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[2003:cb:c73c:8100:600:a1e5:da94:a13a]) by smtp.gmail.com with ESMTPSA id g11-20020a05600c4ecb00b00412e293bee9sm6357567wmq.38.2024.03.05.01.10.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 05 Mar 2024 01:10:09 -0800 (PST) Message-ID: Date: Tue, 5 Mar 2024 10:10:08 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/5] Remove some races around folio_test_hugetlb To: "Matthew Wilcox (Oracle)" , linux-mm@kvack.org Cc: Oscar Salvador References: <20240301214712.2853147-1-willy@infradead.org> 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: <20240301214712.2853147-1-willy@infradead.org> 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-Stat-Signature: nrmd8dkxgm319b8ak8a1ghy7u5kcxxdn X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 9CBA640006 X-Rspam-User: X-HE-Tag: 1709629814-257243 X-HE-Meta: U2FsdGVkX19FGIAoPjPhVkNLTCffZZCHEwNhBuJQPMRZJzYXFa6+5K1eU0hLIoh9gz+OJCL3jKlqUM8U2dGhgs8nbbVTq9MGVtsgQ500gLgNdx3wPd8DHgtymg8CtHyz9WY4merlAYgOanWnCgCNQ/jMImntnGHWWJPnMLn8PCz67Pux5s7g1rTVwbldUYScf0leTNcjhiPTSdB6VbPXAzyaDeMKLt0GSLiCrkxeIZbAzqNMcD+z6KfAcAhobT15TemF7SBhCZZKxSBrVnDxQxpIEV8qsd+yuFQ66dInCpRswh1uh1m2uA4CHqscEYAfzCzqKFVVmDb+46GP8VCfu3hiLXLBbujo6rN1+eyofxrezQ+XzSSBl7lY1IWgp0qX3NLddz8Oxlb9DDfijZwEaS4IwcP4i74M7I6noSz9bleoNGw4lh5iQ7HLJynH8LCM6/FKPezwtE0O9MP7Y9PZxgfaUScwbjZNyKZh5ZT4virg31+mIOEQGGJZ67+CcFQyJOJX9W8SYbZjOA9iGXlONRfs9k9DCcRtnbvRhaC63Bk2RDDWiwEKbKcrOyHTX8mqZlgvdi/+Uf8W1+AxRNFsjyeheGTNt3bOhqC/Bl2RgPWAMFxQXiEt5CpAAJYYIXIguIE+FbjOY2TUwuNRw9SYyK13ZsfYRZXriRVgR3W2XizGxuxjKN5q6Ldue9eSzVuKfnx+Z5GYm21oyx+vND5BFqYwnIbDtV3ogeq0LlVEseTEr/AlvpBPvvIsjndHae0IngjF50JsZFP5t2hIcJ9XJkmilIcbk9YMTHGSLB4Bm4S4y2Anu6BYUuHxmxFl0NDBZBUtRItirT9kpujTl0O+0H1fV/hV2mFpKDLA6WhDBq01MAM0u+WAFdN+DSZIAMNU0FLmOVg6NvHMIG2xy1Gp9u2rOM22mdQ/LY9MMoHKT+1WW421ngBtv2f4TGKFCfaQUqvKZx3n0Qukhkk8VGZ 2kjX5w8t F8Z+syoq/8kNVmXJKt6TaOSnDLAjOgkXbuuz8mozgD58Pl+88WUOi2ptnnRc1TNXjoxuQ99nJNKwF8fzeSvjmAT86ndDTebGeFrYnGunTRiyjlqmPOGADVxXdNppruVgIEV/DDGW6wBUU66/4FnO/va5u4TZFxnnT2sR1bshkNi03Td3BJv1Lje0+WB5Lj1lsNy0/rlJPI1hBuf1JrCccTX3gcA== 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 01.03.24 22:47, Matthew Wilcox (Oracle) wrote: > Oscar and I have been exchanging a bit of email recently about the > bug reported here: > https://lore.kernel.org/all/ZXNhGsX32y19a2Xv@casper.infradead.org > > I've come to the conclusion that folio_test_hugetlb() is just too fragile > as it can give both false positives and false negatives, as well as > resulting in the above bug. With this patch series, it becomes a lot > more robust. In the memory-failure case, we always hold the hugetlb_lock > so it's perfectly reliable. In the compaction caase, it's unreliable, but > the failures are acceptable and we recheck after taking the hugetlb_lock. > > The cost of this reliability is that we now consume the word I recently > freed in folio->page[1]. I think this is acceptable; we've still gained > a completely reliable folio_test_hugetlb() (which we didn't have before > I started messing around with the folio dtors). Non-hugetlb users > can use large_id as a pointer to something else entirely, or even as a > non-pointer, as long as they can guarantee it can't conflict (ie don't > use it as a bitfield). That probably means that we have to always set the lowest bit to use it for something else, or use another bit. I was wondering if a) We could move that to another subpage. In hugetlb folios we have plenty of space for such things. I guess we'd have be able to detect the folio size without holding a reference, to make sure we can touch another subpage. b) We could overload _nr_pages_mapped. We'd effectively have to steal one bit from _nr_pages_mapped to make this work. Maybe what works is using the existing mechanism (hugetlb flag), and then storing the pointer in __nr_pages_mapped. So depending on the hugetlb flag, we can interpret __nr_pages_mapped either as the pointer or as the old variant. Mostly only folio_large_is_mapped() would need care for now, to ignore _nr_pages_mapped if the hugetlb flag is set. -- Cheers, David / dhildenb