Re: [PATCH v13 03/12] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Sun, Jan 17, 2021 at 11:13 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> Every HugeTLB has more than one struct page structure. We __know__ that
> we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
> to store metadata associated with each HugeTLB.
>
> 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.
>
> When we allocate a HugeTLB page from the buddy, we can free some vmemmap
> pages associated with each HugeTLB page. It is more appropriate to do it
> in the prep_new_huge_page().
>
> The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
> pages associated with a HugeTLB page can be freed, returns zero for
> now, which means the feature is disabled. We will enable it once all
> the infrastructure is there.
>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  include/linux/bootmem_info.h |  27 +++++-
>  include/linux/mm.h           |   3 +
>  mm/Makefile                  |   1 +
>  mm/hugetlb.c                 |   3 +
>  mm/hugetlb_vmemmap.c         | 211 +++++++++++++++++++++++++++++++++++++++++++
>  mm/hugetlb_vmemmap.h         |  20 ++++
>  mm/sparse-vmemmap.c          | 198 ++++++++++++++++++++++++++++++++++++++++
>  7 files changed, 462 insertions(+), 1 deletion(-)
>  create mode 100644 mm/hugetlb_vmemmap.c
>  create mode 100644 mm/hugetlb_vmemmap.h
>
> diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
> index 4ed6dee1adc9..ec03a624dfa2 100644
> --- a/include/linux/bootmem_info.h
> +++ b/include/linux/bootmem_info.h
> @@ -2,7 +2,7 @@
>  #ifndef __LINUX_BOOTMEM_INFO_H
>  #define __LINUX_BOOTMEM_INFO_H
>
> -#include <linux/mmzone.h>
> +#include <linux/mm.h>
>
>  /*
>   * Types for free bootmem stored in page->lru.next. These have to be in
> @@ -22,6 +22,27 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
>  void get_page_bootmem(unsigned long info, struct page *page,
>                       unsigned long type);
>  void put_page_bootmem(struct page *page);
> +
> +/*
> + * Any memory allocated via the memblock allocator and not via the
> + * buddy will be marked reserved already in the memmap. For those
> + * pages, we can call this function to free it to buddy allocator.
> + */
> +static inline void free_bootmem_page(struct page *page)
> +{
> +       unsigned long magic = (unsigned long)page->freelist;
> +
> +       /*
> +        * The reserve_bootmem_region sets the reserved flag on bootmem
> +        * pages.
> +        */
> +       VM_BUG_ON_PAGE(page_ref_count(page) != 2, page);
> +
> +       if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
> +               put_page_bootmem(page);
> +       else
> +               VM_BUG_ON_PAGE(1, page);
> +}
>  #else
>  static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
>  {
> @@ -35,6 +56,10 @@ static inline void get_page_bootmem(unsigned long info, struct page *page,
>                                     unsigned long type)
>  {
>  }
> +
> +static inline void free_bootmem_page(struct page *page)
> +{
> +}
>  #endif
>
>  #endif /* __LINUX_BOOTMEM_INFO_H */
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index eabe7d9f80d8..f928994ed273 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3005,6 +3005,9 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
>  }
>  #endif
>
> +void vmemmap_remap_free(unsigned long start, unsigned long end,
> +                       unsigned long reuse);
> +
>  void *sparse_buffer_alloc(unsigned long size);
>  struct page * __populate_section_memmap(unsigned long pfn,
>                 unsigned long nr_pages, int nid, struct vmem_altmap *altmap);
> diff --git a/mm/Makefile b/mm/Makefile
> index ed4b88fa0f5e..056801d8daae 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -71,6 +71,7 @@ obj-$(CONFIG_FRONTSWAP)       += frontswap.o
>  obj-$(CONFIG_ZSWAP)    += zswap.o
>  obj-$(CONFIG_HAS_DMA)  += dmapool.o
>  obj-$(CONFIG_HUGETLBFS)        += hugetlb.o
> +obj-$(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)        += hugetlb_vmemmap.o
>  obj-$(CONFIG_NUMA)     += mempolicy.o
>  obj-$(CONFIG_SPARSEMEM)        += sparse.o
>  obj-$(CONFIG_SPARSEMEM_VMEMMAP) += sparse-vmemmap.o
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 1f3bf1710b66..140135fc8113 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -42,6 +42,7 @@
>  #include <linux/userfaultfd_k.h>
>  #include <linux/page_owner.h>
>  #include "internal.h"
> +#include "hugetlb_vmemmap.h"
>
>  int hugetlb_max_hstate __read_mostly;
>  unsigned int default_hstate_idx;
> @@ -1497,6 +1498,8 @@ void free_huge_page(struct page *page)
>
>  static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
>  {
> +       free_huge_page_vmemmap(h, page);
> +
>         INIT_LIST_HEAD(&page->lru);
>         set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
>         set_hugetlb_cgroup(page, NULL);
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> new file mode 100644
> index 000000000000..4ffa2a4ae2a8
> --- /dev/null
> +++ b/mm/hugetlb_vmemmap.c
> @@ -0,0 +1,211 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Free some vmemmap pages of HugeTLB
> + *
> + * Copyright (c) 2020, Bytedance. All rights reserved.
