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[209.132.180.67]) by mx.google.com with ESMTP id y7-v6si24522302pgi.256.2018.10.10.10.39.27; Wed, 10 Oct 2018 10:39:41 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=intel.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727148AbeJKBCO (ORCPT + 99 others); Wed, 10 Oct 2018 21:02:14 -0400 Received: from mga05.intel.com ([192.55.52.43]:33237 "EHLO mga05.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726789AbeJKBCN (ORCPT ); Wed, 10 Oct 2018 21:02:13 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga006.jf.intel.com ([10.7.209.51]) by fmsmga105.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 10 Oct 2018 10:39:02 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.54,365,1534834800"; d="scan'208";a="81538499" Received: from ahduyck-mobl.amr.corp.intel.com (HELO [10.7.198.156]) ([10.7.198.156]) by orsmga006.jf.intel.com with ESMTP; 10 Oct 2018 10:39:01 -0700 Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap To: Michal Hocko Cc: Dan Williams , Linux MM , Andrew Morton , Linux Kernel Mailing List , linux-nvdimm , Pasha Tatashin , Dave Hansen , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , rppt@linux.vnet.ibm.com, Ingo Molnar , "Kirill A. Shutemov" , yi.z.zhang@linux.intel.com References: <20180925200551.3576.18755.stgit@localhost.localdomain> <20180925202053.3576.66039.stgit@localhost.localdomain> <20181009170051.GA40606@tiger-server> <25092df0-b7b4-d456-8409-9c004cb6e422@linux.intel.com> <20181010095838.GG5873@dhcp22.suse.cz> <20181010172451.GK5873@dhcp22.suse.cz> From: Alexander Duyck Message-ID: <98c35e19-13b9-0913-87d9-b3f1ab738b61@linux.intel.com> Date: Wed, 10 Oct 2018 10:39:01 -0700 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: <20181010172451.GK5873@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/10/2018 10:24 AM, Michal Hocko wrote: > On Wed 10-10-18 09:39:08, Alexander Duyck wrote: >> On 10/10/2018 2:58 AM, Michal Hocko wrote: >>> On Tue 09-10-18 13:26:41, Alexander Duyck wrote: >>> [...] >>>> I would think with that being the case we still probably need the call to >>>> __SetPageReserved to set the bit with the expectation that it will not be >>>> cleared for device-pages since the pages are not onlined. Removing the call >>>> to __SetPageReserved would probably introduce a number of regressions as >>>> there are multiple spots that use the reserved bit to determine if a page >>>> can be swapped out to disk, mapped as system memory, or migrated. >>> >>> PageReserved is meant to tell any potential pfn walkers that might get >>> to this struct page to back off and not touch it. Even though >>> ZONE_DEVICE doesn't online pages in traditional sense it makes those >>> pages available for further use so the page reserved bit should be >>> cleared. >> >> So from what I can tell that isn't necessarily the case. Specifically if the >> pagemap type is MEMORY_DEVICE_PRIVATE or MEMORY_DEVICE_PUBLIC both are >> special cases where the memory may not be accessible to the CPU or cannot be >> pinned in order to allow for eviction. > > Could you give me an example please? Honestly I am getting a bit beyond my depth here so maybe Dan could explain better. I am basing the above comment on Dan's earlier comment in this thread combined with the comment that explains the "memory_type" field for the pgmap: https://elixir.bootlin.com/linux/v4.19-rc7/source/include/linux/memremap.h#L28 >> The specific case that Dan and Yi are referring to is for the type >> MEMORY_DEVICE_FS_DAX. For that type I could probably look at not setting the >> reserved bit. Part of me wants to say that we should wait and clear the bit >> later, but that would end up just adding time back to initialization. At >> this point I would consider the change more of a follow-up optimization >> rather than a fix though since this is tailoring things specifically for DAX >> versus the other ZONE_DEVICE types. > > I thought I have already made it clear that these zone device hacks are > not acceptable to the generic hotplug code. If the current reserve bit > handling is not correct then give us a specific reason for that and we > can start thinking about the proper fix. I might have misunderstood your earlier comment then. I thought you were saying that we shouldn't bother with setting the reserved bit. Now it sounds like you were thinking more along the lines of what I was here in my comment where I thought the bit should be cleared later in some code specifically related to DAX when it is exposing it for use to userspace or KVM.