299 lines
13 KiB
C
299 lines
13 KiB
C
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// SPDX-License-Identifier: GPL-2.0
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/*
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* Free some vmemmap pages of HugeTLB
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*
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* Copyright (c) 2020, Bytedance. All rights reserved.
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*
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* Author: Muchun Song <songmuchun@bytedance.com>
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*
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* The struct page structures (page structs) are used to describe a physical
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* page frame. By default, there is a one-to-one mapping from a page frame to
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* it's corresponding page struct.
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*
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* HugeTLB pages consist of multiple base page size pages and is supported by
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* many architectures. See hugetlbpage.rst in the Documentation directory for
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* more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB
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* are currently supported. Since the base page size on x86 is 4KB, a 2MB
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* HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
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* 4096 base pages. For each base page, there is a corresponding page struct.
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*
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* Within the HugeTLB subsystem, only the first 4 page structs are used to
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* contain unique information about a HugeTLB page. __NR_USED_SUBPAGE provides
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* this upper limit. The only 'useful' information in the remaining page structs
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* is the compound_head field, and this field is the same for all tail pages.
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*
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* By removing redundant page structs for HugeTLB pages, memory can be returned
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* to the buddy allocator for other uses.
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*
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* Different architectures support different HugeTLB pages. For example, the
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* following table is the HugeTLB page size supported by x86 and arm64
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* architectures. Because arm64 supports 4k, 16k, and 64k base pages and
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* supports contiguous entries, so it supports many kinds of sizes of HugeTLB
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* page.
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*
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* +--------------+-----------+-----------------------------------------------+
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* | Architecture | Page Size | HugeTLB Page Size |
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* +--------------+-----------+-----------+-----------+-----------+-----------+
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* | x86-64 | 4KB | 2MB | 1GB | | |
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* +--------------+-----------+-----------+-----------+-----------+-----------+
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* | | 4KB | 64KB | 2MB | 32MB | 1GB |
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* | +-----------+-----------+-----------+-----------+-----------+
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* | arm64 | 16KB | 2MB | 32MB | 1GB | |
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* | +-----------+-----------+-----------+-----------+-----------+
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* | | 64KB | 2MB | 512MB | 16GB | |
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* +--------------+-----------+-----------+-----------+-----------+-----------+
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*
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* When the system boot up, every HugeTLB page has more than one struct page
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* structs which size is (unit: pages):
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*
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* struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
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*
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* Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
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* of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
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* relationship.
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*
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* HugeTLB_Size = n * PAGE_SIZE
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*
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* Then,
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*
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* struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
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* = n * sizeof(struct page) / PAGE_SIZE
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*
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* We can use huge mapping at the pud/pmd level for the HugeTLB page.
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*
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* For the HugeTLB page of the pmd level mapping, then
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*
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* struct_size = n * sizeof(struct page) / PAGE_SIZE
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* = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
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* = sizeof(struct page) / sizeof(pte_t)
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* = 64 / 8
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* = 8 (pages)
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*
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* Where n is how many pte entries which one page can contains. So the value of
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* n is (PAGE_SIZE / sizeof(pte_t)).
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*
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* This optimization only supports 64-bit system, so the value of sizeof(pte_t)
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* is 8. And this optimization also applicable only when the size of struct page
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* is a power of two. In most cases, the size of struct page is 64 bytes (e.g.
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* x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
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* size of struct page structs of it is 8 page frames which size depends on the
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* size of the base page.
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*
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* For the HugeTLB page of the pud level mapping, then
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*
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* struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
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* = PAGE_SIZE / 8 * 8 (pages)
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* = PAGE_SIZE (pages)
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*
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* Where the struct_size(pmd) is the size of the struct page structs of a
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* HugeTLB page of the pmd level mapping.
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*
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* E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
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* HugeTLB page consists in 4096.
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*
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* Next, we take the pmd level mapping of the HugeTLB page as an example to
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* show the internal implementation of this optimization. There are 8 pages
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* struct page structs associated with a HugeTLB page which is pmd mapped.
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*
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* Here is how things look before optimization.
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*
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* HugeTLB struct pages(8 pages) page frame(8 pages)
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* +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
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* | | | 0 | -------------> | 0 |
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* | | +-----------+ +-----------+
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* | | | 1 | -------------> | 1 |
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* | | +-----------+ +-----------+
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* | | | 2 | -------------> | 2 |
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* | | +-----------+ +-----------+
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* | | | 3 | -------------> | 3 |
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* | | +-----------+ +-----------+
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* | | | 4 | -------------> | 4 |
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* | PMD | +-----------+ +-----------+
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* | level | | 5 | -------------> | 5 |
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* | mapping | +-----------+ +-----------+
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* | | | 6 | -------------> | 6 |
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* | | +-----------+ +-----------+
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* | | | 7 | -------------> | 7 |
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* | | +-----------+ +-----------+
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* | |
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* | |
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* | |
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* +-----------+
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*
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* The value of page->compound_head is the same for all tail pages. The first
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* page of page structs (page 0) associated with the HugeTLB page contains the 4
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* page structs necessary to describe the HugeTLB. The only use of the remaining
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* pages of page structs (page 1 to page 7) is to point to page->compound_head.
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* Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
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* will be used for each HugeTLB page. This will allow us to free the remaining
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* 6 pages to the buddy allocator.
