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[79.224.245.50]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-447f78b0acasm100097975e9.31.2025.05.22.04.59.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 May 2025 04:59:24 -0700 (PDT) Message-ID: Date: Thu, 22 May 2025 13:59:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/khugepaged: Fix race with folio splitting in hpage_collapse_scan_file() To: Baolin Wang , Shivank Garg , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: ziy@nvidia.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, fengwei.yin@intel.com, bharata@amd.com, syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com References: <20250522093452.6379-1-shivankg@amd.com> <3eae6bb7-4865-44c2-8603-dc2bd8e3609b@linux.alibaba.com> 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: <3eae6bb7-4865-44c2-8603-dc2bd8e3609b@linux.alibaba.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: dQpczpVTHDWuagvu4q-cUUQ0_9u4jfX2mAGmhx1g7Ec_1747915165 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 49F04140011 X-Stat-Signature: 3jkfdokga4mcq7errjm4fdcrra7j56sm X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1747915168-324269 X-HE-Meta: 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 TRtGAlpB 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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 22.05.25 12:01, Baolin Wang wrote: > > > On 2025/5/22 17:34, Shivank Garg wrote: >> folio_mapcount() checks folio_test_large() before proceeding to >> folio_large_mapcount(), but there exists a race window where a folio >> could be split between these checks which triggered the >> VM_WARN_ON_FOLIO(!folio_test_large(folio), folio) in >> folio_large_mapcount(). >> >> Take a temporary folio reference in hpage_collapse_scan_file() to prevent >> races with concurrent folio splitting/freeing. This prevent potential >> incorrect large folio detection. >> >> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com >> Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com >> Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single value") >> Suggested-by: David Hildenbrand >> Signed-off-by: Shivank Garg >> --- >> mm/khugepaged.c | 16 ++++++++++++++++ >> 1 file changed, 16 insertions(+) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index cc945c6ab3bd..6e8902f9d88c 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -2295,6 +2295,17 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr, >> continue; >> } >> >> + if (!folio_try_get(folio)) { >> + xas_reset(&xas); >> + continue; >> + } >> + >> + if (unlikely(folio != xas_reload(&xas))) { >> + folio_put(folio); >> + xas_reset(&xas); >> + continue; >> + } >> + >> if (folio_order(folio) == HPAGE_PMD_ORDER && >> folio->index == start) { >> /* Maybe PMD-mapped */ >> @@ -2305,23 +2316,27 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr, >> * it's safe to skip LRU and refcount checks before >> * returning. >> */ >> + folio_put(folio); >> break; >> } >> >> node = folio_nid(folio); >> if (hpage_collapse_scan_abort(node, cc)) { >> result = SCAN_SCAN_ABORT; >> + folio_put(folio); >> break; >> } >> cc->node_load[node]++; >> >> if (!folio_test_lru(folio)) { >> result = SCAN_PAGE_LRU; >> + folio_put(folio); >> break; >> } >> >> if (!is_refcount_suitable(folio)) { > > You add a temporary refcnt for the folio, then the > is_refcount_suitable() will always fail, right? Indeed. Would one of our MADV_COLLAPSE selftests catch that? We should also be converting that code to use folio_expected_ref_count() -- either directly or wrapped in is_refcount_suitable(). Likely just here through if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) ... -- Cheers, David / dhildenb