// SPDX-License-Identifier: GPL-2.0 /* * Free some vmemmap pages of HugeTLB * * Copyright (c) 2020, Bytedance. All rights reserved. * * Author: Muchun Song * * The struct page structures (page structs) are used to describe a physical * page frame. By default, there is a one-to-one mapping from a page frame to * it's corresponding page struct. * * HugeTLB pages consist of multiple base page size pages and is supported by * many architectures. See hugetlbpage.rst in the Documentation directory for * more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB * are currently supported. Since the base page size on x86 is 4KB, a 2MB * HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of * 4096 base pages. For each base page, there is a corresponding page struct. * * Within the HugeTLB subsystem, only the first 4 page structs are used to * contain unique information about a HugeTLB page. __NR_USED_SUBPAGE provides * this upper limit. The only 'useful' information in the remaining page structs * is the compound_head field, and this field is the same for all tail pages. * * By removing redundant page structs for HugeTLB pages, memory can be returned * to the buddy allocator for other uses. * * Different architectures support different HugeTLB pages. For example, the * following table is the HugeTLB page size supported by x86 and arm64 * architectures. Because arm64 supports 4k, 16k, and 64k base pages and * supports contiguous entries, so it supports many kinds of sizes of HugeTLB * page. * * +--------------+-----------+-----------------------------------------------+ * | Architecture | Page Size | HugeTLB Page Size | * +--------------+-----------+-----------+-----------+-----------+-----------+ * | x86-64 | 4KB | 2MB | 1GB | | | * +--------------+-----------+-----------+-----------+-----------+-----------+ * | | 4KB | 64KB | 2MB | 32MB | 1GB | * | +-----------+-----------+-----------+-----------+-----------+ * | arm64 | 16KB | 2MB | 32MB | 1GB | | * | +-----------+-----------+-----------+-----------+-----------+ * | | 64KB | 2MB | 512MB | 16GB | | * +--------------+-----------+-----------+-----------+-----------+-----------+ * * When the system boot up, every HugeTLB page has more than one struct page * structs which size is (unit: pages): * * struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE * * Where HugeTLB_Size is the size of the HugeTLB page. We know that the size * of the HugeTLB page is always n times PAGE_SIZE. So we can get the following * relationship. * * HugeTLB_Size = n * PAGE_SIZE * * Then, * * struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE * = n * sizeof(struct page) / PAGE_SIZE * * We can use huge mapping at the pud/pmd level for the HugeTLB page. * * For the HugeTLB page of the pmd level mapping, then * * struct_size = n * sizeof(struct page) / PAGE_SIZE * = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE * = sizeof(struct page) / sizeof(pte_t) * = 64 / 8 * = 8 (pages) * * Where n is how many pte entries which one page can contains. So the value of * n is (PAGE_SIZE / sizeof(pte_t)). * * This optimization only supports 64-bit system, so the value of sizeof(pte_t) * is 8. And this optimization also applicable only when the size of struct page * is a power of two. In most cases, the size of struct page is 64 bytes (e.g. * x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the * size of struct page structs of it is 8 page frames which size depends on the * size of the base page. * * For the HugeTLB page of the pud level mapping, then * * struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd) * = PAGE_SIZE / 8 * 8 (pages) * = PAGE_SIZE (pages) * * Where the struct_size(pmd) is the size of the struct page structs of a * HugeTLB page of the pmd level mapping. * * E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB * HugeTLB page consists in 4096. * * Next, we take the pmd level mapping of the HugeTLB page as an example to * show the internal implementation of this optimization. There are 8 pages * struct page structs associated with a HugeTLB page which is pmd mapped. * * Here is how things look before optimization. * * HugeTLB struct pages(8 pages) page frame(8 pages) * +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ * | | | 0 | -------------> | 0 | * | | +-----------+ +-----------+ * | | | 1 | -------------> | 1 | * | | +-----------+ +-----------+ * | | | 2 | -------------> | 2 | * | | +-----------+ +-----------+ * | | | 3 | -------------> | 3 | * | | +-----------+ +-----------+ * | | | 4 | -------------> | 4 | * | PMD | +-----------+ +-----------+ * | level | | 5 | -------------> | 5 | * | mapping | +-----------+ +-----------+ * | | | 6 | -------------> | 6 | * | | +-----------+ +-----------+ * | | | 7 | -------------> | 7 | * | | +-----------+ +-----------+ * | | * | | * | | * +-----------+ * * The value of page->compound_head is the same for all tail pages. The first * page of page structs (page 0) associated with the HugeTLB page contains the 4 * page structs