Re: [PATCH v3 05/21] mm/hugetlb: Introduce pgtable allocation/freeing helpers

[Mike Kravetz]

On 11/8/20 6:10 AM, Muchun Song wrote:
> On x86_64, vmemmap is always PMD mapped if the machine has hugepages
> support and if we have 2MB contiguos pages and PMD aligned. If we want
> to free the unused vmemmap pages, we have to split the huge pmd firstly.
> So we should pre-allocate pgtable to split PMD to PTE.
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a0007902fafb..5c7be2ee7e15 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1303,6 +1303,108 @@ static inline void destroy_compound_gigantic_page(struct page *page,
...
> +static inline void vmemmap_pgtable_init(struct page *page)
> +{
> +	page_huge_pte(page) = NULL;
> +}
> +
> +static void vmemmap_pgtable_deposit(struct page *page, pgtable_t pgtable)
> +{
> +	/* FIFO */
> +	if (!page_huge_pte(page))
> +		INIT_LIST_HEAD(&pgtable->lru);
> +	else
> +		list_add(&pgtable->lru, &page_huge_pte(page)->lru);
> +	page_huge_pte(page) = pgtable;
> +}
> +
> +static pgtable_t vmemmap_pgtable_withdraw(struct page *page)
> +{
> +	pgtable_t pgtable;
> +
> +	/* FIFO */
> +	pgtable = page_huge_pte(page);
> +	page_huge_pte(page) = list_first_entry_or_null(&pgtable->lru,
> +						       struct page, lru);
> +	if (page_huge_pte(page))
> +		list_del(&pgtable->lru);
> +	return pgtable;
> +}
> +
> +static int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> +{
> +	int i;
> +	pgtable_t pgtable;
> +	unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> +
> +	if (!nr)
> +		return 0;
> +
> +	vmemmap_pgtable_init(page);
> +
> +	for (i = 0; i < nr; i++) {
> +		pte_t *pte_p;
> +
> +		pte_p = pte_alloc_one_kernel(&init_mm);
> +		if (!pte_p)
> +			goto out;
> +		vmemmap_pgtable_deposit(page, virt_to_page(pte_p));
> +	}
> +
> +	return 0;
> +out:
> +	while (i-- && (pgtable = vmemmap_pgtable_withdraw(page)))
> +		pte_free_kernel(&init_mm, page_to_virt(pgtable));
> +	return -ENOMEM;
> +}
> +
> +static void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> +{
> +	pgtable_t pgtable;
> +	unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> +
> +	if (!nr)
> +		return;
> +
> +	pgtable = page_huge_pte(page);
> +	if (!pgtable)
> +		return;
> +
> +	while (nr-- && (pgtable = vmemmap_pgtable_withdraw(page)))
> +		pte_free_kernel(&init_mm, page_to_virt(pgtable));
> +}

I may be confused.

In patch 9 of this series, the first call to vmemmap_pgtable_free() is made:

> @@ -1645,6 +1799,10 @@ void free_huge_page(struct page *page)
>  
>  static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>  {
> +	free_huge_page_vmemmap(h, page);
> +	/* Must be called before the initialization of @page->lru */
> +	vmemmap_pgtable_free(h, page);
> +
>  	INIT_LIST_HEAD(&page->lru);
>  	set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
>  	set_hugetlb_cgroup(page, NULL);

When I saw that comment in previous patch series, I assumed page->lru was
being used to store preallocated pages and pages to free.  However, unless
I am reading the code incorrectly it does not appear page->lru (of the huge
page) is being used for this purpose.  Is that correct?

If it is correct, would using page->lru of the huge page make this code
simpler?  I am just missing the reason why you are using
page_huge_pte(page)->lru

-- 
Mike Kravetz