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=unavailable 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 ED899C3F68F for ; Fri, 17 Jan 2020 16:11:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ABB962073A for ; Fri, 17 Jan 2020 16:11:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ZtnEDWbV" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729145AbgAQQLE (ORCPT ); Fri, 17 Jan 2020 11:11:04 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:23149 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727043AbgAQQLD (ORCPT ); Fri, 17 Jan 2020 11:11:03 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1579277461; 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=bkYx3IXLBjWMcH8xaqQwnXdXj9gNFT4cX9S4gKxUK+s=; b=ZtnEDWbVmT3QTLktgbu+oFlvefjkdAyXnYnANZ/xUFLyMNh4vSWwRBfv7SEVZUZA4o7eqC X4JIae8FBbrxsnFzm+Dfdf/pZ9WQdCl2RlnOsp6Ygzv48614lwjnVIw9FiKHpPjnXONgSX Z5lmCV0sksffO7JbQdL7x9L80G83DME= 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-241-HijssW8sNBeEkfLcdl1RIQ-1; Fri, 17 Jan 2020 11:10:57 -0500 X-MC-Unique: HijssW8sNBeEkfLcdl1RIQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id CC5CA800D41; Fri, 17 Jan 2020 16:10:54 +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 236C25D9CD; Fri, 17 Jan 2020 16:10:49 +0000 (UTC) Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul To: Dan Williams , Michal Hocko Cc: Linux Kernel Mailing List , Linux MM , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Greg Kroah-Hartman , "Rafael J. Wysocki" , Andrew Morton , Leonardo Bras , Nathan Lynch , Allison Randal , Nathan Fontenot , Thomas Gleixner , Stephen Rothwell , Anshuman Khandual , lantianyu1986@gmail.com, linuxppc-dev References: <20200117105759.27905-1-david@redhat.com> <20200117113353.GT19428@dhcp22.suse.cz> <20200117145233.GB19428@dhcp22.suse.cz> <65606e2e-1cf7-de3b-10b1-33653cb41a52@redhat.com> <20200117152947.GK19428@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: <25a94f61-46a1-59a6-6b54-8cc6b35790d2@redhat.com> Date: Fri, 17 Jan 2020 17:10:49 +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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 17.01.20 16:54, Dan Williams wrote: > On Fri, Jan 17, 2020 at 7:30 AM Michal Hocko wrote: >> >> On Fri 17-01-20 15:58:26, David Hildenbrand wrote: >>> On 17.01.20 15:52, Michal Hocko wrote: >>>> On Fri 17-01-20 14:08:06, David Hildenbrand wrote: >>>>> 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 memo= ry >>>>>>> blocks. Add a new function is_mem_section_offlineable() for that = and >>>>>>> make it call is_mem_section_offlineable() for each contained sect= ion. >>>>>>> Within is_mem_section_offlineable(), add some more sanity checks = and >>>>>>> directly bail out if the section contains holes or if it spans mu= ltiple >>>>>>> zones. >>>>>> >>>>>> I didn't read the patch (yet) but I am wondering. If we want to to= uch >>>>>> this code, can we simply always return true there? I mean whoever >>>>>> depends on this check is racy and the failure can happen even afte= r >>>>>> the sysfs says good to go, right? The check is essentially as expe= nsive >>>>>> as calling the offlining code itself. So the only usecase I can th= ink of >>>>>> 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 an= other >>>>>> is essentially going to achieve the same. >>>>> >>>>> Some thoughts: >>>>> >>>>> 1. It allows you to check if memory is likely to be offlineable wit= hout >>>>> 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-ut= ils) >>>>> makes use of /sys/.../removable to speed up the search AFAIK. >>>> >>>> Well, while I do see those points I am not really sure they are wort= h >>>> having a broken (by-definition) interface. >>> >>> It's a pure speedup. And for that, the interface has been working >>> perfectly fine for years? >>> >>>> >>>>> 4. lsmem displays/groups by "removable". >>>> >>>> Is anybody really using that? >>> >>> Well at least I am using that when testing to identify which >>> (ZONE_NORMAL!) block I can easily offline/re-online (e.g., to validat= e >>> all the zone shrinking stuff I have been fixing) >>> >>> So there is at least one user ;) >> >> Fair enough. But I would argue that there are better ways to do the sa= me >> solely for testing purposes. Rather than having a subtly broken code t= o >> maintain. >> >>>> >>>>>> 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, les= s >>>>> performant as you would try to offline any memory block). >>>> >>>> I would argue that the whole interface is broken ;). Not the first t= ime >>>> in the kernel development history and not the last time either. What= I >>>> am trying to say here is that unless there are _real_ usecases depen= ding >>>> on knowing that something surely is _not_ offlineable then I would j= ust >>>> try to drop the functionality while preserving the interface and see >>>> what happens. >>> >>> I can see that, but I can perfectly well understand why - especially >>> powerpc - wants a fast way to sense which blocks actually sense to tr= y >>> to online. >>> >>> The original patch correctly states >>> "which sections of >>> memory are likely to be removable before attempting the potential= ly >>> expensive operation." >>> >>> It works as designed I would say. >> >> Then I would just keep it crippled the same way it has been for years >> without anybody noticing. >=20 > I tend to agree. At least the kmem driver that wants to unplug memory > could not use an interface that does not give stable answers. It just > relies on remove_memory() to return a definitive error. >=20 Just because kmem cannot reuse such an interface doesn't mean we should not touch it (or I am not getting your point). Especially, this interface is about "can it be likely be offlined and then eventually be removed (if there is a HW interface for that)" (as documented), not about "will remove_memory()" work. We do have users and if we agree to keep it (what I think we should as I expressed) then I think we should un-cripple and fix it. After all we have to maintain it. The current interface provides what was documented - "likely to be offlineable". (the chosen name was just horribly bad - as I expressed a while ago already :) ) --=20 Thanks, David / dhildenb 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=-7.