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[2003:d8:2f0d:f000:eec9:2b8d:4913:f32a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4efe740bbsm18632212f8f.54.2025.06.03.08.12.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Jun 2025 08:12:08 -0700 (PDT) Message-ID: <305bde6b-dd5a-4eb4-afc4-f7ed2b46d5b8@redhat.com> Date: Tue, 3 Jun 2025 17:12:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/3] mm, hugetlb: Clean up locking in hugetlb_fault and hugetlb_wp To: Oscar Salvador , Peter Xu Cc: Andrew Morton , Muchun Song , James Houghton , Gavin Guo , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250602141610.173698-1-osalvador@suse.de> <20250602141610.173698-2-osalvador@suse.de> 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: EKiwhcigJ25xaeXO9StScDp70mueL7dwke4qGzz6fjc_1748963529 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 5278F18000E X-Rspamd-Server: rspam09 X-Stat-Signature: f33fytra9dptqm9nofofksz4fcfc1fa1 X-HE-Tag: 1748963532-244110 X-HE-Meta: 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 Ucq+WkfE sjobhuBUxC94GdjP8z4scu84aWE3xEPznqpkrdT1ZKz3pgcURHk3awly+yslVFsTpglx/mwKAxWSvIf4LlsbAJLDFipxT8xlDYl1dcrHkjxU/kNpjsCpyddFz3IRmyXjK8t3+20wysZr1bTcvJP6W5KCSqlCg+NLH448Lams/s44JMsH8rIIgNQFHYzcJRe4sRCQfsVSgkj03L3EBHtdthw1MC+MOyPztpPqgH0+hKyUKL7pEHvGK0WAmJvHqRv6R6vQvELPJnPAO1GOKmawDnX1FMiTL8w9kTy4rQPtjwFFoiPO7DN52swLuRhU43yziYh7BpYybMMHee4X6A00aBoEZpDiXz+VisuFFEJAeyt3MUwsTbbv+woKBcxW7qloRZ0IdHtWv2jVRbbvFf9FZCzlJIqB5Gsk80QeMnKvHlE7qd0acJ1aHk4kudrPW8YoeIqqq 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 03.06.25 15:50, Oscar Salvador wrote: > On Mon, Jun 02, 2025 at 05:30:19PM -0400, Peter Xu wrote: >> Right, and thanks for the git digging as usual. I would agree hugetlb is >> more challenge than many other modules on git archaeology. :) >> >> Even if I mentioned the invalidate_lock, I don't think I thought deeper >> than that. I just wished whenever possible we still move hugetlb code >> closer to generic code, so if that's the goal we may still want to one day >> have a closer look at whether hugetlb can also use invalidate_lock. Maybe >> it isn't worthwhile at last: invalidate_lock is currently a rwsem, which >> normally at least allows concurrent fault, but that's currently what isn't >> allowed in hugetlb anyway.. >> >> If we start to remove finer grained locks that work will be even harder, >> and removing folio lock in this case in fault path also brings hugetlbfs >> even further from other file systems. That might be slightly against what >> we used to wish to do, which is to make it closer to others. Meanwhile I'm >> also not yet sure the benefit of not taking folio lock all across, e.g. I >> don't expect perf would change at all even if lock is avoided. We may want >> to think about that too when doing so. > > Ok, I have to confess I was not looking things from this perspective, > but when doing so, yes, you are right, we should strive to find > replacements wherever we can for not using hugetlb-specific code. > > I do not know about this case though, not sure what other options do we > have when trying to shut concurrent faults while doing other operation. > But it is something we should definitely look at. > > Wrt. to the lock. > There were two locks, old_folio (taken in hugetlb_fault) and > pagecache_folio one. > The thing was not about worry as how much perf we leave on the table > because of these locks, as I am pretty sure is next to 0, but my drive > was to understand what are protection and why, because as the discussion > showed, none of us really had a good idea about it and it turns out that this > goes back more than ~20 years ago. > > Another topic for the lock (old_folio, so the one we copy from), > when we compare it to generic code, we do not take the lock there. > Looking at do_wp_page(), we do __get__ a reference on the folio we copy > from, but not the lock, so AFAIU, the lock seems only to please > folio_move_anon_rmap() from hugetlb_wp. > > Taking a look at do_wp_page()->wp_can_reuse_anon_folio() which also > calls folio_move_anon_rmap() in case we can re-use the folio, it only > takes the lock before the call to folio_move_anon_rmap(), and then > unlocks it. No. It takes the lock around "folio_ref_count(folio) != 1" as well. IOW, if the ref_count is 1, the mapcount must be <= 1, and as the page *is* mapped, we know the mapcount is >= 1. So if the ref_count == mapcount == 1 and the folio is locked, we cannot have concurrent unmapping/splitting/migration of the folio that could affect the mapcount/refcount. -- Cheers, David / dhildenb