> + *
> + *     Author: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> + *
> + * 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.
> + *
> + * The 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. HUGETLB_CGROUP_MIN_ORDER
> + * 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. Becasue 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 whose 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 (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 pages whose 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.
> + *
> + * 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;
> +
> +       vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
> +}
> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> new file mode 100644
> index 000000000000..6923f03534d5
> --- /dev/null
> +++ b/mm/hugetlb_vmemmap.h
> @@ -0,0 +1,20 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Free some vmemmap pages of HugeTLB
> + *
> + * Copyright (c) 2020, Bytedance. All rights reserved.
> + *
> + *     Author: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> + */
> +#ifndef _LINUX_HUGETLB_VMEMMAP_H
> +#define _LINUX_HUGETLB_VMEMMAP_H
> +#include <linux/hugetlb.h>
> +
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> +#else
> +static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> +{
> +}
> +#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
> +#endif /* _LINUX_HUGETLB_VMEMMAP_H */
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index 16183d85a7d5..ce4be1fa93c2 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -27,8 +27,206 @@
>  #include <linux/spinlock.h>
>  #include <linux/vmalloc.h>
>  #include <linux/sched.h>
> +#include <linux/pgtable.h>
> +#include <linux/bootmem_info.h>
> +
>  #include <asm/dma.h>
>  #include <asm/pgalloc.h>
> +#include <asm/tlbflush.h>
> +
> +/**
> + * vmemmap_remap_walk - walk vmemmap page table
> + *
> + * @remap_pte:         called for each non-empty PTE (lowest-level) entry.
> + * @reuse_page:                the page which is reused for the tail vmemmap pages.
> + * @reuse_addr:                the virtual address of the @reuse_page page.
> + * @vmemmap_pages:     the list head of the vmemmap pages that can be freed.
> + */
> +struct vmemmap_remap_walk {
> +       void (*remap_pte)(pte_t *pte, unsigned long addr,
> +                         struct vmemmap_remap_walk *walk);
> +       struct page *reuse_page;
> +       unsigned long reuse_addr;
> +       struct list_head *vmemmap_pages;
> +};
> +
> +static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
> +                             unsigned long end,
> +                             struct vmemmap_remap_walk *walk)
> +{
> +       pte_t *pte;
> +
> +       pte = pte_offset_kernel(pmd, addr);
> +
> +       /*
> +        * The reuse_page is found 'first' in table walk before we start
> +        * remapping (which is calling @walk->remap_pte).
> +        */
> +       if (walk->reuse_addr == addr) {
> +               BUG_ON(pte_none(*pte));
> +
> +               walk->reuse_page = pte_page(*pte++);
> +               /*
> +                * Becasue the reuse address is part of the range that we are
> +                * walking, skip the reuse address range.
