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(p200300d82f2bb200607dd3d232711be0.dip0.t-ipconnect.de. [2003:d8:2f2b:b200:607d:d3d2:3271:1be0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-459e075805csm34786315e9.4.2025.08.05.02.50.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 05 Aug 2025 02:50:37 -0700 (PDT) Message-ID: <5ac2ec58-3908-4d0e-a29b-8b4d776410e3@redhat.com> Date: Tue, 5 Aug 2025 11:50:36 +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] mm: Fix the race between collapse and PT_RECLAIM under per-vma lock To: Qi Zheng , Baolin Wang , Barry Song <21cnbao@gmail.com>, akpm@linux-foundation.org, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, Barry Song , "Lai, Yi" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn , Suren Baghdasaryan , Lokesh Gidra , Tangquan Zheng , Lance Yang , Zi Yan , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain References: <20250805035447.7958-1-21cnbao@gmail.com> <35417160-86bf-4580-8ae9-5cadd4f6401d@bytedance.com> <064cca31-442d-4847-b353-26dc5fd0603c@bytedance.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/g1oFAmgsLPQFCRvGjuMACgkQTd4Q 9wD/g1o0bxAAqYC7gTyGj5rZwvy1VesF6YoQncH0yI79lvXUYOX+Nngko4v4dTlOQvrd/vhb 02e9FtpA1CxgwdgIPFKIuXvdSyXAp0xXuIuRPQYbgNriQFkaBlHe9mSf8O09J3SCVa/5ezKM OLW/OONSV/Fr2VI1wxAYj3/Rb+U6rpzqIQ3Uh/5Rjmla6pTl7Z9/o1zKlVOX1SxVGSrlXhqt kwdbjdj/csSzoAbUF/duDuhyEl11/xStm/lBMzVuf3ZhV5SSgLAflLBo4l6mR5RolpPv5wad GpYS/hm7HsmEA0PBAPNb5DvZQ7vNaX23FlgylSXyv72UVsObHsu6pT4sfoxvJ5nJxvzGi69U s1uryvlAfS6E+D5ULrV35taTwSpcBAh0/RqRbV0mTc57vvAoXofBDcs3Z30IReFS34QSpjvl Hxbe7itHGuuhEVM1qmq2U72ezOQ7MzADbwCtn+yGeISQqeFn9QMAZVAkXsc9Wp0SW/WQKb76 FkSRalBZcc2vXM0VqhFVzTb6iNqYXqVKyuPKwhBunhTt6XnIfhpRgqveCPNIasSX05VQR6/a OBHZX3seTikp7A1z9iZIsdtJxB88dGkpeMj6qJ5RLzUsPUVPodEcz1B5aTEbYK6428H8MeLq NFPwmknOlDzQNC6RND8Ez7YEhzqvw7263MojcmmPcLelYbfOwU0EVcufkQEQAOfX3n0g0fZz 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+DWgUCaCwtJQUJG8aPFAAKCRBN3hD3AP+DWlDnD/4k2TW+HyOOOePVm23F5HOhNNd7nNv3 Vq2cLcW1DteHUdxMO0X+zqrKDHI5hgnE/E2QH9jyV8mB8l/ndElobciaJcbl1cM43vVzPIWn 01vW62oxUNtEvzLLxGLPTrnMxWdZgxr7ACCWKUnMGE2E8eca0cT2pnIJoQRz242xqe/nYxBB /BAK+dsxHIfcQzl88G83oaO7vb7s/cWMYRKOg+WIgp0MJ8DO2IU5JmUtyJB+V3YzzM4cMic3 bNn8nHjTWw/9+QQ5vg3TXHZ5XMu9mtfw2La3bHJ6AybL0DvEkdGxk6YHqJVEukciLMWDWqQQ RtbBhqcprgUxipNvdn9KwNpGciM+hNtM9kf9gt0fjv79l/FiSw6KbCPX9b636GzgNy0Ev2UV m00EtcpRXXMlEpbP4V947ufWVK2Mz7RFUfU4+ETDd1scMQDHzrXItryHLZWhopPI4Z+ps0rB CQHfSpl+wG4XbJJu1D8/Ww3FsO42TMFrNr2/cmqwuUZ0a0uxrpkNYrsGjkEu7a+9MheyTzcm vyU2knz5/stkTN2LKz5REqOe24oRnypjpAfaoxRYXs+F8wml519InWlwCra49IUSxD1hXPxO WBe5lqcozu9LpNDH/brVSzHCSb7vjNGvvSVESDuoiHK8gNlf0v+epy5WYd7CGAgODPvDShGN g3eXuA== Organization: Red Hat In-Reply-To: <064cca31-442d-4847-b353-26dc5fd0603c@bytedance.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 05.08.25 11:30, Qi Zheng wrote: > > > On 8/5/25 4:56 PM, Baolin Wang wrote: >> >> >> On 2025/8/5 16:17, Qi Zheng wrote: >>> Hi Baolin, >>> >>> On 8/5/25 3:53 PM, Baolin Wang wrote: >>>> >>>> >>>> On 2025/8/5 14:42, Qi Zheng wrote: >>>>> Hi Barry, >>>>> >>>>> On 8/5/25 11:54 AM, Barry Song wrote: >>>>>> From: Barry Song >>>>>> >>>>>> The check_pmd_still_valid() call during collapse is currently only >>>>>> protected by the mmap_lock in write mode, which was sufficient when >>>>>> pt_reclaim always ran under mmap_lock in read mode. However, since >>>>>> madvise_dontneed can now execute under a per-VMA lock, this assumption >>>>>> is no longer valid. As a result, a race condition can occur between >>>>>> collapse and PT_RECLAIM, potentially leading to a kernel panic. >>>>> >>>>> There is indeed a race condition