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[2003:cb:c707:5c00:e650:bcd7:e2a0:54fe]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4282b8adaadsm63082695e9.12.2024.08.01.08.35.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 01 Aug 2024 08:35:21 -0700 (PDT) Message-ID: <934885c5-512b-41bf-8501-b568ece34e18@redhat.com> Date: Thu, 1 Aug 2024 17:35:20 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/hugetlb: fix hugetlb vs. core-mm PT locking To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, James Houghton , stable@vger.kernel.org, Oscar Salvador , Muchun Song , Baolin Wang References: <20240731122103.382509-1-david@redhat.com> <541f6c23-77ad-4d46-a8ed-fb18c9b635b3@redhat.com> From: David Hildenbrand Content-Language: en-US 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi Peter, >> - if (huge_page_size(h) == PMD_SIZE) >> + unsigned long size = huge_page_size(h); >> + >> + VM_WARN_ON(size == PAGE_SIZE); >> + >> + /* >> + * hugetlb must use the exact same PT locks as core-mm page table >> + * walkers would. When modifying a PTE table, hugetlb must take the >> + * PTE PT lock, when modifying a PMD table, hugetlb must take the PMD >> + * PT lock etc. >> + * >> + * The expectation is that any hugetlb folio smaller than a PMD is >> + * always mapped into a single PTE table and that any hugetlb folio >> + * smaller than a PUD (but at least as big as a PMD) is always mapped >> + * into a single PMD table. >> + * >> + * If that does not hold for an architecture, then that architecture >> + * must disable split PT locks such that all *_lockptr() functions >> + * will give us the same result: the per-MM PT lock. >> + * >> + * Note that with e.g., CONFIG_PGTABLE_LEVELS=2 where >> + * PGDIR_SIZE==P4D_SIZE==PUD_SIZE==PMD_SIZE, we'd use the MM PT lock >> + * directly with a PMD hugetlb size, whereby core-mm would call >> + * pmd_lockptr() instead. However, in such configurations split PMD >> + * locks are disabled -- split locks don't make sense on a single >> + * PGDIR page table -- and the end result is the same. >> + */ >> + if (size >= P4D_SIZE) >> + return &mm->page_table_lock; > > I'd drop this so the mm lock fallback will be done below (especially in > reality the pud lock is always mm lock for now..). Also this line reads > like there can be P4D size huge page but in reality PUD is the largest > (nopxx doesn't count). We also same some cycles in most cases if removed. The compiler should be smart enough to optimize most of that out. But I agree that there is no need to be that future-proof here. These two are interesting: arch/powerpc/mm/hugetlbpage.c: if (!mm_pud_folded(mm) && sz >= P4D_SIZE) arch/riscv/mm/hugetlbpage.c: else if (sz >= P4D_SIZE) But I assume they are only fordward-looking. (GUP would already be pretty broken if that would exist) >> + else if (size >= PUD_SIZE) >> + return pud_lockptr(mm, (pud_t *) pte); >> + else if (size >= PMD_SIZE || IS_ENABLED(CONFIG_HIGHPTE)) > > I thought this HIGHPTE can also be dropped? Because in HIGHPTE it should > never have lower-than-PMD huge pages or we're in trouble. That's why I > kept one WARN_ON() in my pesudo code but only before trying to take the pte > lockptr. Then the compiler won't optimize out the ptep_lockptr() call and we'll run into a build error. And I think the HIGHPTE builderror serves good purpose. In file included from : In function 'ptep_lockptr', inlined from 'huge_pte_lockptr' at ./include/linux/hugetlb.h:974:9, inlined from 'huge_pte_lock' at ./include/linux/hugetlb.h:1248:8, inlined from 'pagemap_scan_hugetlb_entry' at fs/proc/task_mmu.c:2581:8: ././include/linux/compiler_types.h:510:45: error: call to '__compiletime_assert_256' declared with attribute error: BUILD_BUG_ON failed: IS_ENABLED(CONFIG_HIGHPTE) 510 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__) | ^ ././include/linux/compiler_types.h:491:25: note: in definition of macro '__compiletime_assert' 491 | prefix ## suffix(); \ | ^~~~~~ ././include/linux/compiler_types.h:510:9: note: in expansion of macro '_compiletime_assert' 510 | _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__) | ^~~~~~~~~~~~~~~~~~~ ./include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert' 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg) | ^~~~~~~~~~~~~~~~~~ ./include/linux/build_bug.h:50:9: note: in expansion of macro 'BUILD_BUG_ON_MSG' 50 | BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition) | ^~~~~~~~~~~~~~~~ ./include/linux/mm.h:2874:9: note: in expansion of macro 'BUILD_BUG_ON' 2874 | BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHPTE)); It would be even better to have a way to know whether an arch even has hugetlb < PMD_SIZE (config option?) and use that instead here; but I'll leave that as an optimization. -- Cheers, David / dhildenb