The patch titled Subject: mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Jane Chu jane.chu@oracle.com Subject: mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count Date: Tue, 9 Sep 2025 12:43:57 -0600

commit 59d9094df3d79 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 (commit bcd51a3c679d), fork()'ing with hugetlb PMD sharing quickly locks up -

[ 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] <TASK> [ 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. remove the PMD sharing awareness from copy_hugetlb_page_range(), 2. fix it. This patch opts for the second option.

Link: https://lkml.kernel.org/r/20250909184357.569259-1-jane.chu@oracle.com Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count") Signed-off-by: Jane Chu jane.chu@oracle.com Cc: Cc: David Hildenbrand david@redhat.com Cc: Jane Chu jane.chu@oracle.com Cc: Liu Shixin liushixin2@huawei.com Cc: Muchun Song muchun.song@linux.dev Cc: Oscar Salvador osalvador@suse.de Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/hugetlb.c | 13 ++++--------- 1 file changed, 4 insertions(+), 9 deletions(-)

--- a/mm/hugetlb.c~mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count +++ a/mm/hugetlb.c @@ -5594,18 +5594,13 @@ int copy_hugetlb_page_range(struct mm_st 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_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) { addr |= last_addr_mask; continue; } +#endif

dst_ptl = huge_pte_lock(h, dst, dst_pte); src_ptl = huge_pte_lockptr(h, src, src_pte); _

Patches currently in -mm which might be from jane.chu@oracle.com are

mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count.patch