Re: [PATCH v2 08/11] hugetlb: batch freeing of vmemmap pages

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

On 2023/9/6 05:44, 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 | 60 ++++++++++++++++++++++++++++----------------
>   1 file changed, 38 insertions(+), 22 deletions(-)
>
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 79de984919ef..a715712df831 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -306,18 +306,21 @@ 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.
> + * @vmemmap_pages: list to deposit vmemmap pages to be freed.  It is callers
> + *		responsibility to free pages.
>    *
>    * 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 *vmemmap_pages)
>   {
>   	int ret;
> -	LIST_HEAD(vmemmap_pages);
> +	LIST_HEAD(freed_pages);

IIUC, we could reuse the parameter of @vmemmap_pages directly instead of
a temporary variable, could it be dropped?

>   	struct vmemmap_remap_walk walk = {
>   		.remap_pte	= vmemmap_remap_pte,
>   		.reuse_addr	= reuse,
> -		.vmemmap_pages	= &vmemmap_pages,
> +		.vmemmap_pages	= &freed_pages,
>   	};
>   	int nid = page_to_nid((struct page *)start);
>   	gfp_t gfp_mask = GFP_KERNEL | __GFP_THISNODE | __GFP_NORETRY |
> @@ -335,7 +338,7 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
>   	if (walk.reuse_page) {
>   		copy_page(page_to_virt(walk.reuse_page),
>   			  (void *)walk.reuse_addr);
> -		list_add(&walk.reuse_page->lru, &vmemmap_pages);
> +		list_add(&walk.reuse_page->lru, &freed_pages);
>   	}
>   
>   	/*
> @@ -366,15 +369,14 @@ static int vmemmap_remap_free(unsigned long start, unsigned long end,
>   		walk = (struct vmemmap_remap_walk) {
>   			.remap_pte	= vmemmap_restore_pte,
>   			.reuse_addr	= reuse,
> -			.vmemmap_pages	= &vmemmap_pages,
> +			.vmemmap_pages	= &freed_pages,
>   		};
>   
>   		vmemmap_remap_range(reuse, end, &walk);
>   	}
>   	mmap_read_unlock(&init_mm);
>   
> -	free_vmemmap_page_list(&vmemmap_pages);
> -
> +	list_splice(&freed_pages, vmemmap_pages);
>   	return ret;
>   }
>   
> @@ -553,17 +555,9 @@ 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 *vmemmap_pages)
>   {
>   	unsigned long vmemmap_start = (unsigned long)head, vmemmap_end;
>   	unsigned long vmemmap_reuse;
> @@ -580,21 +574,43 @@ void hugetlb_vmemmap_optimize(const struct hstate *h, struct page *head)
>   
>   	/*
>   	 * Remap the vmemmap virtual address range [@vmemmap_start, @vmemmap_end)
> -	 * to the page which @vmemmap_reuse is mapped to, then free the pages
> -	 * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
> +	 * to the page which @vmemmap_reuse is mapped to.  Add pages previously
> +	 * mapping the range to vmemmap_pages list so that they can be freed by
> +	 * the caller.
>   	 */
> -	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse))
> +	if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, vmemmap_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)
> +{
> +	LIST_HEAD(vmemmap_pages);
> +
> +	__hugetlb_vmemmap_optimize(h, head, &vmemmap_pages);
> +	free_vmemmap_page_list(&vmemmap_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(h, &folio->page, &vmemmap_pages);
> +
> +	free_vmemmap_page_list(&vmemmap_pages);
>   }
>   
>   static struct ctl_table hugetlb_vmemmap_sysctls[] = {