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(p200300cbc7062000e44cbc46d8d30be5.dip0.t-ipconnect.de. [2003:cb:c706:2000:e44c:bc46:d8d3:be5]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43989087517sm121288765e9.8.2025.02.20.14.31.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Feb 2025 14:31:25 -0800 (PST) Message-ID: Date: Thu, 20 Feb 2025 23:31:23 +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>, Lokesh Gidra Cc: Suren Baghdasaryan , 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: KwGXaga3LV6fO1fLTWXFR3j-QlJqv5_92JE73qIuOzI_1740090687 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: 9nja914p7tfgdgimji5pf13gihbe7cbj X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: D128C1C0010 X-Rspam-User: X-HE-Tag: 1740090690-224283 X-HE-Meta: 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 uf4cWPic 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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 23:26, Barry Song wrote: > On Fri, Feb 21, 2025 at 11:20 AM Lokesh Gidra wrote: >> >> On Thu, Feb 20, 2025 at 1:45 PM Barry Song <21cnbao@gmail.com> wrote: >>> >>> On Thu, Feb 20, 2025 at 10:36 PM David Hildenbrand wrote: >>>> >>>> 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. >>> >>> With patch v2[1], as discussed in my previous email, I have observed that >>> small folios consistently succeed without crashing. Similarly, mTHP no >>> longer crashes; however, it still returns -EBUSY during the raced time >>> window, even after adding folio_wait_writeback. While I previously >>> mentioned that folio_writeback prevents mTHP from splitting, this is not >>> the only factor. The split_folio() function still returns -EBUSY because >>> folio_get_anon_vma(folio) returns NULL when the folio is not mapped. >>> >>> int split_huge_page_to_list_to_order(struct page *page, struct list_head *list, >>> unsigned int new_order) >>> { >>> anon_vma = folio_get_anon_vma(folio); >>> if (!anon_vma) { >>> ret = -EBUSY; >>> goto out; >>> } >>> >>> end = -1; >>> mapping = NULL; >>> anon_vma_lock_write(anon_vma); >>> } >>> >>> Even if mTHP is not from TAO's virtual zone, userfaultfd_move() will still >>> fail when performing sub-mTHP moving in the swap cache case due to: >> >> Just to clarify my doubt. What do you mean by sub-mTHP? Also when you >> say 'small folio' above, do you mean single-page folios? > > This means any moving size smaller than the size of mTHP, or moving > a partial mTHP. > >> >> Am I understanding correctly that your patch correctly handles moving >> single swap-cache page case? > > Yes, the crash is fixed for both small and large folios, and for small > folios, moving is consistently successful(even for the swapcache case). > The only issue is that sub-mTHP moving constantly fails for the swapcache > case because split_folio() fails, even after waiting for writeback as > split_folio() > can only split mapped folios - which is false for swapcache since > try_to_unmap_one() has been done. I mean, we (as the caller of split_folio()) have the VMA + anon_vma in our hands. Do we only have to bypass that mapping check, or is there something else that would block us? -- Cheers, David / dhildenb