From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f197.google.com (mail-qt0-f197.google.com [209.85.216.197]) by kanga.kvack.org (Postfix) with ESMTP id 0B3936B000A for ; Mon, 30 Jul 2018 08:11:37 -0400 (EDT) Received: by mail-qt0-f197.google.com with SMTP id d14-v6so10158029qtn.12 for ; Mon, 30 Jul 2018 05:11:37 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id q30-v6si7930626qtb.403.2018.07.30.05.11.35 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 30 Jul 2018 05:11:36 -0700 (PDT) Subject: Re: [PATCH v1] mm: inititalize struct pages when adding a section References: <20180727165454.27292-1-david@redhat.com> <20180730113029.GM24267@dhcp22.suse.cz> <6cc416e7-522c-a67e-2706-f37aadff084f@redhat.com> <20180730120529.GN24267@dhcp22.suse.cz> From: David Hildenbrand Message-ID: <7b58af7b-5187-2c76-b458-b0f49875a1fc@redhat.com> Date: Mon, 30 Jul 2018 14:11:30 +0200 MIME-Version: 1.0 In-Reply-To: <20180730120529.GN24267@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Greg Kroah-Hartman , Ingo Molnar , Pavel Tatashin , Andrew Morton , Dan Williams , Jan Kara , Matthew Wilcox , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Souptick Joarder , "Kirill A. Shutemov" , Vlastimil Babka , Oscar Salvador , YASUAKI ISHIMATSU , Mathieu Malaterre , Mel Gorman , Joonsoo Kim On 30.07.2018 14:05, Michal Hocko wrote: > On Mon 30-07-18 13:53:06, David Hildenbrand wrote: >> On 30.07.2018 13:30, Michal Hocko wrote: >>> On Fri 27-07-18 18:54:54, David Hildenbrand wrote: >>>> Right now, struct pages are inititalized when memory is onlined, not >>>> when it is added (since commit d0dc12e86b31 ("mm/memory_hotplug: optimize >>>> memory hotplug")). >>>> >>>> remove_memory() will call arch_remove_memory(). Here, we usually access >>>> the struct page to get the zone of the pages. >>>> >>>> So effectively, we access stale struct pages in case we remove memory that >>>> was never onlined. So let's simply inititalize them earlier, when the >>>> memory is added. We only have to take care of updating the zone once we >>>> know it. We can use a dummy zone for that purpose. >>> >>> I have considered something like this when I was reworking memory >>> hotplug to not associate struct pages with zone before onlining and I >>> considered this to be rather fragile. I would really not like to get >>> back to that again if possible. >>> >>>> So effectively, all pages will already be initialized and set to >>>> reserved after memory was added but before it was onlined (and even the >>>> memblock is added). We only inititalize pages once, to not degrade >>>> performance. >>> >>> To be honest, I would rather see d0dc12e86b31 reverted. It is late in >>> the release cycle and if the patch is buggy then it should be reverted >>> rather than worked around. I found the optimization not really >>> convincing back then and this is still the case TBH. >>> >> >> If I am not wrong, that's already broken in 4.17, no? What about that? > > Ohh, I thought this was merged in 4.18. > $ git describe --contains d0dc12e86b31 --match="v*" > v4.17-rc1~99^2~44 > > proves me wrong. This means that the fix is not so urgent as I thought. > If you can figure out a reasonable fix then it should be preferable to > the revert. > > Fake zone sounds too hackish to me though. > If I am not wrong, that's the same we had before d0dc12e86b31 but now it is explicit and only one single value for all kernel configs ("ZONE_NORMAL"). Before d0dc12e86b31, struct pages were initialized to 0. So it was (depending on the config) ZONE_DMA, ZONE_DMA32 or ZONE_NORMAL. Now the value is random and might not even be a valid zone. -- Thanks, David / dhildenb