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[2003:cb:c713:a600:7ca0:23b3:d48a:97c7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4280ca10aa6sm13416515e9.26.2024.07.26.01.04.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Jul 2024 01:04:19 -0700 (PDT) Message-ID: Date: Fri, 26 Jul 2024 10:04:18 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/2] mm/hugetlb: fix hugetlb vs. core-mm PT locking To: Baolin Wang , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Andrew Morton , Muchun Song , Peter Xu , Oscar Salvador , stable@vger.kernel.org References: <20240725183955.2268884-1-david@redhat.com> <20240725183955.2268884-3-david@redhat.com> <0067dfe6-b9a6-4e98-9eef-7219299bfe58@linux.alibaba.com> Content-Language: en-US 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: <0067dfe6-b9a6-4e98-9eef-7219299bfe58@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 26.07.24 04:33, Baolin Wang wrote: > > > On 2024/7/26 02:39, David Hildenbrand wrote: >> We recently made GUP's common page table walking code to also walk >> hugetlb VMAs without most hugetlb special-casing, preparing for the >> future of having less hugetlb-specific page table walking code in the >> codebase. Turns out that we missed one page table locking detail: page >> table locking for hugetlb folios that are not mapped using a single >> PMD/PUD. >> >> Assume we have hugetlb folio that spans multiple PTEs (e.g., 64 KiB >> hugetlb folios on arm64 with 4 KiB base page size). GUP, as it walks the >> page tables, will perform a pte_offset_map_lock() to grab the PTE table >> lock. >> >> However, hugetlb that concurrently modifies these page tables would >> actually grab the mm->page_table_lock: with USE_SPLIT_PTE_PTLOCKS, the >> locks would differ. Something similar can happen right now with hugetlb >> folios that span multiple PMDs when USE_SPLIT_PMD_PTLOCKS. >> >> Let's make huge_pte_lockptr() effectively uses the same PT locks as any >> core-mm page table walker would. > > Thanks for raising the issue again. I remember fixing this issue 2 years > ago in commit fac35ba763ed ("mm/hugetlb: fix races when looking up a > CONT-PTE/PMD size hugetlb page"), but it seems to be broken again. > Ah, right! We fixed it by rerouting to hugetlb code that we then removed :D Did we have a reproducer back then that would make my live easier? >> There is one ugly case: powerpc 8xx, whereby we have an 8 MiB hugetlb >> folio being mapped using two PTE page tables. While hugetlb wants to take >> the PMD table lock, core-mm would grab the PTE table lock of one of both >> PTE page tables. In such corner cases, we have to make sure that both >> locks match, which is (fortunately!) currently guaranteed for 8xx as it >> does not support SMP. >> >> Fixes: 9cb28da54643 ("mm/gup: handle hugetlb in the generic follow_page_mask code") >> Cc: >> Signed-off-by: David Hildenbrand >> --- >> include/linux/hugetlb.h | 25 ++++++++++++++++++++++--- >> 1 file changed, 22 insertions(+), 3 deletions(-) >> >> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h >> index c9bf68c239a01..da800e56fe590 100644 >> --- a/include/linux/hugetlb.h >> +++ b/include/linux/hugetlb.h >> @@ -944,10 +944,29 @@ static inline bool htlb_allow_alloc_fallback(int reason) >> static inline spinlock_t *huge_pte_lockptr(struct hstate *h, >> struct mm_struct *mm, pte_t *pte) >> { >> - if (huge_page_size(h) == PMD_SIZE) >> + VM_WARN_ON(huge_page_size(h) == PAGE_SIZE); >> + VM_WARN_ON(huge_page_size(h) >= P4D_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. > > ARM64 also supports cont-PMD size hugetlb, which is 32MiB size with a 4 > KiB base page size. This means the PT locks for 32MiB hugetlb may race > again, as we currently only hold one PMD lock for several PMD entries of > a cont-PMD size hugetlb. Exactly, that's the case where all cont-PMD entries fall into the same page table. That's also what I will try reproducing with (migration racing with GUP). But the race window is small and a lot of other stuff is protected by the VMA lock. -- Cheers, David / dhildenb