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*
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* Here is how things look after remapping.
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*
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* HugeTLB struct pages(8 pages) page frame(8 pages)
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* +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
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* | | | 0 | -------------> | 0 |
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* | | +-----------+ +-----------+
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* | | | 1 | -------------> | 1 |
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* | | +-----------+ +-----------+
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* | | | 2 | ----------------^ ^ ^ ^ ^ ^
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* | | +-----------+ | | | | |
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* | | | 3 | ------------------+ | | | |
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* | | +-----------+ | | | |
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* | | | 4 | --------------------+ | | |
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* | PMD | +-----------+ | | |
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* | level | | 5 | ----------------------+ | |
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* | mapping | +-----------+ | |
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* | | | 6 | ------------------------+ |
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* | | +-----------+ |
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* | | | 7 | --------------------------+
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* | | +-----------+
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* | |
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* | |
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* | |
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* +-----------+
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*
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* When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
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* vmemmap pages and restore the previous mapping relationship.
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*
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* For the HugeTLB page of the pud level mapping. It is similar to the former.
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* We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
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*
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* Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
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* (e.g. aarch64) provides a contiguous bit in the translation table entries
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* that hints to the MMU to indicate that it is one of a contiguous set of
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* entries that can be cached in a single TLB entry.
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*
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* The contiguous bit is used to increase the mapping size at the pmd and pte
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* (last) level. So this type of HugeTLB page can be optimized only when its
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* size of the struct page structs is greater than 2 pages.
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*/
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#define pr_fmt(fmt) "HugeTLB: " fmt
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#include "hugetlb_vmemmap.h"
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/*
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* There are a lot of struct page structures associated with each HugeTLB page.
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* For tail pages, the value of compound_head is the same. So we can reuse first
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* page of tail page structures. We map the virtual addresses of the remaining
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* pages of tail page structures to the first tail page struct, and then free
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* these page frames. Therefore, we need to reserve two pages as vmemmap areas.
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*/
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#define RESERVE_VMEMMAP_NR 2U
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#define RESERVE_VMEMMAP_SIZE (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
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bool hugetlb_free_vmemmap_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
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static int __init early_hugetlb_free_vmemmap_param(char *buf)
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{
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/* We cannot optimize if a "struct page" crosses page boundaries. */
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if ((!is_power_of_2(sizeof(struct page)))) {
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pr_warn("cannot free vmemmap pages because \"struct page\" crosses page boundaries\n");
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return 0;
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}
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if (!buf)
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return -EINVAL;
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if (!strcmp(buf, "on"))
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hugetlb_free_vmemmap_enabled = true;
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else if (!strcmp(buf, "off"))
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hugetlb_free_vmemmap_enabled = false;
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else
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return -EINVAL;
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return 0;
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}
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early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
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static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
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{
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return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
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}
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/*
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* Previously discarded vmemmap pages will be allocated and remapping
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* after this function returns zero.
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*/
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int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
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{
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int ret;
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unsigned long vmemmap_addr = (unsigned long)head;
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unsigned long vmemmap_end, vmemmap_reuse;
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if (!HPageVmemmapOptimized(head))
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return 0;
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vmemmap_addr += RESERVE_VMEMMAP_SIZE;
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vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
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vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
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/*
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* The pages which the vmemmap virtual address range [@vmemmap_addr,
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* @vmemmap_end) are mapped to are freed to the buddy allocator, and
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* the range is mapped to the page which @vmemmap_reuse is mapped to.
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* When a HugeTLB page is freed to the buddy allocator, previously
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* discarded vmemmap pages must be allocated and remapping.
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*/
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ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
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GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
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if (!ret)
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ClearHPageVmemmapOptimized(head);
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return ret;
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}
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void free_huge_page_vmemmap(struct hstate *h, struct page *head)
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{
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unsigned long vmemmap_addr = (unsigned long)head;
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unsigned long vmemmap_end, vmemmap_reuse;
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if (!free_vmemmap_pages_per_hpage(h))
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return;
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vmemmap_addr += RESERVE_VMEMMAP_SIZE;
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vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
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vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
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/*
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* Remap the vmemmap virtual address range [@vmemmap_addr, @vmemmap_end)
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* to the page which @vmemmap_reuse is mapped to, then free the pages
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* which the range [@vmemmap_addr, @vmemmap_end] is mapped to.
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*/
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if (!vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse))
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SetHPageVmemmapOptimized(head);
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}
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void __init hugetlb_vmemmap_init(struct hstate *h)
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{
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unsigned int nr_pages = pages_per_huge_page(h);
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unsigned int vmemmap_pages;
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/*
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* There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
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* page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP,
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* so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.
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*/
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BUILD_BUG_ON(__NR_USED_SUBPAGE >=
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RESERVE_VMEMMAP_SIZE / sizeof(struct page));
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if (!hugetlb_free_vmemmap_enabled)
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return;
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vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
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/*
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* The head page and the first tail page are not to be freed to buddy
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* allocator, the other pages will map to the first tail page, so they
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* can be freed.
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*
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* Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
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* on some architectures (e.g. aarch64). See Documentation/arm64/
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* hugetlbpage.rst for more details.
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*/
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if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR))
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h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
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pr_info("can free %d vmemmap pages for %s\n", h->nr_free_vmemmap_pages,
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h->name);
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}
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