Re: [PATCH 08/12] hugetlb: batch freeing of vmemmap pages

[Muchun Song <muchun.song@linux.dev>]

On 2023/8/26 03:04, Mike Kravetz wrote:
> Now that batching of hugetlb vmemmap optimization processing is possible,
> batch the freeing of vmemmap pages.  When freeing vmemmap pages for a
> hugetlb page, we add them to a list that is freed after the entire batch
> has been processed.
>
> This enhances the ability to return contiguous ranges of memory to the
> low level allocators.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>   mm/hugetlb_vmemmap.c | 56 ++++++++++++++++++++++++++++++++------------
>   1 file changed, 41 insertions(+), 15 deletions(-)
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index d5e6b6c76dce..e390170c0887 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -305,11 +305,14 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
>    * @end:	end address of the vmemmap virtual address range that we want to
>    *		remap.
>    * @reuse:	reuse address.
> + * @bulk_pages: list to deposit vmemmap pages to be freed in bulk operations
> + *		or NULL in non-bulk case;

I'd like to rename bulk_pages to vmemmap_pages. Always add the vmemmap
pages to this list and let the caller (hugetlb_vmemmap_optimize and
hugetlb_vmemmap_optimize_folios) to help us to free them. It will be
clear to me.

>    *
>    * Return: %0 on success, negative error code otherwise.
>    */
>   static int vmemmap_remap_free(unsigned long start, unsigned long end,
> -			      unsigned long reuse)
> +			      unsigned long reuse,
> +			      struct list_head *bulk_pages)
>   {
>   	int ret;
>   	LIST_HEAD(vmemmap_pages);
> @@ -372,7 +375,14 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
>   	}
>   	mmap_read_unlock(&init_mm);
>   
> -	free_vmemmap_page_list(&vmemmap_pages);
> +	/*
> +	 * if performing bulk operation, do not free pages here.
> +	 * rather add them to the bulk list
> +	 */
> +	if (!bulk_pages)
> +		free_vmemmap_page_list(&vmemmap_pages);
> +	else
> +		list_splice(&vmemmap_pages, bulk_pages);

Here, always add vmemmap_pages to the list.

>   
>   	return ret;
>   }
> @@ -546,17 +556,9 @@ static bool vmemmap_should_optimize(const struct hstate *h, const struct page *h
>   	return true;
>   }
>   
> -/**
> - * hugetlb_vmemmap_optimize - optimize @head page's vmemmap pages.
> - * @h:		struct hstate.
> - * @head:	the head page whose vmemmap pages will be optimized.
> - *
> - * This function only tries to optimize @head's vmemmap pages and does not
> - * guarantee that the optimization will succeed after it returns. The caller
> - * can use HPageVmemmapOptimized(@head) to detect if @head's vmemmap pages
> - * have been optimized.
> - */
> -void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
> +static void __hugetlb_vmemmap_optimize(const struct hstate *h,
> +					struct page *head,
> +					struct list_head *bulk_pages)

Also struct list_head *vmemmap_pages.

>   {
>   	unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
>   	unsigned long vmemmap_reuse;
> @@ -575,18 +577,42 @@ void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
>   	 * to the page which @vmemmap_reuse is mapped to, then free the pages
>   	 * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
>   	 */
> -	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse))
> +	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, bulk_pages))
>   		static_branch_dec(&hugetlb_optimize_vmemmap_key);
>   	else
>   		SetHPageVmemmapOptimized(head);
>   }
>   
> +/**
> + * hugetlb_vmemmap_optimize - optimize @head page's vmemmap pages.
> + * @h:		struct hstate.
> + * @head:	the head page whose vmemmap pages will be optimized.
> + *
> + * This function only tries to optimize @head's vmemmap pages and does not
> + * guarantee that the optimization will succeed after it returns. The caller
> + * can use HPageVmemmapOptimized(@head) to detect if @head's vmemmap pages
> + * have been optimized.
> + */
> +void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
> +{
> +	__hugetlb_vmemmap_optimize(h, head, NULL);

Use free_vmemmap_page_list to free vmemmap pages here.

> +}
> +
> +void hugetlb_vmemmap_optimize_bulk(const struct hstate *h, struct page *head,
> +			struct list_head *bulk_pages)
> +{
> +	__hugetlb_vmemmap_optimize(h, head, bulk_pages);
> +}
> +
>   void hugetlb_vmemmap_optimize_folios(struct hstate *h, struct list_head *folio_list)
>   {
>   	struct folio *folio;
> +	LIST_HEAD(vmemmap_pages);
>   
>   	list_for_each_entry(folio, folio_list, lru)
> -		hugetlb_vmemmap_optimize(h, &folio->page);
> +		hugetlb_vmemmap_optimize_bulk(h, &folio->page, &vmemmap_pages);

Directly use __hugetlb_vmemmap_optimize and delete 
hugetlb_vmemmap_optimize_bulk.
In the future, we could rename hugetlb_vmemmap_optimize to 
hugetlb_vmemmap_optimize_folio,
then, both function names are more consistent. E.g.

   1) hugetlb_vmemmap_optimize_folio(): used to free one folio's vmemmap 
pages.
   2) hugetlb_vmemmap_optimize_folios(): used to free multiple folio's 
vmemmap pages.

Thanks.

> +
> +	free_vmemmap_page_list(&vmemmap_pages);
>   }
>   
>   static struct ctl_table hugetlb_vmemmap_sysctls[] = {