here. And after applying this patch, I >>>>> can no longer reproduce the problem locally (I was able to reproduce it >>>>> stably locally last night). >>>>> >>>>> But I still can't figure out how this race condtion causes the >>>>> following panic: >>>>> >>>>> exit_mmap >>>>> --> mmap_read_lock() >>>>>      unmap_vmas() >>>>>      --> pte_offset_map_lock >>>>>          --> rcu_read_lock() >>>>>              check if the pmd entry is a PTE page >>>>>              ptl = pte_lockptr(mm, &pmdval)  <-- ptl is NULL >>>>>              spin_lock(ptl)                  <-- PANIC!! >>>>> >>>>> If this PTE page is freed by pt_reclaim (via RCU), then the ptl can >>>>> not be NULL. >>>>> >>>>> The collapse holds mmap write lock, so it is impossible to be >>>>> concurrent >>>>> with exit_mmap(). >>>>> >>>>> Confusing. :( >>>> >>>> IIUC, the issue is not caused by the concurrency between exit_mmap >>>> and collapse, but rather by the concurrency between pt_reclaim and >>>> collapse. >>>> >>>> Before this patch, khugepaged might incorrectly restore a PTE >>>> pagetable that had already been freed. >>>> >>>> pt_reclaim has cleared the pmd entry and freed the PTE page table. >>>> However, due to the race condition, check_pmd_still_valid() still >>>> passes and continues to attempt the collapse: >>>> >>>> _pmd = pmdp_collapse_flush(vma, address, pmd); ---> returns a none >>>> pmd entry (the original pmd entry has been cleared) >>>> >>>> pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); ---> returns >>>> pte == NULL >>>> >>>> Then khugepaged will restore the old PTE pagetable with an invalid >>>> pmd entry: >>>> >>>> pmd_populate(mm, pmd, pmd_pgtable(_pmd)); >>>> >>>> So when the process exits and trys to free the mapping of the >>>> process, traversing the invalid pmd table will lead to a crash. >>> >>> CPU0                         CPU1 >>> ====                         ==== >>> >>> collapse >>> --> pmd_populate(mm, pmd, pmd_pgtable(_pmd)); >>>      mmap_write_unlock >>>                               exit_mmap >>>                               --> hold mmap lock >>>                                   __pte_offset_map_lock >>>                                   --> pte = __pte_offset_map(pmd, >>> addr, &pmdval); >>>                                       if (unlikely(!pte)) >>>                                           return pte;   <-- will return >> >> __pte_offset_map() might not return NULL? Because the 'pmd_populate(mm, >> pmd, pmd_pgtable(_pmd))' could populate a valid page (although the >> '_pmd' entry is NONE), but it is not the original pagetable page. > > CPU0 CPU1 > ==== ==== > > collapse > --> check_pmd_still_valid > vma read lock > pt_reclaim clear the pmd entry and will > free the PTE page (via RCU) > vma read unlock > > vma write lock > _pmd = pmdp_collapse_flush(vma, address, pmd) <-- pmd_none(_pmd) > pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl); <-- pte is > NULL > pmd_populate(mm, pmd, pmd_pgtable(_pmd)); <-- populate a valid page? > vma write unlock > > The above is the concurrent scenario you mentioned, right? > > What types of this 'valid page' could be? If __pte_offset_map() returns > non-NULL, then it is a PTE page. Even if it is not the original one, it > should not cause panic. Did I miss some key information? :( Wasn't the original issue all about a NULL-pointer de-reference while *locking*? Note that in that kernel config [1] we have CONFIG_DEBUG_SPINLOCK=y, so likely we will have ALLOC_SPLIT_PTLOCKS set. [1] https://github.com/laifryiee/syzkaller_logs/blob/main/250803_193026___pte_offset_map_lock/.config -- Cheers, David / dhildenb