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Tue, 2 Feb 2021 12:56:17 +0000 (UTC) Received: from [10.36.114.148] (ovpn-114-148.ams2.redhat.com [10.36.114.148]) by smtp.corp.redhat.com (Postfix) with ESMTP id EA6405D9DC; Tue, 2 Feb 2021 12:56:11 +0000 (UTC) Subject: Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory To: Will Deacon Cc: Anshuman Khandual , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Catalin Marinas , Ard Biesheuvel , Mark Rutland , James Morse , Robin Murphy , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Dan Williams , Mike Rapoport 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> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <4d8f5156-8628-5531-1485-322ad92aa15c@redhat.com> Date: Tue, 2 Feb 2021 13:56:10 +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: <20210202125152.GC16868@willie-the-truck> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 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 02.02.21 13:51, Will Deacon wrote: > On Tue, Feb 02, 2021 at 01:39:29PM +0100, David Hildenbrand wrote: >> On 02.02.21 13:35, Will Deacon wrote: >>> On Tue, Feb 02, 2021 at 12:32:15PM +0000, Will Deacon wrote: >>>> On Tue, Feb 02, 2021 at 09:41:53AM +0530, Anshuman Khandual wrote: >>>>> pfn_valid() validates a pfn but basically it checks for a valid struct page >>>>> backing for that pfn. It should always return positive for memory ranges >>>>> backed with struct page mapping. But currently pfn_valid() fails for all >>>>> ZONE_DEVICE based memory types even though they have struct page mapping. >>>>> >>>>> pfn_valid() asserts that there is a memblock entry for a given pfn without >>>>> MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is >>>>> that they do not have memblock entries. Hence memblock_is_map_memory() will >>>>> invariably fail via memblock_search() for a ZONE_DEVICE based address. This >>>>> eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs >>>>> to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged >>>>> into the system via memremap_pages() called from a driver, their respective >>>>> memory sections will not have SECTION_IS_EARLY set. >>>>> >>>>> Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock >>>>> regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set >>>>> for firmware reserved memory regions. memblock_is_map_memory() can just be >>>>> skipped as its always going to be positive and that will be an optimization >>>>> for the normal hotplug memory. Like ZONE_DEVICE based memory, all normal >>>>> hotplugged memory too will not have SECTION_IS_EARLY set for their sections >>>>> >>>>> Skipping memblock_is_map_memory() for all non early memory sections would >>>>> fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its >>>>> performance for normal hotplug memory as well. >>>> >>>> Hmm. Although I follow your logic, this does seem to rely on an awful lot of >>>> assumptions to continue to hold true as the kernel evolves. In particular, >>>> how do we ensure that early sections are always fully backed with >>> >>> Sorry, typo here: ^^^ should be *non-early* sections. >> >> It might be a good idea to have a look at generic >> include/linux/mmzone.h:pfn_valid() > > The generic implementation already makes assumptions that aren't true on > arm64, so that's why we've ended up with our own implementation. But the > patches here put us in a position where I worry that pfn_valid() may return > 'true' in future for cases where the underlying struct page is either > non-existent or bogus, and debugging those failures really sucks. We had a > raft of those back when NOMAP was introduced and I don't want to re-live > that experience. Yeah, and I agree when it comes to boot mem. However, the way generic memory hotplug/memremap infrastructure (->!early sections) works does not allow for such special cases you mention and would break quite some other code if messed up. So I wouldn't worry about that part too much for now. > >> 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 be 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 to backporting :) -- Thanks, David / dhildenb