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hugetlb_vmemmap.c
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hugetlb_vmemmap.c
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// SPDX-License-Identifier: GPL-2.0
/*
* Free some vmemmap pages of HugeTLB
*
* Copyright (c) 2020, Bytedance. All rights reserved.
*
* Author: Muchun Song <[email protected]>
*
* 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 = IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
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"))
hugetlb_free_vmemmap_enabled = false;
else
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.
*/
if (!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);
}