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 7D55538DDB for ; Tue, 16 Sep 2025 03:08:17 +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=1757992098; cv=none; b=XZleqWeS7CAEkvib1EerQZscfYUm4YXVovhsj3C8+Pxj7ZkOdcslOt+rMu8aIBALdijA+y9qQpIxPlQdL0iNbTyNiioECf1k/HMv2A2rWe4EF/jQjqPX08QSsgfvYdRb+HExZKNeuWXkU7iKh/aoN7zBmzfYZBZ+uQjCeQT43K8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757992098; c=relaxed/simple; bh=SzU9zd6gKPgO0BdktrwqDgUBQcmLllVJGEvbli/LrUs=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=oFsHpRTrDaeC4/OqprENiTroTWfMDEvsS4SE4Z0qgaVoBOobenbIO0xX4GPe10maGp6Ydb2NPAm3SHUOgcL7nIb28Vzru1eHBW9a55P3iLWPmcOhrnlJFP0adVjFrvEEWqu2qzjuJnwKzDaSe1TwiKHrplNfdxdoMV+V9AoC54Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=N8A+NpMj; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="N8A+NpMj" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 90EC9C4CEF1; Tue, 16 Sep 2025 03:08:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1757992096; bh=SzU9zd6gKPgO0BdktrwqDgUBQcmLllVJGEvbli/LrUs=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=N8A+NpMjB+OO2qFkCikHUYaXyurnUzhTBVaKoQWOUESt4QplPSf6y5gNUrLZdTnzH NJHtWZhaZ92UXB7/08B/2cODdTaYBEELsRs1Okg4QdAKEJQVeQ3prjctU0f7uZBUuA xZZ6jGQOsShojE+eX88M6C+jgKtO7J0y2OQceE3o= Date: Mon, 15 Sep 2025 20:08:15 -0700 From: Andrew Morton To: Jane Chu Cc: david@redhat.com, harry.yoo@oracle.com, osalvador@suse.de, liushixin2@huawei.com, muchun.song@linux.dev, jannh@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count Message-Id: <20250915200815.e2b844e0a3291fa994d333b4@linux-foundation.org> In-Reply-To: <20250916004520.1604530-1-jane.chu@oracle.com> References: <20250916004520.1604530-1-jane.chu@oracle.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 15 Sep 2025 18:45:20 -0600 Jane Chu wrote: > commit 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count") > introduced ->pt_share_count dedicated to hugetlb PMD share count tracking, > but omitted fixing copy_hugetlb_page_range(), leaving the function relying on > page_count() for tracking that no longer works. > > When lazy page table copy for hugetlb is disabled, that is, revert > commit bcd51a3c679d ("hugetlb: lazy page table copies in fork()") > fork()'ing with hugetlb PMD sharing quickly lockup - > > [ 239.446559] watchdog: BUG: soft lockup - CPU#75 stuck for 27s! > [ 239.446611] RIP: 0010:native_queued_spin_lock_slowpath+0x7e/0x2e0 > [ 239.446631] Call Trace: > [ 239.446633] > [ 239.446636] _raw_spin_lock+0x3f/0x60 > [ 239.446639] copy_hugetlb_page_range+0x258/0xb50 > [ 239.446645] copy_page_range+0x22b/0x2c0 > [ 239.446651] dup_mmap+0x3e2/0x770 > [ 239.446654] dup_mm.constprop.0+0x5e/0x230 > [ 239.446657] copy_process+0xd17/0x1760 > [ 239.446660] kernel_clone+0xc0/0x3e0 > [ 239.446661] __do_sys_clone+0x65/0xa0 > [ 239.446664] do_syscall_64+0x82/0x930 > [ 239.446668] ? count_memcg_events+0xd2/0x190 > [ 239.446671] ? syscall_trace_enter+0x14e/0x1f0 > [ 239.446676] ? syscall_exit_work+0x118/0x150 > [ 239.446677] ? arch_exit_to_user_mode_prepare.constprop.0+0x9/0xb0 > [ 239.446681] ? clear_bhb_loop+0x30/0x80 > [ 239.446684] ? clear_bhb_loop+0x30/0x80 > [ 239.446686] entry_SYSCALL_64_after_hwframe+0x76/0x7e > > There are two options to resolve the potential latent issue: > 1. warn against PMD sharing in copy_hugetlb_page_range(), > 2. fix it. > This patch opts for the second option. > While at it, simplify the comment, the details are not actually relevant > anymore. Thanks. I dropped the v2 patch from mm-hotfixes-stable and added this. > Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count") > Signed-off-by: Jane Chu > Reviewed-by: Harry Yoo > --- > include/linux/mm_types.h | 5 +++++ > mm/hugetlb.c | 15 +++++---------- > 2 files changed, 10 insertions(+), 10 deletions(-) > It's conventional (and useful) to explain (beneath the "---") what changed since the previous version. Here's the v2->v3 diff, which appears to be based on David's review comments: --- a/include/linux/mm_types.h~mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count +++ a/include/linux/mm_types.h @@ -631,6 +631,11 @@ static inline int ptdesc_pmd_pts_count(s { return atomic_read(&ptdesc->pt_share_count); } + +static inline bool ptdesc_pmd_is_shared(struct ptdesc *ptdesc) +{ + return !!ptdesc_pmd_pts_count(ptdesc); +} #else static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc) { --- a/mm/hugetlb.c~mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count +++ a/mm/hugetlb.c @@ -5595,8 +5595,8 @@ int copy_hugetlb_page_range(struct mm_st } #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING - /* If the pagetables are shared don't copy or take references. */ - if (ptdesc_pmd_pts_count(virt_to_ptdesc(dst_pte)) > 0) { + /* If the pagetables are shared, there is nothing to do */ + if (ptdesc_pmd_is_shared(virt_to_ptdesc(dst_pte))) { addr |= last_addr_mask; continue; } @@ -7597,7 +7597,7 @@ int huge_pmd_unshare(struct mm_struct *m hugetlb_vma_assert_locked(vma); if (sz != PMD_SIZE) return 0; - if (!ptdesc_pmd_pts_count(virt_to_ptdesc(ptep))) + if (!ptdesc_pmd_is_shared(virt_to_ptdesc(ptep))) return 0; pud_clear(pud); _