> +                */
> +               addr += PAGE_SIZE;
> +       }
> +
> +       for (; addr != end; addr += PAGE_SIZE, pte++) {
> +               BUG_ON(pte_none(*pte));
> +
> +               walk->remap_pte(pte, addr, walk);
> +       }
> +}
> +
> +static void vmemmap_pmd_range(pud_t *pud, unsigned long addr,
> +                             unsigned long end,
> +                             struct vmemmap_remap_walk *walk)
> +{
> +       pmd_t *pmd;
> +       unsigned long next;
> +
> +       pmd = pmd_offset(pud, addr);
> +       do {
> +               BUG_ON(pmd_none(*pmd));
> +
> +               next = pmd_addr_end(addr, end);
> +               vmemmap_pte_range(pmd, addr, next, walk);
> +       } while (pmd++, addr = next, addr != end);
> +}
> +
> +static void vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
> +                             unsigned long end,
> +                             struct vmemmap_remap_walk *walk)
> +{
> +       pud_t *pud;
> +       unsigned long next;
> +
> +       pud = pud_offset(p4d, addr);
> +       do {
> +               BUG_ON(pud_none(*pud));
> +
> +               next = pud_addr_end(addr, end);
> +               vmemmap_pmd_range(pud, addr, next, walk);
> +       } while (pud++, addr = next, addr != end);
> +}
> +
> +static void vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
> +                             unsigned long end,
> +                             struct vmemmap_remap_walk *walk)
> +{
> +       p4d_t *p4d;
> +       unsigned long next;
> +
> +       p4d = p4d_offset(pgd, addr);
> +       do {
> +               BUG_ON(p4d_none(*p4d));
> +
> +               next = p4d_addr_end(addr, end);
> +               vmemmap_pud_range(p4d, addr, next, walk);
> +       } while (p4d++, addr = next, addr != end);
> +}
> +
> +static void vmemmap_remap_range(unsigned long start, unsigned long end,
> +                               struct vmemmap_remap_walk *walk)
> +{
> +       unsigned long addr = start;
> +       unsigned long next;
> +       pgd_t *pgd;
> +
> +       VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
> +       VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
> +
> +       pgd = pgd_offset_k(addr);
> +       do {
> +               BUG_ON(pgd_none(*pgd));
> +
> +               next = pgd_addr_end(addr, end);
> +               vmemmap_p4d_range(pgd, addr, next, walk);
> +       } while (pgd++, addr = next, addr != end);
> +
> +       /*
> +        * We do not change the mapping of the vmemmap virtual address range
> +        * [@start, @start + PAGE_SIZE) which is belong to the reuse range.
> +        * So we not need to flush the TLB.
> +        */
> +       flush_tlb_kernel_range(start - PAGE_SIZE, end);

Sorry. Here should be "flush_tlb_kernel_range(start + PAGE_SIZE, end)".
Will be fixed in the next version.

> +}
> +
> +/*
> + * Free a vmemmap page. A vmemmap page can be allocated from the memblock
> + * allocator or buddy allocator. If the PG_reserved flag is set, it means
> + * that it allocated from the memblock allocator, just free it via the
> + * free_bootmem_page(). Otherwise, use __free_page().
> + */
> +static inline void free_vmemmap_page(struct page *page)
> +{
> +       if (PageReserved(page))
> +               free_bootmem_page(page);
> +       else
> +               __free_page(page);
> +}
> +
> +/* Free a list of the vmemmap pages */
> +static void free_vmemmap_page_list(struct list_head *list)
> +{
> +       struct page *page, *next;
> +
> +       list_for_each_entry_safe(page, next, list, lru) {
> +               list_del(&page->lru);
> +               free_vmemmap_page(page);
> +       }
> +}
> +
> +static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
> +                             struct vmemmap_remap_walk *walk)
> +{
> +       /*
> +        * Remap the tail pages as read-only to catch illegal write operation
> +        * to the tail pages.
> +        */
> +       pgprot_t pgprot = PAGE_KERNEL_RO;
> +       pte_t entry = mk_pte(walk->reuse_page, pgprot);
> +       struct page *page = pte_page(*pte);
> +
> +       list_add(&page->lru, walk->vmemmap_pages);
> +       set_pte_at(&init_mm, addr, pte, entry);
> +}
> +
> +/**
> + * vmemmap_remap_free - remap the vmemmap virtual address range [@start, @end)
> + *                     to the page which @reuse is mapped, then free vmemmap
> + *                     pages.
> + * @start:     start address of the vmemmap virtual address range.
> + * @end:       end address of the vmemmap virtual address range.
> + * @reuse:     reuse address.
> + */
> +void vmemmap_remap_free(unsigned long start, unsigned long end,
> +                       unsigned long reuse)
> +{
> +       LIST_HEAD(vmemmap_pages);
> +       struct vmemmap_remap_walk walk = {
> +               .remap_pte      = vmemmap_remap_pte,
> +               .reuse_addr     = reuse,
> +               .vmemmap_pages  = &vmemmap_pages,
> +       };
> +
> +       /*
> +        * In order to make remapping routine most efficient for the huge pages,
> +        * the routine of vmemmap page table walking has the following rules
> +        * (see more details from the vmemmap_pte_range()):
> +        *
> +        * - The @reuse address is part of the range that we are walking.
> +        * - The @reuse address is the first in the complete range.
> +        *
> +        * So we need to make sure that @start and @reuse meet the above rules.
> +        */
> +       BUG_ON(start - reuse != PAGE_SIZE);
> +
> +       vmemmap_remap_range(reuse, end, &walk);
> +       free_vmemmap_page_list(&vmemmap_pages);
> +}
>
>  /*
>   * Allocate a block of memory to be used to back the virtual memory map
> --
> 2.11.0
>