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[2003:d8:2f3c:3a00:5662:26b3:3e5d:438e]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a36a83b63bsm6795831f8f.97.2025.05.19.05.50.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 May 2025 05:50:51 -0700 (PDT) Message-ID: <04c32758-af9b-492a-909a-ad1bc6c0777d@redhat.com> Date: Mon, 19 May 2025 14:50:50 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] arm64: Restrict pagetable teardown to avoid false warning To: Ryan Roberts , Dev Jain , catalin.marinas@arm.com, will@kernel.org Cc: 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: <20250518095445.31044-1-dev.jain@arm.com> <5763d921-f8a8-4ca6-b5b5-ad96eb5cda11@arm.com> <7680e775-d277-45ea-9b6c-1f16b8b55a3f@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 19.05.25 14:47, Ryan Roberts wrote: > On 19/05/2025 13:16, David Hildenbrand wrote: >> On 19.05.25 11:08, Ryan Roberts wrote: >>> On 18/05/2025 10:54, Dev Jain wrote: >>>> Commit 9c006972c3fe removes the pxd_present() checks because the caller >>> >>> nit: please use the standard format for describing commits: Commit 9c006972c3fe >>> ("arm64: mmu: drop pXd_present() checks from pXd_free_pYd_table()") >>> >>>> 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. Thus it is possible to >>>> hit a warning in the latter, since pmd_none => !pmd_table(). Thus, add >>>> a pmd_present() check in pud_free_pmd_page(). >>>> >>>> This problem was found by code inspection. >>>> >>>> This patch is based on 6.15-rc6. >>> >>> nit: please remove this to below the "---", its not part of the commit log. >>> >>>> >>>> Fixes: 9c006972c3fe (arm64: mmu: drop pXd_present() checks from >>>> pXd_free_pYd_table()) >>>> >>> >>> nit: remove empty line; the tags should all be in a single block with no empty >>> lines. >>> >>>> Cc: >>>> Reported-by: Ryan Roberts >>>> Signed-off-by: Dev Jain >>>> --- >>>> v1->v2: >>>>   - Enforce check in caller >>>> >>>>   arch/arm64/mm/mmu.c | 3 ++- >>>>   1 file changed, 2 insertions(+), 1 deletion(-) >>>> >>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>> index ea6695d53fb9..5b1f4cd238ca 100644 >>>> --- a/arch/arm64/mm/mmu.c >>>> +++ b/arch/arm64/mm/mmu.c >>>> @@ -1286,7 +1286,8 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr) >>>>       next = addr; >>>>       end = addr + PUD_SIZE; >>>>       do { >>>> -        pmd_free_pte_page(pmdp, next); >>>> +        if (pmd_present(*pmdp)) >>> >>> pmd_free_pte_page() is using READ_ONCE() to access the *pmdp to ensure it can't >>> be torn. I suspect we don't technically need that in these functions because >>> there can be no race with a writer. >> >> Yeah, if there is no proper locking in place the function would already >> seriously mess up (double freeing etc). > > Indeed; there is no locking, but this portion of the vmalloc VA space has been > allocated to us exclusively, so we know there can be no one else racing. > >> >>> But the arm64 arch code always uses >>> READ_ONCE() for dereferencing pgtable entries for safely. Perhaps we should be >>> consistent here? >> >> mm/vmalloc.c:   if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr)) > > Yes, I saw that. I know that we don't technically need READ_ONCE(). I'm just > proposng that for arm64 code we should be consistent with what it already does. > See Commit 20a004e7b017 ("arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing > page tables") The more READ_ONCE() we add, the less the compiler can optimize (e.g., when inlining). Apparently, for no good reason in this page table handling code, because there cannot be concurrent changes that would turn it !present etc. Anyhow, something for the arm maintainers to decide. -- Cheers, David / dhildenb