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Diffstat (limited to 'mm/hugetlb_vmemmap.c')
| -rw-r--r-- | mm/hugetlb_vmemmap.c | 218 |
1 files changed, 218 insertions, 0 deletions
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c new file mode 100644 index 000000000000..e45a138a7f85 --- /dev/null +++ b/mm/hugetlb_vmemmap.c @@ -0,0 +1,218 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Free some vmemmap pages of HugeTLB + * + * Copyright (c) 2020, Bytedance. All rights reserved. + * + * Author: Muchun Song <songmuchun@bytedance.com> + * + * 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. + */ +#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) + +/* + * How many vmemmap pages associated with a HugeTLB page that can be freed + * to the buddy allocator. + * + * Todo: Returns zero for now, which means the feature is disabled. We will + * enable it once all the infrastructure is there. + */ +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h) +{ + return 0; +} + +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h) +{ + return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT; +} + +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); +} |