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[185.25.65.24]) by mx.google.com with ESMTPS id g8si342880edg.448.2019.03.12.10.41.41 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 12 Mar 2019 10:41:42 -0700 (PDT) Received-SPF: pass (google.com: domain of prvs=9679d707e=andrew.cooper3@citrix.com designates 185.25.65.24 as permitted sender) client-ip=185.25.65.24; Authentication-Results: mx.google.com; spf=pass (google.com: domain of prvs=9679d707e=andrew.cooper3@citrix.com designates 185.25.65.24 as permitted sender) smtp.mailfrom="prvs=9679d707e=Andrew.Cooper3@citrix.com" X-IronPort-AV: E=Sophos;i="5.58,471,1544486400"; d="scan'208";a="87170659" Subject: Re: [Xen-devel] xen: Can't insert balloon page into VM userspace (WAS Re: [linux-linus bisection] complete test-arm64-arm64-xl-xsm) To: David Hildenbrand , Matthew Wilcox , Julien Grall CC: Juergen Gross , , Stefano Stabellini , Kees Cook , Konrad Rzeszutek Wilk , "VMware, Inc." , osstest service owner , "linux-kernel@vger.kernel.org" , , Julien Freche , Nadav Amit , , Boris Ostrovsky References: <80211e70-5f54-9421-8e8f-2a4fc758ce39@arm.com> <46118631-61d4-adb6-6ffc-4e7c62ea3da9@arm.com> <20190312171421.GJ19508@bombadil.infradead.org> From: Andrew Cooper Openpgp: preference=signencrypt Autocrypt: addr=andrew.cooper3@citrix.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Content-Language: en-GB X-ClientProxiedBy: AMSPEX02CAS01.citrite.net (10.69.22.112) To AMSPEX02CL02.citrite.net (10.69.22.126) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 12/03/2019 17:18, David Hildenbrand wrote: > On 12.03.19 18:14, Matthew Wilcox wrote: >> On Tue, Mar 12, 2019 at 05:05:39PM +0000, Julien Grall wrote: >>> On 3/12/19 3:59 PM, Julien Grall wrote: >>>> It looks like all the arm test for linus [1] and next [2] tree >>>> are now failing. x86 seems to be mostly ok. >>>> >>>> The bisector fingered the following commit: >>>> >>>> commit 0ee930e6cafa048c1925893d0ca89918b2814f2c >>>> Author: Matthew Wilcox >>>> Date: Tue Mar 5 15:46:06 2019 -0800 >>>> >>>> mm/memory.c: prevent mapping typed pages to userspace >>>> Pages which use page_type must never be mapped to userspace as it would >>>> destroy their page type. Add an explicit check for this instead of >>>> assuming that kernel drivers always get this right. >> Oh good, it found a real problem. >> >>> It turns out the problem is because the balloon driver will call >>> __SetPageOffline() on allocated page. Therefore the page has a type and >>> vm_insert_pages will deny the insertion. >>> >>> My knowledge is quite limited in this area. So I am not sure how we can >>> solve the problem. >>> >>> I would appreciate if someone could provide input of to fix the mapping. >> I don't know the balloon driver, so I don't know why it was doing this, >> but what it was doing was Wrong and has been since 2014 with: >> >> commit d6d86c0a7f8ddc5b38cf089222cb1d9540762dc2 >> Author: Konstantin Khlebnikov >> Date: Thu Oct 9 15:29:27 2014 -0700 >> >> mm/balloon_compaction: redesign ballooned pages management >> >> If ballooned pages are supposed to be mapped into userspace, you can't mark >> them as ballooned pages using the mapcount field. >> > Asking myself why anybody would want to map balloon inflated pages into > user space (this just sounds plain wrong but my understanding to what > XEN balloon driver does might be limited), but I assume the easy fix > would be to revert I suspect the bug here is that the balloon driver is (ab)used for a second purpose - to create a hole in pfn space to map some other bits of shared memory into. I think at the end of the day, what is needed is a struct page_info which looks like normal RAM, but the backing for which can be altered by hypercall to map other things. ~Andrew