From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-x344.google.com (mail-ot1-x344.google.com [IPv6:2607:f8b0:4864:20::344]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id A7B4921A09130 for ; Wed, 10 Oct 2018 11:19:03 -0700 (PDT) Received: by mail-ot1-x344.google.com with SMTP id c23so4528433otl.9 for ; Wed, 10 Oct 2018 11:19:03 -0700 (PDT) MIME-Version: 1.0 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> In-Reply-To: <20181010172451.GK5873@dhcp22.suse.cz> From: Dan Williams Date: Wed, 10 Oct 2018 11:18:49 -0700 Message-ID: Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-nvdimm-bounces@lists.01.org Sender: "Linux-nvdimm" To: Michal Hocko Cc: Pasha Tatashin , linux-nvdimm , Dave Hansen , Linux Kernel Mailing List , Ingo Molnar , Linux MM , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , rppt@linux.vnet.ibm.com, alexander.h.duyck@linux.intel.com, Andrew Morton , "Kirill A. Shutemov" List-ID: On Wed, Oct 10, 2018 at 10:30 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? > > > 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. > Right, so we're in a situation where a hack is needed for KVM's current interpretation of the Reserved flag relative to dax mapped pages. I'm arguing to push that knowledge / handling as deep as possible into the core rather than hack the leaf implementations like KVM, i.e. disable the Reserved flag for all non-MEMORY_DEVICE_* ZONE_DEVICE types. Here is the KVM thread about why they need a change: https://lkml.org/lkml/2018/9/7/552 ...and where I pushed back on a KVM-local hack: https://lkml.org/lkml/2018/9/19/154 _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm