linux-kernel - Re: [PATCH] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share

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Message-ID: <e9b34151-0879-4900-af9f-2ce0dbb678a6@redhat.com>
Date: Wed, 10 Sep 2025 08:45:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Jane Chu <jane.chu@...cle.com>, harry.yoo@...cle.com, osalvador@...e.de,
 liushixin2@...wei.com, muchun.song@...ux.dev, akpm@...ux-foundation.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: fix copy_hugetlb_page_range() to use
 ->pt_share_count

On 09.09.25 20:43, Jane Chu wrote:
> 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 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. remove the PMD sharing awareness from 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@...cle.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.
> -		 */
> -		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. */

Why remove so much of the original comment?

> +		if (ptdesc_pmd_pts_count(virt_to_ptdesc(dst_pte)) > 0) {
>   			addr |= last_addr_mask;
>   			continue;
>   		}

LGTM, thanks!

-- 
Cheers

David / dhildenb