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[2003:cb:c70a:4d00:b968:9e7a:af8b:adf7]) by smtp.gmail.com with ESMTPSA id bt9-20020a056000080900b0033b75d0993esm7810202wrb.74.2024.02.13.01.25.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 13 Feb 2024 01:25:23 -0800 (PST) Message-ID: Date: Tue, 13 Feb 2024 10:25:22 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/huge_memory: fix swap entry values of tail pages of THP To: Charan Teja Kalla , gregkh@linuxfoundation.org, akpm@linux-foundation.org, willy@infradead.org, vbabka@suse.cz, dhowells@redhat.com, surenb@google.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, # see patch description References: <1707814102-22682-1-git-send-email-quic_charante@quicinc.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: <1707814102-22682-1-git-send-email-quic_charante@quicinc.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 8AD57C0011 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: w3mh9jqq39cg6apbqyifekgr775gz8um X-HE-Tag: 1707816327-901780 X-HE-Meta: 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 4YFP9XX0 8DI2g/2GkHUSMbFymt+BOMbKbbEQBjd1GFW67bleQZjC1LAT0ohXyLWFDWzZTbo2ME0erITw3lQZf0/8LmGBgnxC6terhMV6txzR3L90/Xl9Doz1FK08f/CtvSYWTjP9iQx/90gRVx0ZdNi1+QeafoI5s5jNETCsc5CjL3y7lfqjcmWaf4KkX0mpbT38hOLD6NI9jD2VoNKeZrDbhE0wP1KtIVD2+Ln6aMuAO8qk6H6idqarX4sjVKH7kjQ5V6jawvOR5fpS1WuOm9FVI3A/1ngW4Lk5tbW3kOI+pVX+cEHYZlW0fsMZRKtk0a1F81m9P7inMzJcYbTl+LLHFogWCg8TSfptK7LCVXeTt21lFR7Vc5DK10lzrHpNYbhFdbEC4w/iPpNu8SUAYEz+63W0+vDM9RRFLr73uDF0WIuTkZy+SlY/umPcuSiXQ2khc4edNdki7+KI9hzFt+07ZmFLxOqm6JW72aiqQHYGap47qfXO8E4C8CnVK37G3j2wwWMieW1j2vfkG6BiWhsJkUriOlwnKJaHNndhrGHVvf5X5F7vBbrrU6xrNBia2uhve+Ss6XzRxYw4uj6PE2EtabVtBlbKOxdOwrD09aj8JBu9lZxfTXzE= 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 13.02.24 09:48, Charan Teja Kalla wrote: > An anon THP page is first added to swap cache before reclaiming it. > Initially, each tail page contains the proper swap entry value(stored in > ->private field) which is filled from add_to_swap_cache(). After This is a stable-only fix? In that case, it make sense to indicate that in the patch subject [PATCH STABLE vX.Y]. But it's always odd to have stable-only fixes, the docs [1] don't cover that (maybe they should? or I missed it). [1] https://www.kernel.org/doc/Documentation/process/stable-kernel-rules.rst So we are migrating a THP that was added to the swapcache. Do you have a reproducer? > migrating the THP page sitting on the swap cache, only the swap entry of > the head page is filled(see folio_migrate_mapping()). > > Now when this page is tried to split(one case is when this page is again > migrated, see migrate_pages()->try_split_thp()), the tail pages > ->private is not stored with proper swap entry values. When this tail > page is now try to be freed, as part of it delete_from_swap_cache() is > called which operates on the wrong swap cache index and eventually > replaces the wrong swap cache index with shadow/NULL value, frees the > page. But what if we don't split the THP after migration. Is there anything else that can go wrong? It's sufficient to look where upstream calls page_swap_entry(): For example, do_swap_page() will never be able to make progress, because the swap entry of the page does not correspond to the swap entry in the PTE? It can easily fault on a swap PTE that refers a THP subpage in the swapcache. So unless I am missing something, only fixing this up during the split is insufficient. You have to fix it up during migration. > > This leads to the state with a swap cache containing the freed page. > This issue can manifest in many forms and the most common thing observed > is the rcu stall during the swapin (see mapping_get_entry()). > > On the recent kernels, this issues is indirectly getting fixed with the > series[1], to be specific[2]. > > When tried to back port this series, it is observed many merge > conflicts and also seems dependent on many other changes. As backporting > to LTS branches is not a trivial one, the similar change from [2] is > picked as a fix. The fix is in commit cfeed8ffe55b37fa10286aaaa1369da00cb88440 Author: David Hildenbrand Date: Mon Aug 21 18:08:46 2023 +0200 mm/swap: stop using page->private on tail pages for THP_SWAP > > [1] https://lore.kernel.org/all/20230821160849.531668-1-david@redhat.com/ > [2] https://lore.kernel.org/all/20230821160849.531668-5-david@redhat.com/ > > Closes: https://lore.kernel.org/linux-mm/69cb784f-578d-ded1-cd9f-c6db04696336@quicinc.com/ > Fixes: 3417013e0d18 ("mm/migrate: Add folio_migrate_mapping()") > Cc: # see patch description, applicable to <=6.1 > Signed-off-by: Charan Teja Kalla 3417013e0d18 went into 5.16. cfeed8ffe55b3 went into 6.6. So only 6.1 is affected. Isn't there a way to bite the bullet and backport that series to 6.1 instead? -- Cheers, David / dhildenb