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[2003:cb:c707:7600:5c18:5a7d:c5b7:e7a9]) by smtp.gmail.com with ESMTPSA id l11-20020a5d560b000000b0033df5710fabsm937527wrv.44.2024.02.27.06.11.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 Feb 2024 06:11:48 -0800 (PST) Message-ID: <30ea073d-0ccf-46e1-954d-e22f5cbf69f7@redhat.com> Date: Tue, 27 Feb 2024 15:11:47 +0100 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of THP Content-Language: en-US From: David Hildenbrand To: Matthew Wilcox , Charan Teja Kalla Cc: gregkh@linuxfoundation.org, akpm@linux-foundation.org, vbabka@suse.cz, dhowells@redhat.com, surenb@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, # see patch description , Huang Ying , Zi Yan , Naoya Horiguchi References: <1707814102-22682-1-git-send-email-quic_charante@quicinc.com> <8620c1a0-e091-46e9-418a-db66e621b9c4@quicinc.com> <845ca78f-913b-4a92-8b40-ff772a7ad333@redhat.com> 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 14.02.24 15:34, David Hildenbrand wrote: > On 14.02.24 15:18, Matthew Wilcox wrote: >> On Wed, Feb 14, 2024 at 12:04:10PM +0530, Charan Teja Kalla wrote: >>>> 1) Is it broken in 5.15? Did you actually try to reproduce or is this >>>>    just a guess? >>>> >>> >>> We didn't run the tests with THP enabled on 5.15, __so we didn't >>> encounter this issue__ on older to 6.1 kernels. >>> >>> I mentioned that issue exists is based on my understanding after code >>> walk through. To be specific, I just looked to the >>> migrate_pages()->..->migrate_page_move_mapping() & >>> __split_huge_page_tail() where the ->private field of thp sub-pages is >>> not filled with swap entry. If it could have set, I think these are the >>> only places where it would have done, per my understanding. CMIW. >> >> I think you have a misunderstanding. David's patch cfeed8ffe55b (part >> of 6.6) _stopped_ us using the tail ->private entries. So in 6.1, these >> tail pages should already have page->private set, and I don't understand >> what you're fixing. > > I think the issue is, that migrate_page_move_mapping() / > folio_migrate_mapping() would update ->private for a folio in the > swapcache (head page) > > newfolio->private = folio_get_private(folio); > > but not the ->private of the tail pages. > > So once you migrate a THP that is in the swapcache, ->private of the > tail pages would not be migrated and, therefore, be stale/wrong. > > Even before your patch that was the case. > > Looking at migrate_page_move_mapping(), we had: > > if (PageSwapBacked(page)) { > __SetPageSwapBacked(newpage); > if (PageSwapCache(page)) { > SetPageSwapCache(newpage); > set_page_private(newpage, page_private(page)); > } > } else { > VM_BUG_ON_PAGE(PageSwapCache(page), page); > } > > > I don't immediately see where the tail pages would similarly get updated > (via set_page_private). > > With my patch the problem is gone, because the tail page entries don't > have to be migrated, because they are unused. > > > Maybe this was an oversight from THP_SWAP -- 38d8b4e6bdc8 ("mm, THP, > swap: delay splitting THP during swap out"). > > It did update __add_to_swap_cache(): > > for (i = 0; i < nr; i++) { > set_page_private(page + i, entry.val + i); > error = radix_tree_insert(&address_space->page_tree, > idx + i, page + i); > if (unlikely(error)) > break; > } > > and similarly __delete_from_swap_cache(). > > But I don't see any updates to migration code. > > Now, it could be that THP migration was added later (post 2017), in that > case the introducing commit would not have been 38d8b4e6bdc8. > Let's continue: The introducing commit is likely either (1) 38d8b4e6bdc87 ("mm, THP, swap: delay splitting THP during swap out") That one added THP_SWAP, but THP migration wasn't supported yet AFAIKS. -> v4.13 (2) 616b8371539a6 ("mm: thp: enable thp migration in generic path") Or likely any of the following that actually allocate THP for migration: 8135d8926c08e mm: memory_hotplug: memory hotremove supports thp migration e8db67eb0ded3 mm: migrate: move_pages() supports thp migration c8633798497ce mm: mempolicy: mbind and migrate_pages support thp migration That actually enable THP migration. -> v4.14 So likely we'd have to fix the stable kernels: 4.19 5.4 5.10 5.15 6.1 That's a lot of pre-folio code. A backport of my series likely won't really make any sense. Staring at 4.19.307 code base, we likely have to perform a stable-only fix that properly handles the swapcache of compoud pages in migrate_page_move_mapping(). Ugly. -- Cheers, David / dhildenb