Re: [PATCH v9 09/11] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate

[Oscar Salvador <osalvador@suse.de>]

On Sun, Dec 13, 2020 at 11:45:32PM +0800, Muchun Song wrote:
> All the infrastructure is ready, so we introduce nr_free_vmemmap_pages
> field in the hstate to indicate how many vmemmap pages associated with
> a HugeTLB page that we can free to buddy allocator. And initialize it
"can be freed to buddy allocator"

> in the hugetlb_vmemmap_init(). This patch is actual enablement of the
> feature.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Acked-by: Mike Kravetz <mike.kravetz@oracle.com>

With below nits addressed you can add:

Reviewed-by: Oscar Salvador <osalvador@suse.de>

>  static int __init early_hugetlb_free_vmemmap_param(char *buf)
>  {
> +	/* We cannot optimize if a "struct page" crosses page boundaries. */
> +	if (!is_power_of_2(sizeof(struct page)))
> +		return 0;
> +

I wonder if we should report a warning in case someone wants to enable this
feature and stuct page size it not power of 2.
In case someone wonders why it does not work for him/her.

> +void __init hugetlb_vmemmap_init(struct hstate *h)
> +{
> +	unsigned int nr_pages = pages_per_huge_page(h);
> +	unsigned int vmemmap_pages;
> +
> +	if (!hugetlb_free_vmemmap_enabled)
> +		return;
> +
> +	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
> +	/*
> +	 * The head page and the first tail page are not to be freed to buddy
> +	 * system, the others page will map to the first tail page. So there
> +	 * are the remaining pages that can be freed.
"the other pages will map to the first tail page, so they can be freed."
> +	 *
> +	 * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
> +	 * on some architectures (e.g. aarch64). See Documentation/arm64/
> +	 * hugetlbpage.rst for more details.
> +	 */
> +	if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR))
> +		h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
> +
> +	pr_info("can free %d vmemmap pages for %s\n", h->nr_free_vmemmap_pages,
> +		h->name);

Maybe specify this is hugetlb code:

pr_info("%s: blabla", __func__, ...)
or
pr_info("hugetlb: blalala", ...);

although I am not sure whether we need that at all, or maybe just use
pr_debug().

-- 
Oscar Salvador
SUSE L3