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[84.136.37.227]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-424817b53f2sm189599165e9.25.2024.06.25.12.45.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 25 Jun 2024 12:46:00 -0700 (PDT) Message-ID: <9e8d71f4-69dc-4bf4-a40d-e1b89586f5c9@redhat.com> Date: Tue, 25 Jun 2024 21:45:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable] mm: folio_add_new_anon_rmap() careful __folio_set_swapbacked() To: Hugh Dickins Cc: Andrew Morton , Barry Song <21cnbao@gmail.com>, baolin.wang@linux.alibaba.com, chrisl@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, ryan.roberts@arm.com, shy828301@gmail.com, surenb@google.com, v-songbaohua@oppo.com, willy@infradead.org, ying.huang@intel.com, yosryahmed@google.com, yuanshuai@oppo.com, yuzhao@google.com References: <0a41d5fc-d1a1-4b1b-873e-a701b20bbcb3@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-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: F049240013 X-Stat-Signature: x9m7h9m1dbd75t8kj7zd7nq3m119yzp3 X-Rspam-User: X-HE-Tag: 1719344764-23962 X-HE-Meta: U2FsdGVkX1+QLJDBWrxfYoHTYpOaZ7WHV4c0dD4RSQhV5pypB+5mM+72xnihdKRL2Lo6QUjBcUdVT0lvWSE2oACaGWw81NJVWKsrl71AnE0ysDNmwCqmFGmXwnF3AnQSBWrz/eEn+tX91SgPrmNUtiHxJY1xusnXJBjUmSowkx9sMSfNEtHJd87ByFjCjF99q07rMUPYkAx8fNZrWiUOLXAXw4waVnLeJmpCKR56olcFRZWeTzPRAEQfbDJUExqG7RfLmBcTXBQpQsPx0wcfLUG1YCWE8oYmbQVePCxEGXwAa/MwT4PJcNiABdVnwWNvbh3+Q4PM0fWCL3eCOPz9//iyZA2jn8sRNLWQ0LE2bvLThhOgcE2n7nKfoZ3cXrNVs7x/y0ZMESvo0op4424vwbCDvw91r9LNjuqsYA+rp3EdpQfL+HyuuJQL5NwprFHb6FChSN7Z4mp1k2t8rrfGQZc4yR6sdd5pwxVZ7ddVz1OvwEmHbg4BZvdvB7GQKjCwdCQzuz4v74BDY5Y9CYM4S+IdjQU9FkBWDS8hYaTuOqYoo68AnLEcYtAd0jovBG2vE2IFDKnwnvSb2JonPjSK0RnSpG24yW4i7djq8S990MBU4mksmhxNzi2O5L1VrzISD180FD477Pwvu3EahzlqYONobSqkqEkQXbpNs7pFjjzKeygNJbYyqOt8/lN+fPAuY+ldLRb6zBvCCDoTfod2kWKk1SYC1p/e/LIbyJpMm4hytZqBVyfbOU4rUqZJUgn7pABbVRYdXAMyvcJ2NZo+hrI6I8Pf5NKKTl/s1uJ/KGNzkX4GuWa8gRtwrieP+1Tal2sE3OdvTIbeLuLyxlITJg/khDPetp2JB+jgSQiRbfcbmY3ta9g56kWwhJ4xHevUrcUJl/vYK+VdGb3Qy4IQb3ZTHxnL+a8gunMrheIeiL5l6WzLnqqvUqQfYw+HCel+PSA0XwQjnUTdIxb+20T B1y+PHyb sY/K1N28ZIp9W6ykyZueZ1fHssUB08mRTG7yqAuo6AQUnv79vEzSM+TJEon87Bn5JV0P4usWtABFeX5ldUCyuiGtdPgvRWbdG+dpGkWaIcGSpqUqY0PavppyA59qWyrO+WIwydQOFOME8XdIgWa+Se2QHMqEzg71+n2IZcwhYKbGWuDAAKOVIPgl40m/1HhUsYVDk/YszHVJ05KA33QxlRFdoFUHLOt13GXrX3c0l7S2RaRKe07II1jR3NgwFn9IUPiEntBMFU13KqQ5OTOV3NyunDPrh2CnWdHdJLzpZFxYn/q3SV9B6O2/8eMMeZVHVP6Sw7qOoR4g3ovLOE0xky9/kjHXkGj2qybUsKJXPBR4m58XX8TvwiCIUZovLUwIhXa2BMJuIG9MZNOxzfBKy1e5+9xmdLybJ5c6sHwfCzp4icyyJCdolSRiqD3zM/m2fjbYL2OkJkb/I5oQ= 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 25.06.24 21:37, Hugh Dickins wrote: > On Tue, 25 Jun 2024, David Hildenbrand wrote: >> On 25.06.24 07:00, Hugh Dickins wrote: >>> Commit "mm: use folio_add_new_anon_rmap() if folio_test_anon(folio)== >>> false" has extended folio_add_new_anon_rmap() to use on non-exclusive >>> folios, already visible to others in swap cache and on LRU. >>> >>> That renders its non-atomic __folio_set_swapbacked() unsafe: it risks >>> overwriting concurrent atomic operations on folio->flags, losing bits >>> added or restoring bits cleared. Since it's only used in this risky >>> way when folio_test_locked and !folio_test_anon, many such races are >>> excluded; but, for example, isolations by folio_test_clear_lru() are >>> vulnerable, and setting or clearing active. >>> >>> It could just use the atomic folio_set_swapbacked(); but this function >>> does try to avoid atomics where it can, so use a branch instead: just >>> avoid setting swapbacked when it is already set, that is good enough. >>> (Swapbacked is normally stable once set: lazyfree can undo it, but >>> only later, when found anon in a page table.) >>> >>> This fixes a lot of instability under compaction and swapping loads: >>> assorted "Bad page"s, VM_BUG_ON_FOLIO()s, apparently even page double >>> frees - though I've not worked out what races could lead to the latter. >>> >>> Signed-off-by: Hugh Dickins >>> --- >>> mm/rmap.c | 4 +++- >>> 1 file changed, 3 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index df1a43295c85..5394c1178bf1 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1408,7 +1408,9 @@ void folio_add_new_anon_rmap(struct folio *folio, >>> struct vm_area_struct *vma, >>> VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio); >>> VM_BUG_ON_VMA(address < vma->vm_start || >>> address + (nr << PAGE_SHIFT) > vma->vm_end, vma); >>> - __folio_set_swapbacked(folio); >>> + >>> + if (!folio_test_swapbacked(folio)) >>> + __folio_set_swapbacked(folio); >>> __folio_set_anon(folio, vma, address, exclusive); >>> >>> if (likely(!folio_test_large(folio))) { >> >> LGTM. >> >> I'll point out that it's sufficient for a PFN walker to do a tryget + trylock >> to cause trouble. > > That surprises me. I thought a racer's tryget was irrelevant (touching > a different field) and its trylock not a problem, since "we" hold the > folio lock throughout. If my mental model is too naive there, please > explain in more detail: we all need to understand this better. Sorry, I was imprecise. tryget+trylock should indeed not be a problem, tryget+lock would be, because IIRC folio_wait_bit_common()->folio_set_waiters() would be messing with folio flags. -- Cheers, David / dhildenb