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[91.12.97.62]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a073e4684csm14681657f8f.76.2025.04.29.11.56.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 29 Apr 2025 11:56:04 -0700 (PDT) Message-ID: <1df577bb-eaba-4e34-9050-309ee1c7dc57@redhat.com> Date: Tue, 29 Apr 2025 20:56:03 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm: Fix folio_pte_batch() overcount with zero PTEs To: =?UTF-8?Q?Petr_Van=C4=9Bk?= Cc: linux-kernel@vger.kernel.org, Andrew Morton , Ryan Roberts , linux-mm@kvack.org, stable@vger.kernel.org References: <20250429142237.22138-1-arkamar@atlas.cz> <20250429142237.22138-2-arkamar@atlas.cz> <2025429144547-aBDmGzJBQc9RMBj--arkamar@atlas.cz> <2025429183321-aBEbcQQY3WX6dsNI-arkamar@atlas.cz> 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: <2025429183321-aBEbcQQY3WX6dsNI-arkamar@atlas.cz> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 29.04.25 20:33, Petr Vaněk wrote: > On Tue, Apr 29, 2025 at 05:45:53PM +0200, David Hildenbrand wrote: >> On 29.04.25 16:52, David Hildenbrand wrote: >>> On 29.04.25 16:45, Petr Vaněk wrote: >>>> On Tue, Apr 29, 2025 at 04:29:30PM +0200, 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. >>>> >>>> In the problematic case pte.pte == 0 and expected_pte.pte == 0 as well. >>>> pte_same() returns a.pte == b.pte -> 0 == 0. Both are non-present PTEs. >>> >>> Observe that folio_pte_batch() was called *with a present pte*. >>> >>> do_zap_pte_range() >>> if (pte_present(ptent)) >>> zap_present_ptes() >>> folio_pte_batch() >>> >>> How can we end up with an expected_pte that is !present, if it is based >>> on the provided pte that *is present* and we only used pte_advance_pfn() >>> to advance the pfn? >> >> I've been staring at the code for too long and don't see the issue. >> >> We even have >> >> VM_WARN_ON_FOLIO(!pte_present(pte), folio); >> >> So the initial pteval we got is present. >> >> I don't see how >> >> nr = pte_batch_hint(start_ptep, pte); >> expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags); >> >> would suddenly result in !pte_present(expected_pte). > > The issue is not happening in __pte_batch_clear_ignored but later in > following line: > > expected_pte = pte_advance_pfn(expected_pte, nr); > > The issue seems to be in __pte function which converts PTE value to > pte_t in pte_advance_pfn, because warnings disappears when I change the > line to > > expected_pte = (pte_t){ .pte = pte_val(expected_pte) + (nr << PFN_PTE_SHIFT) }; > > The kernel probably uses __pte function from > arch/x86/include/asm/paravirt.h because it is configured with > CONFIG_PARAVIRT=y: > > static inline pte_t __pte(pteval_t val) > { > return (pte_t) { PVOP_ALT_CALLEE1(pteval_t, mmu.make_pte, val, > "mov %%rdi, %%rax", ALT_NOT_XEN) }; > } > > I guess it might cause this weird magic, but I need more time to > understand what it does :) What XEN does with basic primitives that convert between pteval and pte_t is beyond horrible. How come set_ptes() that uses pte_next_pfn()->pte_advance_pfn() does not run into this? Is it only a problem if we exceed a certain pfn? -- Cheers, David / dhildenb