From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id 33BEF211518C2 for ; Fri, 21 Sep 2018 07:23:23 -0700 (PDT) Subject: Re: [PATCH V5 4/4] kvm: add a check if pfn is from NVDIMM pmem. References: <4e8c2e0facd46cfaf4ab79e19c9115958ab6f218.1536342881.git.yi.z.zhang@linux.intel.com> <20180920224953.GA53363@tiger-server> <20180921224739.GA33892@tiger-server> From: David Hildenbrand Message-ID: Date: Fri, 21 Sep 2018 16:23:19 +0200 MIME-Version: 1.0 In-Reply-To: <20180921224739.GA33892@tiger-server> Content-Language: en-US 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: Dan Williams , KVM list , Linux Kernel Mailing List , linux-nvdimm , Paolo Bonzini , Dave Jiang , "Zhang, Yu C" , Pankaj Gupta , Jan Kara , Christoph Hellwig , Linux MM , rkrcmar@redhat.com, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , "Zhang, Yi Z" List-ID: On 22/09/2018 00:47, Yi Zhang wrote: > On 2018-09-20 at 14:19:17 -0700, Dan Williams wrote: >> On Thu, Sep 20, 2018 at 7:11 AM Yi Zhang wrote: >>> >>> On 2018-09-19 at 09:20:25 +0200, David Hildenbrand wrote: >>>> Am 19.09.18 um 04:53 schrieb Dan Williams: >>>>> >>>>> Should we consider just not setting PageReserved for >>>>> devm_memremap_pages()? Perhaps kvm is not be the only component making >>>>> these assumptions about this flag? >>>> >>>> I was asking the exact same question in v3 or so. >>>> >>>> I was recently going through all PageReserved users, trying to clean up >>>> and document how it is used. >>>> >>>> PG_reserved used to be a marker "not available for the page allocator". >>>> This is only partially true and not really helpful I think. My current >>>> understanding: >>>> >>>> " >>>> PG_reserved is set for special pages, struct pages of such pages should >>>> in general not be touched except by their owner. Pages marked as >>>> reserved include: >>>> - Kernel image (including vDSO) and similar (e.g. BIOS, initrd) >>>> - Pages allocated early during boot (bootmem, memblock) >>>> - Zero pages >>>> - Pages that have been associated with a zone but were not onlined >>>> (e.g. NVDIMM/pmem, online_page_callback used by XEN) >>>> - Pages to exclude from the hibernation image (e.g. loaded kexec images) >>>> - MCA (memory error) pages on ia64 >>>> - Offline pages >>>> Some architectures don't allow to ioremap RAM pages that are not marked >>>> as reserved. Allocated pages might have to be set reserved to allow for >>>> that - if there is a good reason to enforce this. Consequently, >>>> PG_reserved part of a user space table might be the indicator for the >>>> zero page, pmem or MMIO pages. >>>> " >>>> >>>> Swapping code does not care about PageReserved at all as far as I >>>> remember. This seems to be fine as it only looks at the way pages have >>>> been mapped into user space. >>>> >>>> I don't really see a good reason to set pmem pages as reserved. One >>>> question would be, how/if to exclude them from the hibernation image. >>>> But that could also be solved differently (we would have to double check >>>> how they are handled in hibernation code). >>>> >>>> >>>> A similar user of PageReserved to look at is: >>>> >>>> drivers/vfio/vfio_iommu_type1.c:is_invalid_reserved_pfn() >>>> >>>> It will not mark pages dirty if they are reserved. Similar to KVM code. >>> Yes, kvm is not the only one user of the dax reserved page. >>>> >>>>> >>>>> Why is MEMORY_DEVICE_PUBLIC memory specifically excluded? >>>>> >>>>> This has less to do with "dax" pages and more to do with >>>>> devm_memremap_pages() established ranges. P2PDMA is another producer >>>>> of these pages. If either MEMORY_DEVICE_PUBLIC or P2PDMA pages can be >>>>> used in these kvm paths then I think this points to consider clearing >>>>> the Reserved flag. >>> >>> Thanks Dan/David's comments. >>> for MEMORY_DEVICE_PUBLIC memory, since host driver could manager the >>> memory resource to share to guest, Jerome says we could ignore it at >>> this time. >>> >>> And p2pmem, it seems mapped in a PCI bar space which should most likely >>> a mmio. I think kvm should treated as a reserved page. >> >> Ok, but the question you left unanswered is whether it would be better >> for devm_memremap_pages() to clear the PageReserved flag for >> MEMORY_DEVICE_{FS,DEV}_DAX rather than introduce a local kvm-only hack >> for what looks like a global problem. > > Remove the PageReserved flag sounds more reasonable. > And Could we still have a flag to identify it is a device private memory, or > where these pages coming from? We could use a page type for that or what you proposed. (as I said, we might have to change hibernation code to skip the pages once we drop the reserved flag). -- Thanks, David / dhildenb _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm