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Howlett" , Lorenzo Stoakes , Charan Teja Kalla , akpm@linux-foundation.org, shikemeng@huaweicloud.com, kasong@tencent.com, nphamcs@gmail.com, bhe@redhat.com, baohua@kernel.org, chrisl@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Matthew Wilcox References: <20250808092156.1918973-1-quic_charante@quicinc.com> <29e080fd-5f9a-4760-876a-a6ccef33129f@quicinc.com> <9178bf98-2ea7-4ad8-ad43-cdcc02ab863d@redhat.com> <904f85d0-acd6-4f47-ab45-fbf18b80f1c6@lucifer.local> <7e7bfd05-434c-40b7-98ec-8ce352a8147d@redhat.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/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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Y-H_v_uKjPWFe7S9k7XFzFTt4rRddT8ALXu6abNUnP8_1754926774 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 63A2518000C X-Stat-Signature: gsy9fuznxmegxq4stq6eenge6gignbgp X-Rspam-User: X-HE-Tag: 1754926777-444535 X-HE-Meta: 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 QjXPUCLr 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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: >> >> I think it may actually be difficult to do on some level or there was some >> reason we couldn't, but I may be mistaken. > Thanks for the information! > Down the rabbit hole we go.. > > The cloning of the tree happens by copying the tree in DFS and > replacing the old nodes with new nodes. The tree leaves end up being > copied, which contains all the vmas (unless DONT_COPY is set, so > basically always all of them..). When the tree is copied, we have a > duplicate of the tree with pointers to all the vmas in the old process. > > The way the tree fails is that we've been unable to finish cloning it, > usually for out of memory reasons. So, this means we have a tree with > new and exciting vmas that have never been used and old but still active > vmas in oldmm. > > The failure point is then marked with an XA_ZERO_ENTRY, which will > succeed in storing as it's a direct replacement in the tree so no > allocations necessary. Thus this is safe even in -ENOMEM scenarios. > > Clearing out the stale data means we may actually need to allocate to > remove vmas from the new tree, because we use allocated memory in the > maple tree - we'll need to rebalance, new parents, etc, etc. > > So, to remove the stale data - we may actually have to allocate memory. Ah, that sucks. And I assume we cannot preallocate early, because we might actually require multiple alloc+free cycles. > > But we're most likely out of memory.. and we don't want to get the > shrinker involved in a broken task teardown, especially since it has > already been run and failed to help.. > > We could replace all the old vmas with XA_ZERO_ENTRY, but that doesn't > really fix this issue either. No. > > I could make a function that frees all new vmas and destroys the tree > specifically for this failure state? I think the problem is that some page tables were already copied, so we would have to zap them as well. Maybe just factoring stuff from the exit_mmap() function could be one way to do it. > > I'm almost certain this will lead to another whack-a-mole situation, but > those _should_ already be checked or written to work when there are zero > vmas in an mm (going by experience of what the scheduler does with an > empty tree). Syzbot finds these scenarios sometimes via signals or > other corner cases that can happen.. Yes, I rememebr fixing one instance of "0 VMAs". Most code paths just don't apply if a process was never actually ran I guess. > > Then again, I also thought the unstable mm should be checked where > necessary to avoid assumptions on the mm state..? > > This is funny because we already have a (probably) benign race with oom > here. This code may already visit the mm after __oom_reap_task_mm() and > the mm disappearing, but since the anon vmas should be removed, > unuse_mm() will skip them. > > Although, I'm not sure what happens when > mmu_notifier_invalidate_range_start_nonblock() fails AND unuse_mm() is > called on the mm after. Maybe checking the unstable mm is necessary > here anyways? Can we have MMU notifiers active while the process never even ran and we are only halfway through duplicating VMAs? -- Cheers, David / dhildenb