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 CA8C3C33CB1 for ; Fri, 17 Jan 2020 16:13:31 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 43A112072E for ; Fri, 17 Jan 2020 16:13:31 +0000 (UTC) Authentication-Results: mail.kernel.org; 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Fri, 17 Jan 2020 11:10:57 -0500 X-MC-Unique: HijssW8sNBeEkfLcdl1RIQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id CC5CA800D41; Fri, 17 Jan 2020 16:10:54 +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 236C25D9CD; Fri, 17 Jan 2020 16:10:49 +0000 (UTC) Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul To: Dan Williams , Michal Hocko References: <20200117105759.27905-1-david@redhat.com> <20200117113353.GT19428@dhcp22.suse.cz> <20200117145233.GB19428@dhcp22.suse.cz> <65606e2e-1cf7-de3b-10b1-33653cb41a52@redhat.com> <20200117152947.GK19428@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: <25a94f61-46a1-59a6-6b54-8cc6b35790d2@redhat.com> Date: Fri, 17 Jan 2020 17:10:49 +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: Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Content-Transfer-Encoding: quoted-printable X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Nathan Lynch , Stephen Rothwell , Thomas Gleixner , "Rafael J. Wysocki" , Anshuman Khandual , Linux Kernel Mailing List , Linux MM , Paul Mackerras , Greg Kroah-Hartman , Leonardo Bras , Andrew Morton , linuxppc-dev , Nathan Fontenot , Allison Randal , lantianyu1986@gmail.com Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 17.01.20 16:54, Dan Williams wrote: > On Fri, Jan 17, 2020 at 7:30 AM Michal Hocko wrote: >> >> On Fri 17-01-20 15:58:26, David Hildenbrand wrote: >>> On 17.01.20 15:52, Michal Hocko wrote: >>>> On Fri 17-01-20 14:08:06, David Hildenbrand wrote: >>>>> 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 memo= ry >>>>>>> blocks. Add a new function is_mem_section_offlineable() for that = and >>>>>>> make it call is_mem_section_offlineable() for each contained sect= ion. >>>>>>> Within is_mem_section_offlineable(), add some more sanity checks = and >>>>>>> directly bail out if the section contains holes or if it spans mu= ltiple >>>>>>> zones. >>>>>> >>>>>> I didn't read the patch (yet) but I am wondering. If we want to to= uch >>>>>> this code, can we simply always return true there? I mean whoever >>>>>> depends on this check is racy and the failure can happen even afte= r >>>>>> the sysfs says good to go, right? The check is essentially as expe= nsive >>>>>> as calling the offlining code itself. So the only usecase I can th= ink of >>>>>> 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 an= other >>>>>> is essentially going to achieve the same. >>>>> >>>>> Some thoughts: >>>>> >>>>> 1. It allows you to check if memory is likely to be offlineable wit= hout >>>>> 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-ut= ils) >>>>> makes use of /sys/.../removable to speed up the search AFAIK. >>>> >>>> Well, while I do see those points I am not really sure they are wort= h >>>> having a broken (by-definition) interface. >>> >>> It's a pure speedup. And for that, the interface has been working >>> perfectly fine for years? >>> >>>> >>>>> 4. lsmem displays/groups by "removable". >>>> >>>> Is anybody really using that? >>> >>> Well at least I am using that when testing to identify which >>> (ZONE_NORMAL!) block I can easily offline/re-online (e.g., to validat= e >>> all the zone shrinking stuff I have been fixing) >>> >>> So there is at least one user ;) >> >> Fair enough. But I would argue that there are better ways to do the sa= me >> solely for testing purposes. Rather than having a subtly broken code t= o >> maintain. >> >>>> >>>>>> 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, les= s >>>>> performant as you would try to offline any memory block). >>>> >>>> I would argue that the whole interface is broken ;). Not the first t= ime >>>> in the kernel development history and not the last time either. What= I >>>> am trying to say here is that unless there are _real_ usecases depen= ding >>>> on knowing that something surely is _not_ offlineable then I would j= ust >>>> try to drop the functionality while preserving the interface and see >>>> what happens. >>> >>> I can see that, but I can perfectly well understand why - especially >>> powerpc - wants a fast way to sense which blocks actually sense to tr= y >>> to online. >>> >>> The original patch correctly states >>> "which sections of >>> memory are likely to be removable before attempting the potential= ly >>> expensive operation." >>> >>> It works as designed I would say. >> >> Then I would just keep it crippled the same way it has been for years >> without anybody noticing. >=20 > I tend to agree. At least the kmem driver that wants to unplug memory > could not use an interface that does not give stable answers. It just > relies on remove_memory() to return a definitive error. >=20 Just because kmem cannot reuse such an interface doesn't mean we should not touch it (or I am not getting your point). Especially, this interface is about "can it be likely be offlined and then eventually be removed (if there is a HW interface for that)" (as documented), not about "will remove_memory()" work. We do have users and if we agree to keep it (what I think we should as I expressed) then I think we should un-cripple and fix it. After all we have to maintain it. The current interface provides what was documented - "likely to be offlineable". (the chosen name was just horribly bad - as I expressed a while ago already :) ) --=20 Thanks, David / dhildenb