYuU0hs7QVrWJ jQdFxQbRMrYz7uP8KmTK9/Cnvqehgl4EzyRaZppshruKMeyheBgvgJd5On1wWq4ZUV5PFM4x II3QbD3EJfWbaJMR55jI9dMFa+vK7MFz3rhWOkEx/QR959lfdRSTXdxs8V3zDvChcmRVGN8U Vo93d1YNtWnA9w6oCW1dnDZ4kgQZZSBIjp6iHcA08apzh7DPi08jL7M9UQByeYGr8KuR4i6e RZI6xhlZerUScVzn35ONwOC91VdYiQgjemiVLq1WDDZ3B7DIzUZ4RQTOaIWdtXBWb8zWakt/ ztGhsx0e39Gvt3391O1PgcA7ilhvqrBPemJrlb9xSPPRbaNAW39P8ws/UJnzSJqnHMVxbRZC Am4add/SM+OCP0w3xYss1jy9T+XdZa0lhUvJfLy7tNcjVG/sxkBXOaSC24MFPuwnoC9WvCVQ ZBxouph3kqc4Dt5X1EeXVLeba+466P1fe1rC8MbcwDkoUo65Ag0EVcufkQEQAOfX3n0g0fZz 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+w6DABEBAAGJAiUEGAECAA8FAlXLn5ECGwwFCQlmAYAACgkQTd4Q 9wD/g1qA6w/+M+ggFv+JdVsz5+ZIc6MSyGUozASX+bmIuPeIecc9UsFRatc91LuJCKMkD9Uv GOcWSeFpLrSGRQ1Z7EMzFVU//qVs6uzhsNk0RYMyS0B6oloW3FpyQ+zOVylFWQCzoyyf227y GW8HnXunJSC+4PtlL2AY4yZjAVAPLK2l6mhgClVXTQ/S7cBoTQKP+jvVJOoYkpnFxWE9pn4t H5QIFk7Ip8TKr5k3fXVWk4lnUi9MTF/5L/mWqdyIO1s7cjharQCstfWCzWrVeVctpVoDfJWp 4LwTuQ5yEM2KcPeElLg5fR7WB2zH97oI6/Ko2DlovmfQqXh9xWozQt0iGy5tWzh6I0JrlcxJ ileZWLccC4XKD1037Hy2FLAjzfoWgwBLA6ULu0exOOdIa58H4PsXtkFPrUF980EEibUp0zFz GotRVekFAceUaRvAj7dh76cToeZkfsjAvBVb4COXuhgX6N4pofgNkW2AtgYu1nUsPAo+NftU CxrhjHtLn4QEBpkbErnXQyMjHpIatlYGutVMS91XTQXYydCh5crMPs7hYVsvnmGHIaB9ZMfB njnuI31KBiLUks+paRkHQlFcgS2N3gkRBzH7xSZ+t7Re3jvXdXEzKBbQ+dC3lpJB0wPnyMcX FOTT3aZT7IgePkt5iC/BKBk3hqKteTnJFeVIT7EC+a6YUFg= Organization: Red Hat GmbH Message-ID: Date: Fri, 17 Jan 2020 14:08:06 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <20200117113353.GT19428@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Content-Transfer-Encoding: quoted-printable 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: On 17.01.20 12:33, Michal Hocko wrote: > On Fri 17-01-20 11:57:59, David Hildenbrand wrote: >> Let's refactor that code. We want to check if we can offline memory >> blocks. Add a new function is_mem_section_offlineable() for that and >> make it call is_mem_section_offlineable() for each contained section. >> Within is_mem_section_offlineable(), add some more sanity checks and >> directly bail out if the section contains holes or if it spans multipl= e >> zones. >=20 > I didn't read the patch (yet) but I am wondering. If we want to touch > this code, can we simply always return true there? I mean whoever > depends on this check is racy and the failure can happen even after > the sysfs says good to go, right? The check is essentially as expensive > as calling the offlining code itself. So the only usecase I can think o= f > is a dumb driver to crawl over blocks and check which is removable and > try to hotremove it. But just trying to offline one block after another > is essentially going to achieve the same. Some thoughts: 1. It allows you to check if memory is likely to be offlineable without doing expensive locking and trying to isolate pages (meaning: zone->lock, mem_hotplug_lock. but also, calling drain_all_pages() when isolating) 2. There are use cases that want to identify a memory block/DIMM to unplug. One example is PPC DLPAR code (see this patch). Going over all memory block trying to offline them is an expensive operation. 3. powerpc-utils (https://github.com/ibm-power-utilities/powerpc-utils) makes use of /sys/.../removable to speed up the search AFAIK. 4. lsmem displays/groups by "removable". 5. If "removable=3Dfalse" then it usually really is not offlineable. Of course, there could also be races (free the last unmovable page), but it means "don't even try". OTOH, "removable=3Dtrue" is more racy, and gives less guarantees. ("looks okay, feel free to try") >=20 > Or does anybody see any reasonable usecase that would break if we did > that unconditional behavior? If we would return always "true", then the whole reason the interface originally was introduced would be "broken" (meaning, less performant as you would try to offline any memory block). commit 5c755e9fd813810680abd56ec09a5f90143e815b Author: Badari Pulavarty Date: Wed Jul 23 21:28:19 2008 -0700 memory-hotplug: add sysfs removable attribute for hotplug memory remo= ve =20 Memory may be hot-removed on a per-memory-block basis, particularly o= n POWER where the SPARSEMEM section size often matches the memory-block size. A user-level agent must be able to identify which sections of memory are likely to be removable before attempting the potentially expensive operation. This patch adds a file called "removable" to th= e memory directory in sysfs to help such an agent. In this patch, a me= mory block is considered removable if; I'd love to see this go away (just like "valid_zones"), but I don't think it is possible. So this patch makes it work a little more correctly (multiplezones, holes= ), cleans up the code and avoids races with unplug code. It can, however, not give more guarantees if memory offlining will actually succeed. --=20 Thanks, David / dhildenb