On 10/26/20 7:51 AM, Muchun Song wrote:
> On some architectures, the vmemmap areas use huge page mapping.
> 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
> huge pmd.
>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
> arch/x86/include/asm/hugetlb.h | 5 ++
> include/linux/hugetlb.h | 17 +++++
> mm/hugetlb.c | 117 +++++++++++++++++++++++++++++++++
> 3 files changed, 139 insertions(+)
>
> diff --git a/arch/x86/include/asm/hugetlb.h b/arch/x86/include/asm/hugetlb.h
> index 1721b1aadeb1..f5e882f999cd 100644
> --- a/arch/x86/include/asm/hugetlb.h
> +++ b/arch/x86/include/asm/hugetlb.h
> @@ -5,6 +5,11 @@
> #include <asm/page.h>
> #include <asm-generic/hugetlb.h>
>
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +#define VMEMMAP_HPAGE_SHIFT PMD_SHIFT
> +#define arch_vmemmap_support_huge_mapping() boot_cpu_has(X86_FEATURE_PSE)
> +#endif
> +
> #define hugepages_supported() boot_cpu_has(X86_FEATURE_PSE)
>
> #endif /* _ASM_X86_HUGETLB_H */
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index eed3dd3bd626..ace304a6196c 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -593,6 +593,23 @@ static inline unsigned int blocks_per_huge_page(struct hstate *h)
>
> #include <asm/hugetlb.h>
>
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +#ifndef arch_vmemmap_support_huge_mapping
> +static inline bool arch_vmemmap_support_huge_mapping(void)
> +{
> + return false;
> +}
> +#endif
> +
> +#ifndef VMEMMAP_HPAGE_SHIFT
> +#define VMEMMAP_HPAGE_SHIFT PMD_SHIFT
> +#endif
> +#define VMEMMAP_HPAGE_ORDER (VMEMMAP_HPAGE_SHIFT - PAGE_SHIFT)
> +#define VMEMMAP_HPAGE_NR (1 << VMEMMAP_HPAGE_ORDER)
> +#define VMEMMAP_HPAGE_SIZE ((1UL) << VMEMMAP_HPAGE_SHIFT)
> +#define VMEMMAP_HPAGE_MASK (~(VMEMMAP_HPAGE_SIZE - 1))
> +#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
> +
> #ifndef is_hugepage_only_range
> static inline int is_hugepage_only_range(struct mm_struct *mm,
> unsigned long addr, unsigned long len)
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index f1b2b733b49b..d6ae9b6876be 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1295,11 +1295,108 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> #define RESERVE_VMEMMAP_NR 2U
>
> +#define page_huge_pte(page) ((page)->pmd_huge_pte)
> +
I am not good at function names. The following suggestions may be too
verbose. However, they helped me understand purpose of routines.
> static inline unsigned int nr_free_vmemmap(struct hstate *h)
perhaps? free_vmemmap_pages_per_hpage()
> {
> return h->nr_free_vmemmap_pages;
> }
>
> +static inline unsigned int nr_vmemmap(struct hstate *h)
perhaps? vmemmap_pages_per_hpage()
> +{
> + return nr_free_vmemmap(h) + RESERVE_VMEMMAP_NR;
> +}
> +
> +static inline unsigned long nr_vmemmap_size(struct hstate *h)
perhaps? vmemmap_pages_size_per_hpage()
> +{
> + return (unsigned long)nr_vmemmap(h) << PAGE_SHIFT;
> +}
> +
> +static inline unsigned int nr_pgtable(struct hstate *h)
perhaps? pgtable_pages_to_prealloc_per_hpage()
> +{
> + unsigned long vmemmap_size = nr_vmemmap_size(h);
> +
> + if (!arch_vmemmap_support_huge_mapping())
> + return 0;
> +
> + /*
> + * No need pre-allocate page tabels when there is no vmemmap pages
> + * to free.
> + */
> + if (!nr_free_vmemmap(h))
> + return 0;
> +
> + return ALIGN(vmemmap_size, VMEMMAP_HPAGE_SIZE) >> VMEMMAP_HPAGE_SHIFT;
> +}
> +
> +static inline void vmemmap_pgtable_init(struct page *page)
> +{
> + page_huge_pte(page) = NULL;
> +}
> +
I see the following routines follow the pattern for vmemmap manipulation
in dax.
> +static void vmemmap_pgtable_deposit(struct page *page, pte_t *pte_p)
> +{
> + pgtable_t pgtable = virt_to_page(pte_p);
> +
> + /* 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 pte_t *vmemmap_pgtable_withdraw(struct page *page)
> +{
> + pgtable_t pgtable;
> +
> + /* FIFO */
> + pgtable = page_huge_pte(page);
> + if (unlikely(!pgtable))
> + return NULL;
> + page_huge_pte(page) = list_first_entry_or_null(&pgtable->lru,
> + struct page, lru);
> + if (page_huge_pte(page))
> + list_del(&pgtable->lru);
> + return page_to_virt(pgtable);
> +}
> +
> +static int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> +{
> + int i;
> + pte_t *pte_p;
> + unsigned int nr = nr_pgtable(h);
> +
> + if (!nr)
> + return 0;
> +
> + vmemmap_pgtable_init(page);
> +
> + for (i = 0; i < nr; i++) {
> + pte_p = pte_alloc_one_kernel(&init_mm);
> + if (!pte_p)
> + goto out;
> + vmemmap_pgtable_deposit(page, pte_p);
> + }
> +
> + return 0;
> +out:
> + while (i-- && (pte_p = vmemmap_pgtable_withdraw(page)))
> + pte_free_kernel(&init_mm, pte_p);
> + return -ENOMEM;
> +}
> +
> +static inline void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> +{
> + pte_t *pte_p;
> +
> + if (!nr_pgtable(h))
> + return;
> +
> + while ((pte_p = vmemmap_pgtable_withdraw(page)))
> + pte_free_kernel(&init_mm, pte_p);
> +}
> +
> static void __init hugetlb_vmemmap_init(struct hstate *h)
> {
> unsigned int order = huge_page_order(h);
> @@ -1323,6 +1420,15 @@ static void __init hugetlb_vmemmap_init(struct hstate *h)
> static inline void hugetlb_vmemmap_init(struct hstate *h)
> {
> }
> +
> +static inline int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> +{
> + return 0;
> +}
> +
> +static inline void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> +{
> +}
> #endif
>
> static void update_and_free_page(struct hstate *h, struct page *page)
> @@ -1531,6 +1637,9 @@ void free_huge_page(struct page *page)
>
> static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
> {
> + /* 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);
> @@ -1783,6 +1892,14 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
> if (!page)
> return NULL;
>
> + if (vmemmap_pgtable_prealloc(h, page)) {
> + if (hstate_is_gigantic(h))
> + free_gigantic_page(page, huge_page_order(h));
> + else
> + put_page(page);
> + return NULL;
> + }
> +
It seems a bit strange that we will fail a huge page allocation if
vmemmap_pgtable_prealloc fails. Not sure, but it almost seems like we shold
allow the allocation and log a warning? It is somewhat unfortunate that
we need to allocate a page to free pages.
> if (hstate_is_gigantic(h))
> prep_compound_gigantic_page(page, huge_page_order(h));
> prep_new_huge_page(h, page, page_to_nid(page));
>
--
Mike Kravetz
