Re: [PATCH v2] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count

David Hildenbrand
2025-09-11T08:17:33Z

On 10.09.25 21:27, 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] <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.
>
> Fixes: 59d9094df3d79 ("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>
> ---
> mm/hugetlb.c | 13 ++++---------
> 1 file changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 753f99b4c718..8ca5b4f7805f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5594,18 +5594,13 @@ 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.
> - */

I think you ignored my question to v1 regarding the change of comment.

--
Cheers

David / dhildenb