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Thu, 11 Feb 2021 12:35:59 +0000 (UTC) Received: from [10.36.114.52] (ovpn-114-52.ams2.redhat.com [10.36.114.52]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3616360BF1; Thu, 11 Feb 2021 12:35:57 +0000 (UTC) To: Anshuman Khandual , Will Deacon Cc: Mark Rutland , Catalin Marinas , linux-kernel@vger.kernel.org, Mike Rapoport , linux-mm@kvack.org, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , James Morse , Dan Williams , Robin Murphy , Ard Biesheuvel , linux-arm-kernel@lists.infradead.org References: <1612239114-28428-1-git-send-email-anshuman.khandual@arm.com> <1612239114-28428-2-git-send-email-anshuman.khandual@arm.com> <20210202123215.GA16868@willie-the-truck> <20210202123524.GB16868@willie-the-truck> <20210202125152.GC16868@willie-the-truck> <4d8f5156-8628-5531-1485-322ad92aa15c@redhat.com> <0e649f28-4d54-319d-f876-8a93870cda7f@arm.com> <20210205185552.GA23216@willie-the-truck> <20210211115354.GB29894@willie-the-truck> <23e5eb93-a39c-c68e-eac1-c5ccf9036079@arm.com> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory Message-ID: Date: Thu, 11 Feb 2021 13:35:56 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <23e5eb93-a39c-c68e-eac1-c5ccf9036079@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Content-Transfer-Encoding: quoted-printable 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 11.02.21 13:10, Anshuman Khandual wrote: >=20 >=20 > On 2/11/21 5:23 PM, Will Deacon wrote: >> On Fri, Feb 05, 2021 at 06:55:53PM +0000, Will Deacon wrote: >>> On Wed, Feb 03, 2021 at 09:20:39AM +0530, Anshuman Khandual wrote: >>>> On 2/2/21 6:26 PM, David Hildenbrand wrote: >>>>> On 02.02.21 13:51, Will Deacon wrote: >>>>>> On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote: >>>>>>> As I expressed already, long term we should really get rid of the= arm64 >>>>>>> variant and rather special-case the generic one. Then we won't go= out of >>>>>>> sync - just as it happened with ZONE_DEVICE handling here. >>>>>> >>>>>> Why does this have to be long term? This ZONE_DEVICE stuff could b= e the >>>>>> carrot on the stick :) >>>>> >>>>> Yes, I suggested to do it now, but Anshuman convinced me that doing= a >>>>> simple fix upfront might be cleaner --- for example when it comes t= o >>>>> backporting :) >>>> >>>> Right. The current pfn_valid() breaks for ZONE_DEVICE memory and thi= s fixes >>>> the problem in the present context which can be easily backported if= required. >>>> >>>> Changing or rather overhauling the generic code with new configs as = proposed >>>> earlier (which I am planning to work on subsequently) would definite= ly be an >>>> improvement for the current pfn_valid() situation in terms of mainta= inability >>>> but then it should not stop us from fixing the problem now. >>> >>> Alright, I've mulled this over a bit. I don't agree that this patch h= elps >>> with maintainability (quite the opposite, in fact), but perfection is= the >>> enemy of the good so I'll queue the series for 5.12. However, I'll re= vert >>> the changes at the first sign of a problem, so please do work towards= a >>> generic solution which can replace this in the medium term. >> >> ... and dropped. These patches appear to be responsible for a boot >> regression reported by CKI: >=20 > Ahh, boot regression ? These patches only change the behaviour > for non boot memory only. >=20 >> >> https://lore.kernel.org/r/cki.8D1CB60FEC.K6NJMEFQPV@redhat.com >=20 > Will look into the logs and see if there is something pointing to > the problem. >=20 It's strange. One thing I can imagine is a mis-detection of early=20 sections. However, I don't see that happening: In sparse_init_nid(), we: 1. Initialize the memmap 2. Set SECTION_IS_EARLY | SECTION_HAS_MEM_MAP via sparse_init_one_section() Only hotplugged sections (DIMMs, dax/kmem) set SECTION_HAS_MEM_MAP=20 without SECTION_IS_EARLY - which is correct, because these are not early. So once we know that we have valid_section() -- SECTION_HAS_MEM_MAP is=20 set -- early_section() should be correct. Even if someone would be doing a pfn_valid() after=20 memblocks_present()->memory_present() but before sparse_init_nid(), we should be fine (!valid_section() -> return 0). As it happens early during boot, I doubt that some NVDIMMs that get=20 detected and added early during boot as system RAM (via dax/kmem). Are=20 the problem. --=20 Thanks, David / dhildenb