linux-kernel - Re: [PATCH v3] mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share

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Message-ID: <377db013-4850-4501-a810-552e2f821dd9@redhat.com>
Date: Wed, 17 Sep 2025 09:40:52 +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,
 jannh@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/hugetlb: fix copy_hugetlb_page_range() to use
 ->pt_share_count

On 16.09.25 02:45, 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.
> While at it, simplify the comment, the details are not actually relevant
> anymore.
> 
> Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared count")
> Signed-off-by: Jane Chu <jane.chu@...cle.com>
> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> ---
>   include/linux/mm_types.h |  5 +++++
>   mm/hugetlb.c             | 15 +++++----------
>   2 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 08bc2442db93..a643fae8a349 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -631,6 +631,11 @@ static inline int ptdesc_pmd_pts_count(struct ptdesc *ptdesc)
>   {
>   	return atomic_read(&ptdesc->pt_share_count);
>   }
> +
> +static inline bool ptdesc_pmd_is_shared(struct ptdesc *ptdesc)
> +{
> +	return !!ptdesc_pmd_pts_count(ptdesc);

No need for the !!, the compiler will do that automatically.

LGTM

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb