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[2003:d8:2f0d:f000:eec9:2b8d:4913:f32a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4f00972f3sm18884082f8f.69.2025.06.03.08.08.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Jun 2025 08:08:56 -0700 (PDT) Message-ID: Date: Tue, 3 Jun 2025 17:08:55 +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: Peter Xu , Oscar Salvador 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: EiojQjmFP1yYqYGtKNyn0UNXZjqG6Wz7fMHAYQ97BXE_1748963337 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: 432BD180009 X-Stat-Signature: gx6jorxqn4axzmfdqm6bm48dcgcxup1a X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1748963342-170470 X-HE-Meta: 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 4p9IBQJI XxaqeHSFEPWBHQjMTNeCc49XBeizv3qeiVRHILU4vSBxX+XNfGBYqJxr7c5mI60D/Czct3qvprOQGklun+EWi7vPLTYMNDijMEN1kigDM+Pi7Hoy4JlvkvpNbD0zRp8xfHDLoYaIwSkNLE84B+60mg6l0Aj+fueCCgcX7Dgxlj7wPMJayxV1zuvpgPsPvXJTqUNgCWUgmY1WenP1+E7P7oC2SAGSO4cKQMJWyaOko4TVTomrbynCnZ9hTSzgaqvQXF097InrOWASe3YMz5JVluRYiCo0r0KjMDxd+OaMAKnF7Wq34PPyDe8dYxYIwL6fbMf7Roime0Zphgo79NMsCn9+DYwKjZQ6Qw6Sik8VYCewYdHnHtfWJqoP5MaRckuswf/lbxrMLu9hTw9zQuQoNO5431DI3wW4Z3spu7ZFS4XwM3MaLCna6MmmbO09tallcbsOt 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 16:57, Peter Xu wrote: > On Tue, Jun 03, 2025 at 03:50:54PM +0200, 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. > > There're actually three places this patch touched, the 3rd one is > hugetlb_no_page(), in which case I also think we should lock it, not only > because file folios normally does it (see do_fault(), for example), but > also that's exactly what James mentioned I believe on possible race of > !uptodate hugetlb folio being injected by UFFDIO_CONTINUE, along the lines: > > folio = alloc_hugetlb_folio(vma, vmf->address, false); > ... > folio_zero_user(folio, vmf->real_address); > __folio_mark_uptodate(folio); > >> 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 > > Yes this is a good point; for CoW path alone maybe we don't need to lock > old_folio. > >> 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. > > IMHO, do_wp_page() took the folio lock not for folio_move_anon_rmap(), but > for checking swapcache/ksm stuff which needs to be serialized with folio > lock. > > So I'm not 100% confident on the folio_move_anon_rmap(), but I _think_ it > deserves a data_race() and IIUC it only work not because of the folio lock, > but because of how anon_vma is managed as a tree as of now, so that as long > as WRITE_ONCE() even a race is benign (because the rmap walker will either > see a complete old anon_vma that includes the parent process's anon_vma, or > the child's). What really protects the anon_vma should really be anon_vma > lock.. That can definitely be a separate topic. I'm not sure whether you'd > like to dig this part out, but if you do I'd also be more than happy to > know whether my understanding needs correction here.. :) > > In general, I still agree with you that if hugetlb CoW path can look closer > to do_wp_page then it's great. As stated elsewhere, the mapcount check + folio_move_anon_rmap need the folio lock. -- Cheers, David / dhildenb