Re: [RFC PATCH 05/24] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate

[Mike Kravetz <mike.kravetz@oracle.com>]

On 9/15/20 5:59 AM, Muchun Song wrote:
> If the size of hugetlb page is 2MB, we need 512 struct page structures
> (8 pages) to be associated with it. As far as I know, we only use the
> first 3 struct page structures and only read the compound_dtor members

Actually, the first 4 pages can be used if CONFIG_CGROUP_HUGETLB.
/*
 * Minimum page order trackable by hugetlb cgroup.
 * At least 4 pages are necessary for all the tracking information.
 * The second tail page (hpage[2]) is the fault usage cgroup.
 * The third tail page (hpage[3]) is the reservation usage cgroup.
 */
#define HUGETLB_CGROUP_MIN_ORDER        2

However, this still easily fits within the first page of struct page
structures.

> of the remaining struct page structures. For tail page, the value of
> compound_dtor is the same. So we can reuse first tail page. We map the
> virtual addresses of the remaining 6 tail pages to the first tail page,
> and then free these 6 pages. Therefore, we need to reserve at least 2
> pages as vmemmap areas.

I got confused the first time I read the above sentences.  Perhaps it
should be more explicit with something like:

For tail pages, the value of compound_dtor is the same. So we can reuse
first page of tail page structs. We map the virtual addresses of the
remaining 6 pages of tail page structs to the first tail page struct,
and then free these 6 pages. Therefore, we need to reserve at least 2
pages as vmemmap areas.

It still does not sound great, but hopefully avoids some confusion.
-- 
Mike Kravetz

> So we introduce a new 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 system.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/linux/hugetlb.h |  3 +++
>  mm/hugetlb.c            | 35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index d5cc5f802dd4..eed3dd3bd626 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -492,6 +492,9 @@ struct hstate {
>  	unsigned int nr_huge_pages_node[MAX_NUMNODES];
>  	unsigned int free_huge_pages_node[MAX_NUMNODES];
>  	unsigned int surplus_huge_pages_node[MAX_NUMNODES];
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +	unsigned int nr_free_vmemmap_pages;
> +#endif
>  #ifdef CONFIG_CGROUP_HUGETLB
>  	/* cgroup control files */
>  	struct cftype cgroup_files_dfl[7];
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 81a41aa080a5..f1b2b733b49b 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1292,6 +1292,39 @@ static inline void destroy_compound_gigantic_page(struct page *page,
>  						unsigned int order) { }
>  #endif
>  
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +#define RESERVE_VMEMMAP_NR	2U
> +
> +static inline unsigned int nr_free_vmemmap(struct hstate *h)
> +{
> +	return h->nr_free_vmemmap_pages;
> +}
> +
> +static void __init hugetlb_vmemmap_init(struct hstate *h)
> +{
> +	unsigned int order = huge_page_order(h);
> +	unsigned int vmemmap_pages;
> +
> +	vmemmap_pages = ((1 << order) * sizeof(struct page)) >> PAGE_SHIFT;
> +	/*
> +	 * The head page and the first tail page not free to buddy system,
> +	 * the others page will map to the first tail page. So there are
> +	 * (@vmemmap_pages - RESERVE_VMEMMAP_NR) pages can be freed.
> +	 */
> +	if (vmemmap_pages > RESERVE_VMEMMAP_NR)
> +		h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
> +	else
> +		h->nr_free_vmemmap_pages = 0;
> +
> +	pr_info("HugeTLB: can free %d vmemmap pages for %s\n",
> +		h->nr_free_vmemmap_pages, h->name);
> +}
> +#else
> +static inline void hugetlb_vmemmap_init(struct hstate *h)
> +{
> +}
> +#endif
> +
>  static void update_and_free_page(struct hstate *h, struct page *page)
>  {
>  	int i;
> @@ -3285,6 +3318,8 @@ void __init hugetlb_add_hstate(unsigned int order)
>  	snprintf(h->name, HSTATE_NAME_LEN, "hugepages-%lukB",
>  					huge_page_size(h)/1024);
>  
> +	hugetlb_vmemmap_init(h);
> +
>  	parsed_hstate = h;
>  }
>  
>