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[2003:cb:c73b:fa00:8909:2d07:8909:6a5a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a073e5e345sm14319611f8f.94.2025.04.29.07.46.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 29 Apr 2025 07:46:20 -0700 (PDT) Message-ID: Date: Tue, 29 Apr 2025 16:46:20 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm: Fix folio_pte_batch() overcount with zero PTEs To: Ryan Roberts , =?UTF-8?Q?Petr_Van=C4=9Bk?= , linux-kernel@vger.kernel.org Cc: Andrew Morton , linux-mm@kvack.org, stable@vger.kernel.org References: <20250429142237.22138-1-arkamar@atlas.cz> <20250429142237.22138-2-arkamar@atlas.cz> From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: ITPV_YulF31zsUH5wrYc_FY_vkBcl6SbkiWaF5IiQBY_1745937982 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: E9113120003 X-Stat-Signature: ya9od66t9gpji9pf8b13cbiojb6bp6yz X-Rspam-User: X-HE-Tag: 1745937984-760183 X-HE-Meta: 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 R1iZKlPA 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 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: List-Subscribe: List-Unsubscribe: On 29.04.25 16:41, Ryan Roberts wrote: > On 29/04/2025 15:29, David Hildenbrand wrote: >> On 29.04.25 16:22, Petr Vaněk wrote: >>> folio_pte_batch() could overcount the number of contiguous PTEs when >>> pte_advance_pfn() returns a zero-valued PTE and the following PTE in >>> memory also happens to be zero. The loop doesn't break in such a case >>> because pte_same() returns true, and the batch size is advanced by one >>> more than it should be. >>> >>> To fix this, bail out early if a non-present PTE is encountered, >>> preventing the invalid comparison. >>> >>> This issue started to appear after commit 10ebac4f95e7 ("mm/memory: >>> optimize unmap/zap with PTE-mapped THP") and was discovered via git >>> bisect. >>> >>> Fixes: 10ebac4f95e7 ("mm/memory: optimize unmap/zap with PTE-mapped THP") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Petr Vaněk >>> --- >>>   mm/internal.h | 2 ++ >>>   1 file changed, 2 insertions(+) >>> >>> diff --git a/mm/internal.h b/mm/internal.h >>> index e9695baa5922..c181fe2bac9d 100644 >>> --- a/mm/internal.h >>> +++ b/mm/internal.h >>> @@ -279,6 +279,8 @@ static inline int folio_pte_batch(struct folio *folio, >>> unsigned long addr, >>>               dirty = !!pte_dirty(pte); >>>           pte = __pte_batch_clear_ignored(pte, flags); >>>   +        if (!pte_present(pte)) >>> +            break; >>>           if (!pte_same(pte, expected_pte)) >>>               break; >> >> How could pte_same() suddenly match on a present and non-present PTE. >> >> Something with XEN is really problematic here. >> > > We are inside a lazy MMU region (arch_enter_lazy_mmu_mode()) at this point, > which I believe XEN uses. If a PTE was written then read back while in lazy mode > you could get a stale value. > > See > https://lore.kernel.org/all/912c7a32-b39c-494f-a29c-4865cd92aeba@agordeev.local/ > for an example bug. So if we cannot trust ptep_get() output, then, ... how could we trust anything here and ever possibly batch? -- Cheers, David / dhildenb