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Thu, 4 Mar 2021 08:12:32 +0000 (UTC) Subject: Re: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory To: Anshuman Khandual , Will Deacon , Catalin Marinas Cc: Mark Rutland , 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: <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> <20210303190428.GB24035@arm.com> <20210303212406.GB20055@willie-the-truck> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <9872a864-15b1-12a7-6aac-0e68554bc744@redhat.com> Date: Thu, 4 Mar 2021 09:12:31 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Stat-Signature: 9odueh5cjh8dh1xsr1j6ujyfptxswtox X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 2DA72C0007C7 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf06; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1614845564-619059 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 04.03.21 04:31, Anshuman Khandual wrote: > > > On 3/4/21 2:54 AM, Will Deacon wrote: >> On Wed, Mar 03, 2021 at 07:04:33PM +0000, Catalin Marinas wrote: >>> On Thu, Feb 11, 2021 at 01:35:56PM +0100, David Hildenbrand wrote: >>>> On 11.02.21 13:10, Anshuman Khandual wrote: >>>>> On 2/11/21 5:23 PM, Will Deacon wrote: >>>>>> ... and dropped. These patches appear to be responsible for a boot >>>>>> regression reported by CKI: >>>>> >>>>> Ahh, boot regression ? These patches only change the behaviour >>>>> for non boot memory only. >>>>> >>>>>> https://lore.kernel.org/r/cki.8D1CB60FEC.K6NJMEFQPV@redhat.com >>>>> >>>>> Will look into the logs and see if there is something pointing to >>>>> the problem. >>>> >>>> It's strange. One thing I can imagine is a mis-detection of early 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 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 set >>>> -- early_section() should be correct. >>>> >>>> Even if someone would be doing a pfn_valid() after >>>> memblocks_present()->memory_present() but before >>>> sparse_init_nid(), we should be fine (!valid_section() -> return 0). >>> >>> I couldn't figure out how this could fail with Anshuman's patches. >>> Will's suspicion is that some invalid/null pointer gets dereferenced >>> before being initialised but the only case I see is somewhere in >>> pfn_section_valid() (ms->usage) if valid_section() && !early_section(). >>> >>> Assuming that we do get a valid_section(ms) && !early_section(ms), is >>> there a case where ms->usage is not initialised? I guess races with >>> section_deactivate() are not possible this early. >>> >>> Another situation could be that pfn_valid() returns true when no memory >>> is mapped for that pfn. >> >> The case I wondered about was __pfn_to_section() with a bogus pfn, since >> with patch 2/2 we call that *before* checking that pfn_to_section_nr() is >> sane. > > Right, that is problematic. __pfn_to_section() should not be called without > first validating pfn_to_section_nr(), as it could cause out-of-bound access > on mem_section buffer. Will fix that order but as there is no test scenario > which is definitive for this reported regression, how should we ensure that > it fixes the problem ? Oh, right, I missed that in patch #2. (and when comparing to generic pfn_valid()). I thought bisecting pointed at patch #1, that's why I didn't even have another look at patch #2. Makes sense. -- Thanks, David / dhildenb