From 59cd6535e7ed89e6f27a06d4849169e6c2be4436 Mon Sep 17 00:00:00 2001 From: Jane Chu Date: Mon, 15 Sep 2025 18:45:20 -0600 Subject: [PATCH] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count commit 14967a9c7d247841b0312c48dcf8cd29e55a4cc8 upstream. 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. Link: https://lkml.kernel.org/r/20250916004520.1604530-1-jane.chu@oracle.com Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count") Signed-off-by: Jane Chu Reviewed-by: Harry Yoo Acked-by: Oscar Salvador Acked-by: David Hildenbrand Cc: Jann Horn Cc: Liu Shixin Cc: Muchun Song Signed-off-by: Andrew Morton [ David: We don't have ptdesc and the wrappers, so work directly on the page->pt_share_count. CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING is still called CONFIG_ARCH_WANT_HUGE_PMD_SHARE. ] Signed-off-by: David Hildenbrand (Arm) Signed-off-by: Greg Kroah-Hartman --- mm/hugetlb.c | 13 ++++--------- 1 file changed, 4 insertions(+), 9 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index c0719ece2b7f..8268943e05eb 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -4341,16 +4341,11 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, break; } - /* - * If the pagetables are shared don't copy or take references. - * - * dst_pte == src_pte is the common case of src/dest sharing. - * However, src could have 'unshared' and dst shares with - * another vma. So page_count of ptep page is checked instead - * to reliably determine whether pte is shared. - */ - if (page_count(virt_to_page(dst_pte)) > 1) +#ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE + /* If the pagetables are shared, there is nothing to do */ + if (atomic_read(&virt_to_page(dst_pte)->pt_share_count)) continue; +#endif dst_ptl = huge_pte_lock(h, dst, dst_pte); src_ptl = huge_pte_lockptr(h, src, src_pte);