Re: [PATCH v5 04/17] mm/sparse-vmemmap: initialize shared tail vmemmap pages on allocation

[Qi Zheng <qi.zheng@linux.dev>]

On 8/25/26 4:45 PM, Muchun Song wrote:
> The shared tail vmemmap page allocated in vmemmap_get_tail() used to be
> left uninitialized, because memmap_init_range() would later overwrite
> it.  That forced users such as HugeTLB to defer the initialization to
> their own setup paths.
> 
> Now that memmap_init_range() skips shared tail vmemmap pages, initialize
> them immediately in vmemmap_get_tail() with init_compound_tail()
> instead.
> 
> This moves the initialization to the point where the shared tail page is
> allocated and avoids relying on deferred handling in individual users.
> The remaining deferred initialization in HugeTLB will be removed once it
> switches to the section-based vmemmap optimization mechanism.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>   mm/sparse-vmemmap.c | 12 ++----------
>   1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index aa6a4a2fae98..107215cf8488 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -338,19 +338,11 @@ static __meminit struct page *vmemmap_get_tail(unsigned int order, struct zone *
>   	tail = zone->vmemmap_tails[idx];
>   	if (tail)
>   		return tail;
> -
> -	/*
> -	 * Only allocate the page, but do not initialize it.
> -	 *
> -	 * Any initialization done here will be overwritten by memmap_init().
> -	 *
> -	 * hugetlb_bootmem_struct_page_init() will take care of initialization
> -	 * after memmap_init().
> -	 */
> -
>   	p = vmemmap_alloc_block_zero(PAGE_SIZE, node);
>   	if (!p)
>   		return NULL;
> +	for (int i = 0; i < PAGE_SIZE / sizeof(struct page); i++)
> +		init_compound_tail(p + i, NULL, order, zone);

Until section->order is set, vmemmap_optimizable_pfn() (introduced by
patch #3) will always return false. As a result, the initialization here
won't actually take effect until patch #10.

Acked-by: Qi Zheng <qi.zheng@linux.dev>

Thanks,
Qi

>   
>   	tail = virt_to_page(p);
>   	zone->vmemmap_tails[idx] = tail;