From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by kanga.kvack.org (Postfix) with ESMTP id 319C06B0007 for ; Wed, 10 Oct 2018 13:53:58 -0400 (EDT) Received: by mail-ed1-f72.google.com with SMTP id b13-v6so3648337edb.1 for ; Wed, 10 Oct 2018 10:53:58 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id s26-v6si1609826edi.218.2018.10.10.10.53.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 10 Oct 2018 10:53:56 -0700 (PDT) Date: Wed, 10 Oct 2018 19:53:54 +0200 From: Michal Hocko Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap Message-ID: <20181010175354.GO5873@dhcp22.suse.cz> 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> <98c35e19-13b9-0913-87d9-b3f1ab738b61@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <98c35e19-13b9-0913-87d9-b3f1ab738b61@linux.intel.com> Sender: owner-linux-mm@kvack.org List-ID: To: Alexander Duyck Cc: Dan Williams , Linux MM , Andrew Morton , Linux Kernel Mailing List , linux-nvdimm , Pasha Tatashin , Dave Hansen , =?iso-8859-1?B?Suly9G1l?= Glisse , rppt@linux.vnet.ibm.com, Ingo Molnar , "Kirill A. Shutemov" , yi.z.zhang@linux.intel.com On Wed 10-10-18 10:39:01, Alexander Duyck wrote: > > > 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. I was referring to my earlier comment that if you need to do something about struct page initialization (move_pfn_range_to_zone) outside of the lock (with the appropriate ground work that is needed) rather than pulling more zone device hacks into the generic hotplug code [1] [1] http://lkml.kernel.org/r/20180926075540.GD6278@dhcp22.suse.cz -- Michal Hocko SUSE Labs