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(p200300cbc7062000e44cbc46d8d30be5.dip0.t-ipconnect.de. [2003:cb:c706:2000:e44c:bc46:d8d3:be5]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4398e84efb9sm86595125e9.10.2025.02.20.01.36.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Feb 2025 01:36:06 -0800 (PST) Message-ID: Date: Thu, 20 Feb 2025 10:36:05 +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> Cc: Suren Baghdasaryan , Lokesh Gidra , 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> <50566d42-7754-4017-b290-f29d92e69231@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/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: G4Ql2nwY9Bzlvj2F9IuD851DiaD4WJhpDbbnJr2GqRI_1740044168 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 8B65E20004 X-Stat-Signature: pzpgd4c3ehp4unf54qm7uzx6wwdcwj4j X-HE-Tag: 1740044173-291781 X-HE-Meta: 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 tLW0pA1N 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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 20.02.25 10:31, Barry Song wrote: > On Thu, Feb 20, 2025 at 9:51 PM David Hildenbrand wrote: >> >> On 19.02.25 21:37, Barry Song wrote: >>> On Thu, Feb 20, 2025 at 7:27 AM Suren Baghdasaryan wrote: >>>> >>>> On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@gmail.com> wrote: >>>>> >>>>> From: Barry Song >>>>> >>>>> userfaultfd_move() checks whether the PTE entry is present or a >>>>> swap entry. >>>>> >>>>> - If the PTE entry is present, move_present_pte() handles folio >>>>> migration by setting: >>>>> >>>>> src_folio->index = linear_page_index(dst_vma, dst_addr); >>>>> >>>>> - 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. > > This is a useful suggestion. I’ve heard that Lokesh is also interested in > modifying ART to perform moves at the mTHP granularity, which would require > kernel modifications as well. It’s likely the direction we’ll take after > fixing the current urgent bugs. The current split_folio() really isn’t ideal. > > The corner cases you mentioned are definitely worth considering. However, > once we can perform batch UFFDIO_MOVE, I believe that in most cases, > the conflict between userfaultfd_move() and TAO will be resolved ? Well, as soon as you would have varying mTHP sizes, you'd still run into the split with TAO. Maybe that doesn't apply with Android today, but I can just guess that performing sub-mTHP moving would still be required for GC at some point. -- Cheers, David / dhildenb