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