Re: [External] Re: [PATCH v13 03/12] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page
From: Muchun Song
Date: Fri Jan 22 2021 - 22:24:34 EST
On Sat, Jan 23, 2021 at 9:00 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> X-Gm-Spam: 0
> X-Gm-Phishy: 0
>
> On 1/17/21 7:10 AM, Muchun Song 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
>
> Thank you for the continued updates! Just some comments below.
> I am hoping that others can take a look so we can move this forward.
> I do not see any obvious issues.
Yeah, hope more reviewers will participate in this. :)
>
> > 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>
> > +
>
> We made the decision to disable PMD mapping of the vmemmap if this feature
> is enabled. However, that is not until later in the series. And, the code
> which disables PMD mapping is done in arch specific init code. So, a reader
> of this new code in sparse-vmemmap.c might not be aware of this. But, the
> code below depends on vmemmap being base page mapped.
>
> I know your plan is to perhaps remove this restriction in the future.
> Perhaps we should have a big comment in the code (?and commit message?)
> noting that this is designed to only work with base page mappings so that
> people do not get confused?
Agree. Will add some comments in the next version.
>
> > +/**
> > + * 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);
> > +}
> > +
> > +/*
> > + * 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.
> > + */
>
> Thanks for adding this comment.
>
> For now this code only works for huge pages. We need to make sure that is
> clear to reviewers and people just reading the code.
>
> --
> Mike Kravetz
>
> > + 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
> >