[PATCH 5.15.y 2/6] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count

David Hildenbrand (Arm)
2026-02-18T11:01:46Z

From: Jane Chu <jane.chu@oracle.com>

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] <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. 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 <jane.chu@oracle.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 14967a9c7d247841b0312c48dcf8cd29e55a4cc8)
[ 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) <david@kernel.org>
---
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);
--
2.43.0