linux/mm/hugetlb_vmemmap.c

// 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)

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);
}