From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1D03A39A041 for ; Wed, 13 May 2026 10:15:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778667328; cv=none; b=n7bzsnqJL3FhxVqC1rWij0N9g9fK2+VT9Wm+zrgttZiI5qPB7zhFtAIDJHnYTqnBQ6AQ50MqFvXZ7F6ysD1YletUltuhwNo5dmEqmVS+CNrPEPwtPtL/SgS/CWLg77LeZpSR8U4RlwWLHQfdEmnxFhZ1pes9Yowfd4yXlY8MepY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778667328; c=relaxed/simple; bh=q2O5EU1M9LAwDFxvC1kACL2BPQVBEFSmu2p0v2BeLTU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bRR/E/9HX3rSzmyl2UYqka9D1QuZP63JS12KBP7bEJyM0+rl1Bd0R/6Bv66QQYhmY2v/mCfgDVKe0QMltaOiMFyzgR+JKatl5SqGv2/EfFi8pMdYYNbFFpkVS5N7OB6Po2/EDzjV9MRlza+VppsTAuHTz0xIXorjU83pqLZ5si4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SuIPOCoI; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SuIPOCoI" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AC99AC2BCB7; Wed, 13 May 2026 10:15:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778667327; bh=q2O5EU1M9LAwDFxvC1kACL2BPQVBEFSmu2p0v2BeLTU=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 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 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; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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