Re: [PATCH 5.10 420/491] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count

Ben Hutchings
2026-04-13T20:37:57Z

[-- Attachment #1: Type: text/plain, Size: 4108 bytes --]

On Mon, 2026-04-13 at 18:01 +0200, Greg Kroah-Hartman wrote:
> 5.10-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Jane Chu <jane.chu@oracle.com>

This one was commit 14967a9c7d247841b0312c48dcf8cd29e55a4cc8 upstream.

Ben.

> 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>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
> mm/hugetlb.c | 13 ++++---------
> 1 file changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a2cab8f2190f8..8fa34032bc173 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3865,16 +3865,11 @@ 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);

--
Ben Hutchings
When in doubt, use brute force. - Ken Thompson

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]