necessary to describe the HugeTLB. The only use of the remaining * pages of page structs (page 1 to page 7) is to point to page->compound_head. * Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs * will be used for each HugeTLB page. This will allow us to free the remaining * 6 pages to the buddy allocator. * * Here is how things look after remapping. * * HugeTLB struct pages(8 pages) page frame(8 pages) * +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ * | | | 0 | -------------> | 0 | * | | +-----------+ +-----------+ * | | | 1 | -------------> | 1 | * | | +-----------+ +-----------+ * | | | 2 | ----------------^ ^ ^ ^ ^ ^ * | | +-----------+ | | | | | * | | | 3 | ------------------+ | | | | * | | +-----------+ | | | | * | | | 4 | --------------------+ | | | * | PMD | +-----------+ | | | * | level | | 5 | ----------------------+ | | * | mapping | +-----------+ | | * | | | 6 | ------------------------+ | * | | +-----------+ | * | | | 7 | --------------------------+ * | | +-----------+ * | | * | | * | | * +-----------+ * * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for * vmemmap pages and restore the previous mapping relationship. * * For the HugeTLB page of the pud level mapping. It is similar to the former. * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages. * * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures * (e.g. aarch64) provides a contiguous bit in the translation table entries * that hints to the MMU to indicate that it is one of a contiguous set of * entries that can be cached in a single TLB entry. * * The contiguous bit is used to increase the mapping size at the pmd and pte * (last) level. So this type of HugeTLB page can be optimized only when its * size of the struct page structs is greater than 2 pages. */ #define pr_fmt(fmt) "HugeTLB: " fmt #include "hugetlb_vmemmap.h" /* * There are a lot of struct page structures associated with each HugeTLB page. * For tail pages, the value of compound_head is the same. So we can reuse first * page of tail page structures. We map the virtual addresses of the remaining * pages of tail page structures to the first tail page struct, and then free * these page frames. Therefore, we need to reserve two pages as vmemmap areas. */ #define RESERVE_VMEMMAP_NR 2U #define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT) bool hugetlb_free_vmemmap_enabled; 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)))) { pr_warn("cannot free vmemmap pages because \"struct page\" crosses page boundaries\n"); return 0; } if (!buf) return -EINVAL; if (!strcmp(buf, "on")) hugetlb_free_vmemmap_enabled = true; else if (strcmp(buf, "off")) return -EINVAL; return 0; } early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param); static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h) { return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT; } /* * Previously discarded vmemmap pages will be allocated and remapping * after this function returns zero. */ int alloc_huge_page_vmemmap(struct hstate *h, struct page *head) { int ret; unsigned long vmemmap_addr = (unsigned long)head; unsigned long vmemmap_end, vmemmap_reuse; if (!HPageVmemmapOptimized(head)) return 0; vmemmap_addr += RESERVE_VMEMMAP_SIZE; vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h); vmemmap_reuse = vmemmap_addr - PAGE_SIZE; /* * The pages which the vmemmap virtual address range [@vmemmap_addr, * @vmemmap_end) are mapped to are freed to the buddy allocator, and * the range is mapped to the page which @vmemmap_reuse is mapped to. * When a HugeTLB page is freed to the buddy allocator, previously * discarded vmemmap pages must be allocated and remapping. */ ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse, GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE); if (!ret) ClearHPageVmemmapOptimized(head); return ret; } void free_huge_page_vmemmap(struct hstate *h, struct page *head) { unsigned long vmemmap_addr = (unsigned long)head; unsigned long vmemmap_end, vmemmap_reuse; if (!free_vmemmap_pages_per_hpage(h)) return; vmemmap_addr += RESERVE_VMEMMAP_SIZE; vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h); vmemmap_reuse = vmemmap_addr - PAGE_SIZE; /* * Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end) * to the page which @vmemmap_reuse is mapped to, then free the pages * which the range [@vmemmap_addr, @vmemmap_end] is mapped to. */ vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse); SetHPageVmemmapOptimized(head); } void __init hugetlb_vmemmap_init(struct hstate *h) { unsigned int nr_pages = pages_per_huge_page(h); unsigned int vmemmap_pages; /* * There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct * page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, * so add a BUILD_BUG_ON to catch invalid usage of the tail struct page. */ BUILD_BUG_ON(__NR_USED_SUBPAGE >= RESERVE_VMEMMAP_SIZE / sizeof(struct page)); 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 * allocator, 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); }