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[91.12.97.149]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38f259f7979sm19709544f8f.83.2025.02.20.00.51.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Feb 2025 00:51:47 -0800 (PST) Message-ID: <50566d42-7754-4017-b290-f29d92e69231@redhat.com> Date: Thu, 20 Feb 2025 09:51:45 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache To: Barry Song <21cnbao@gmail.com>, Suren Baghdasaryan , Lokesh Gidra Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, zhengtangquan@oppo.com, Barry Song , Andrea Arcangeli , Al Viro , Axel Rasmussen , Brian Geffon , Christian Brauner , Hugh Dickins , Jann Horn , Kalesh Singh , "Liam R . Howlett" , Matthew Wilcox , Michal Hocko , Mike Rapoport , Nicolas Geoffray , Peter Xu , Ryan Roberts , Shuah Khan , ZhangPeng , Yu Zhao References: <20250219112519.92853-1-21cnbao@gmail.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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: HgZ2Q9FxCXFjBudFS1wDGso1zug50Xjp6MvpwvwAggA_1740041509 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; 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>>> >>> - If the PTE entry is a swap entry, move_swap_pte() simply copies >>> the PTE to the new dst_addr. >>> >>> This approach is incorrect because even if the PTE is a swap >>> entry, it can still reference a folio that remains in the swap >>> cache. >>> >>> If do_swap_page() is triggered, it may locate the folio in the >>> swap cache. However, during add_rmap operations, a kernel panic >>> can occur due to: >>> page_pgoff(folio, page) != linear_page_index(vma, address) >> >> Thanks for the report and reproducer! >> >>> >>> $./a.out > /dev/null >>> [ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c >>> [ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0 >>> [ 13.337716] memcg:ffff00000405f000 >>> [ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff) >>> [ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 >>> [ 13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 >>> [ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 >>> [ 13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 >>> [ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001 >>> [ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000 >>> [ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address)) >>> [ 13.340190] ------------[ cut here ]------------ >>> [ 13.340316] kernel BUG at mm/rmap.c:1380! >>> [ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP >>> [ 13.340969] Modules linked in: >>> [ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299 >>> [ 13.341470] Hardware name: linux,dummy-virt (DT) >>> [ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) >>> [ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0 >>> [ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0 >>> [ 13.342018] sp : ffff80008752bb20 >>> [ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001 >>> [ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001 >>> [ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00 >>> [ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff >>> [ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f >>> [ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0 >>> [ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40 >>> [ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8 >>> [ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000 >>> [ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f >>> [ 13.343876] Call trace: >>> [ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P) >>> [ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320 >>> [ 13.344333] do_swap_page+0x1060/0x1400 >>> [ 13.344417] __handle_mm_fault+0x61c/0xbc8 >>> [ 13.344504] handle_mm_fault+0xd8/0x2e8 >>> [ 13.344586] do_page_fault+0x20c/0x770 >>> [ 13.344673] do_translation_fault+0xb4/0xf0 >>> [ 13.344759] do_mem_abort+0x48/0xa0 >>> [ 13.344842] el0_da+0x58/0x130 >>> [ 13.344914] el0t_64_sync_handler+0xc4/0x138 >>> [ 13.345002] el0t_64_sync+0x1ac/0x1b0 >>> [ 13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000) >>> [ 13.345504] ---[ end trace 0000000000000000 ]--- >>> [ 13.345715] note: a.out[107] exited with irqs disabled >>> [ 13.345954] note: a.out[107] exited with preempt_count 2 >>> >>> Fully fixing it would be quite complex, requiring similar handling >>> of folios as done in move_present_pte. >> >> How complex would that be? Is it a matter of adding >> folio_maybe_dma_pinned() checks, doing folio_move_anon_rmap() and >> folio->index = linear_page_index like in move_present_pte() or >> something more? > > My main concern is still with large folios that require a split_folio() > during move_pages(), as the entire folio shares the same index and > anon_vma. However, userfaultfd_move() moves pages individually, > making a split necessary. > > However, in split_huge_page_to_list_to_order(), there is a: > > if (folio_test_writeback(folio)) > return -EBUSY; > > This is likely true for swapcache, right? However, even for move_present_pte(), > it simply returns -EBUSY: > > move_pages_pte() > { > /* at this point we have src_folio locked */ > if (folio_test_large(src_folio)) { > /* split_folio() can block */ > pte_unmap(&orig_src_pte); > pte_unmap(&orig_dst_pte); > src_pte = dst_pte = NULL; > err = split_folio(src_folio); > if (err) > goto out; > > /* have to reacquire the folio after it got split */ > folio_unlock(src_folio); > folio_put(src_folio); > src_folio = NULL; > goto retry; > } > } > > Do we need a folio_wait_writeback() before calling split_folio()? > > By the way, I have also reported that userfaultfd_move() has a fundamental > conflict with TAO (Cc'ed Yu Zhao), which has been part of the Android common > kernel. In this scenario, folios in the virtual zone won’t be split in > split_folio(). Instead, the large folio migrates into nr_pages small folios. > > Thus, the best-case scenario would be: > > mTHP -> migrate to small folios in split_folio() -> move small folios to > dst_addr > > While this works, it negates the performance benefits of > userfaultfd_move(), as it introduces two PTE operations (migration in > split_folio() and move in userfaultfd_move() while retry), nr_pages memory > allocations, and still requires one memcpy(). This could end up > performing even worse than userfaultfd_copy(), I guess. > > The worst-case scenario would be failing to allocate small folios in > split_folio(), then userfaultfd_move() might return -ENOMEM? Although that's an Android problem and not an upstream problem, I'll note that there are other reasons why the split / move might fail, and user space either must retry or fallback to a COPY. Regarding mTHP, we could move the whole folio if the user space-provided range allows for batching over multiple PTEs (nr_ptes), they are in a single VMA, and folio_mapcount() == nr_ptes. There are corner cases to handle, such as moving mTHPs such that they suddenly cross two page tables I assume, that are harder to handle when not moving individual PTEs where that cannot happen. -- Cheers, David / dhildenb