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[2003:d8:2f4a:8900:884a:b3af:e3c9:ec88]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-442f396c093sm60263885e9.27.2025.05.15.01.49.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 15 May 2025 01:49:37 -0700 (PDT) Message-ID: <61da67fe-ba7a-4b9c-acb2-f1488f00a804@redhat.com> Date: Thu, 15 May 2025 10:49:36 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] arm64: Check pxd_leaf() instead of !pxd_table() while tearing down page tables To: Dev Jain , catalin.marinas@arm.com, will@kernel.org Cc: ryan.roberts@arm.com, anshuman.khandual@arm.com, mark.rutland@arm.com, yang@os.amperecomputing.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, stable@vger.kernel.org References: <20250515063450.86629-1-dev.jain@arm.com> <332ecda7-14c4-4dc3-aeff-26801b74ca04@redhat.com> <4904d02f-6595-4230-a321-23327596e085@arm.com> <6fe7848c-485e-4639-b65c-200ed6abe119@redhat.com> <73a67d06-b497-40a2-8cb2-3b80c6ba59d1@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250515_014942_250502_17977C47 X-CRM114-Status: GOOD ( 16.17 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 15.05.25 10:40, Dev Jain wrote: > > > On 15/05/25 2:06 pm, David Hildenbrand wrote: >> On 15.05.25 10:22, Dev Jain wrote: >>> >>> >>> On 15/05/25 1:43 pm, David Hildenbrand wrote: >>>> On 15.05.25 08:34, Dev Jain wrote: >>>>> Commit 9c006972c3fe removes the pxd_present() checks because the caller >>>>> checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller >>>>> only >>>>> checks pud_present(); pud_free_pmd_page() recurses on each pmd through >>>>> pmd_free_pte_page(), wherein the pmd may be none. >>>> The commit states: "The core code already has a check for pXd_none()", >>>> so I assume that assumption was not true in all cases? >>>> >>>> Should that one problematic caller then check for pmd_none() instead? >>> >>>   From what I could gather of Will's commit message, my interpretation is >>> that the concerned callers are vmap_try_huge_pud and vmap_try_huge_pmd. >>> These individually check for pxd_present(): >>> >>> if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr)) >>>     return 0; >>> >>> The problem is that vmap_try_huge_pud will also iterate on pte entries. >>> So if the pud is present, then pud_free_pmd_page -> pmd_free_pte_page >>> may encounter a none pmd and trigger a WARN. >> >> Yeah, pud_free_pmd_page()->pmd_free_pte_page() looks shaky. >> >> I assume we should either have an explicit pmd_none() check in >> pud_free_pmd_page() before calling pmd_free_pte_page(), or one in >> pmd_free_pte_page(). >> >> With your patch, we'd be calling pte_free_kernel() on a NULL pointer, >> which sounds wrong -- unless I am missing something important. >> >>> >>>> >>>> If you were able to trigger this WARN, it's always a good idea to >>>> include the splat in the commit. >>> >>> I wasn't able to, it is just an observation from code inspection. >> >> That better be included in the patch description :) > > I did, actually. My bad for not putting in spaces, I notice now that the > description looks horrible to the eye :) Ahh, there it is. Sorry :) Yeah, some empty lines won't hurt! -- Cheers, David / dhildenb