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b=c39XhitkGKdQ/yXBH5DDEEOWKDiqpWzdQvRaLfGE0PbANlGxw3t6EWIwyiiGPY6/a 5iONMk1hgoZFGooMXpm2iSxvYrW0gbGPkpbdkYU5fqNalHrPKmOoWDIZtqT2KEUvm6 98c61JZfXQ9uNRxkzSIwvdIuqK/HkIE77N6mNBd7f92BX1xqAneUSTjBz+FDf0wj6Q RhCqBYOIinjgtNh8N7msgGmQse1xAwbAG+hKZng0Iz6lUaSRLIIwhQ7Krg4q1roAH2 OOuL/d6430Qpme+7Mrcvis+5vRvFxhm4MwV+lY/wRDF9JcEB7ZEcEoExcOe4aMl3sz kZqp+4Vb1nOiQ== Message-ID: <8f509b48-db3e-44ff-8423-1e43a35daa01@kernel.org> Date: Wed, 30 Sep 2026 12:03:36 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hugetlb: avoid recursive i_mmap_rwsem in PMD sharing To: Li Zhe , akpm@linux-foundation.org, muchun.song@linux.dev, osalvador@suse.de Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Oscar Salvador , "Lorenzo Stoakes (Arm)" References: <20260930064308.58159-1-lizhe.67@bytedance.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260930064308.58159-1-lizhe.67@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: a4okrz9x8k4wyu93w6sm8owhtdoqcp8i X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: F23B780008 X-HE-Tag: 1790762621-811196 X-HE-Meta: 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 aFQqG99t V2ktXVRjEJu/8YnZ9NvY73rrckUzukicdJbaSvhGTBa41GouWYQTdedK/CF8YaBMrnGZGX2nvIx9+iJxjhmm/6Dzj6TKQVwjeuYzyymQWtQUeSzgbVkOyqogRWasd4udMGG9jVeJuWxWKXcLmR6FuCfEXSaEajtooss+RxyTTv6JAfxeVZy7fQWPsMTyf6YOHmFKrVFGI/RIj9be0K6NCH3grMmlDnOVMHFiu4pGiIc92RMMOHu0FzxwrnGa4hG8g1EGODJmtOlwDkG5fJPm+gOLtwX0glk1TR7nOrd7cjiNBTXRtdKXaftjpVlsRdCIhxIYz9Vg+xoi2+mA= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/30/26 08:43, Li Zhe wrote: > huge_pmd_share() always takes i_mmap_rwsem for read before looking for a > shareable PMD page table. > > That is unsafe for callers that already hold the same mapping lock for > write. In particular, move_hugetlb_page_tables() takes i_mmap_rwsem for > write to prevent truncation races, then calls huge_pte_alloc() for the > destination address. If the destination PUD is empty and PMD sharing is > possible, huge_pte_alloc() can call huge_pmd_share(), which then tries to > take the same rwsem for read and can deadlock on itself. > > PMD sharing is only an optimization. If the read side of i_mmap_rwsem > cannot be acquired immediately, fall back to allocating a private PMD > table instead of blocking in the sharing path. > > Signed-off-by: Li Zhe > --- > mm/hugetlb.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index cea25773a6c95..75631101a6807 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -6995,7 +6995,15 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma, > pte_t *spte = NULL; > pte_t *pte; > > - i_mmap_lock_read(mapping); > + /* > + * Some callers already hold i_mmap_rwsem for write, for example > + * move_hugetlb_page_tables(). PMD sharing is only an optimization, so > + * fall back to a private PMD table instead of blocking on the same > + * rwsem in read mode. > + */ "just an optimization" is not entirely true. There are workloads that depend on huge pmd sharing to work. So I don't think this is acceptable. There was a private discussion on this issue recently (for some weird reason the deadlock was reported as a security issue), 2 months ago I think. Oscar, I recall that you wanted to send a proper patch? -- Cheers, David