Re: [PATCH] mm/hugetlb: init tails before init_migratetype

[Kiryl Shutsemau <kas@kernel.org>]

On Mon, Jun 22, 2026 at 12:19:01PM +0200, Michal Clapinski wrote:
> Currently, if you enable HVO, DEFERRED_STRUCT_PAGE_INIT and VM_DEBUG
> the kernel will crash with the following stack trace
> 
> get_pfnblock_bitmap_bitidx
> __set_pfnblock_flags_mask
> hugetlb_bootmem_init_migratetype
> prep_and_add_bootmem_folios
> gather_bootmem_prealloc_node
> gather_bootmem_prealloc_parallel
> padata_do_multithreaded
> gather_bootmem_prealloc
> hugetlb_init
> 
> on this code
> 
> VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
> 
> This code looks inside the struct page which will be uninitialized
> for hugetlb tail pages, which will cause a false positive.
> 
> So let's initialize the tail pages before this happens.
> 
> Fixes: 622026e87c40 ("mm/hugetlb: remove fake head pages")
> Signed-off-by: Michal Clapinski <mclapinski@google.com>

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Tested-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

One nit below.

> ---
> I think this one check is the only thing that fails, so I can probably
> just remove it instead.

Please don't. It caught a legitimate issue.

> ---
>  mm/hugetlb.c         |  1 +
>  mm/hugetlb_vmemmap.c | 14 +++++++++-----
>  mm/hugetlb_vmemmap.h |  5 +++++
>  3 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 571212b80835..de5e7c104c10 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4127,6 +4127,7 @@ static int __init hugetlb_init(void)
>  	}
>  
>  	hugetlb_init_hstates();
> +	hugetlb_vmemmap_init_tails();
>  	gather_bootmem_prealloc();
>  	report_hugepages();
>  
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 133b46dfb09f..cf485c101926 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -867,14 +867,10 @@ static const struct ctl_table hugetlb_vmemmap_sysctls[] = {
>  	},
>  };
>  
> -static int __init hugetlb_vmemmap_init(void)
> +void __init hugetlb_vmemmap_init_tails(void)

While you're touching this, please also fix up the now-stale comment in
vmemmap_get_tail() in mm/sparse-vmemmap.c, which still points at the
old function.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov