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b=SuIPOCoIGr7Ez0zRklgTOxZeEf5a4KuTjvQEHwRuH7qe579n73wzkYJVUkyZuLTcC PYUOvVhj4dTRXFqR3j/B6y282fRhKCg9lZdOL0IerEk49jJvwjE1RkwIEKM7BlaiSz 8FvBzs3Qsrlsr+R03e8dSeskq82c3yVH6cfpEPoaEiccOKTISiOx9qmmwcKo3SzQgu KonCtkImdVpAprhnP026Lbvhbdgyz5T4CLhiR/zxCfWHIfN677n4Ejb7ubuHEqG6/k DW+txvCTPFmkzu5vKxJUyo5YmL87aBcRgXbfBZoUg8BD9lkP6AaXN2LH6IhCTR0cEB 3Ff/+S9BSJ6Kw== Message-ID: <291cd4df-7c52-426e-a8cc-b0cf77654c52@kernel.org> Date: Wed, 13 May 2026 12:15:23 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-hotfixes] mm/hugetlb: avoid false positive lockdep assertion To: Lorenzo Stoakes , Andrew Morton Cc: Muchun Song , Oscar Salvador , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260513085658.45264-1-ljs@kernel.org> 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: <20260513085658.45264-1-ljs@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: BFE46C0005 X-Rspam-User: X-Stat-Signature: arjn3o3sgzfres3mket1wx4zggo54k7s X-HE-Tag: 1778667328-392656 X-HE-Meta: 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 uKVA9lcu KthzLN85w0XRcANJPpiu0wRJZSXs3Q0ogg28HCXM0T9ZN/oW/23m+4eQOJRmG5Rq9A3mm45Icp2suQ9zTYp1iXmLv186III1TED6rgSblRHqqTZlPnQNKAXjrf7I/kfzTIYaK/wRyAMgyUTmZAcj+z367hRhIxPbiio93SDXQu13ZGo0F1qIw/0+Xo0QrvLxTLHgd6xMz4iO1zOBAg9hGIh1sKablzhwig+fpF5n8dIdqCo54Mu2J53p6fHQ58CuktwjxobWeqSFHIWuU6c4sdrZCBaPUN8KC6RYL63d+Gltp/x2orP4+IGrCig== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/13/26 10:56, Lorenzo Stoakes wrote: > Commit 081056dc00a2 ("mm/hugetlb: unshare page tables during VMA split, not > before") changed the locking model around hugetlbfs PMD unsharing on VMA > split, but did not update the function which asserts the locks, > hugetlb_vma_assert_locked(). > > This function asserts that either the hugetlb VMA lock is held (if a shared > mapping) or that the reservation map lock is held (if private). > > If you get an unfortunate race between something which results in one of > these locks being released and a hugetlb split and you have CONFIG_LOCKDEP "hugetlb split": I assume you used that terminology because of hugetlb_split(). Which is all just rather nasty #justhugetlbthings "hugetlb VMA split" is probably easier to get. > enabled, you can therefore see a false positive assertion arise when there > is in fact no issue. > > Since this change introduced a new take_locks parameter to > hugetlb_unshare_pmds(), which, when set to false, indicates that locking is > sufficient, simply pass this to the unsharing logic and predicate the > lock assertions on this. > > This is safe, as we already asserted the file rmap lock and the VMA write > lock prior to this (implying exclusive mmap write lock), so we cannot be > raced by either rmap or page fault page table walkers which the asserted > locks are intended to protect against (we don't mind GUP-fast). > > Separate out huge_pmd_unshare() into __huge_pmd_unshare() to add a > check_locks parameter, and update hugetlb_unshare_pmds() to pass this > parameter to it. > > This leaves all other callers of huge_pmd_unshare() still correctly > asserting the locks. > > The below reproducer will trigger the assert in a kernel with > CONFIG_LOCKDEP enabled by racing process teardown (which will release the > hugetlb lock) against a hugetlb split. > > void execute_one(void) > { > void *ptr; > pid_t pid; > > /* > * Create a hugetlb mapping spanning a PUD entry. > * > * We force the hugetlb page allocation with populate and > * noreserve. > * > * |---------------------| > * | | > * |---------------------| > * 0 PUD boundary > */ > ptr = mmap(0, PUD_SIZE, PROT_READ | PROT_WRITE, > MAP_FIXED | MAP_SHARED | MAP_ANON | > MAP_NORESERVE | MAP_HUGETLB | MAP_POPULATE, > -1, 0); > if (ptr == MAP_FAILED) { > perror("mmap"); > exit(EXIT_FAILURE); > } > > /* > * Fork but with a bogus stack pointer so we try to execute code in > * a non-VM_EXEC VMA, causing segfault + teardown via exit_mmap(). > * > * The clone will cause PMD page table sharing between the > * processes first via: > * copy_process() -> ... -> huge_pte_alloc() -> huge_pmd_share() > * > * Then tear down and release the hugetlb 'VMA' lock via: > * exit_mmap() -> ... -> vma_close() -> hugetlb_vma_lock_free() > */ > pid = syscall(__NR_clone, 0, 2 * PMD_SIZE, 0, 0, 0); > if (pid < 0) { > perror("clone"); > exit(EXIT_FAILURE); > } if (pid == 0) { > /* Pop stack... */ > return; > } > > /* > * We are the parent process. > * > * Race the child process's teardown with a PMD unshare. > * > * We do this by triggering: > * > * __split_vma() -> hugetlb_split() -> hugetlb_unshare_pmds() > * > * Which, importantly, doesn't hold the hugetlb VMA lock (nor can > * it), meaning we assert in hugetlb_vma_assert_locked(). > * > * . > * |----------.----------| > * | . | > * |----------.----------| > * 0 . PUD boundary > */ > mmap(0, PUD_SIZE / 2, PROT_READ | PROT_WRITE, > MAP_FIXED | MAP_ANON | MAP_PRIVATE, -1, 0); > } > > int main(void) > { > int i; > > /* Kick off fork children. */ > for (i = 0; i < NUM_FORKS; i++) { > pid_t pid = fork(); > > if (pid < 0) { > perror("fork"); > exit(EXIT_FAILURE); > } > > /* Fork children do their work and exit. */ > if (!pid) { > int j; > > for (j = 0; j < NUM_ITERS; j++) > execute_one(); > return EXIT_SUCCESS; > } > } > > /* If we succeeded, wait on children. */ > for (i = 0; i < NUM_FORKS; i++) > wait(NULL); > > return EXIT_SUCCESS; > } > > Fixes: 081056dc00a2 ("mm/hugetlb: unshare page tables during VMA split, not before") > Cc: > Signed-off-by: Lorenzo Stoakes > --- LGTM, all rather nasty with "take_locks" parameters ... Acked-by: David Hildenbrand (Arm) -- Cheers, David