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Subject: Re: [PATCH v17 3/9] mm: hugetlb: free the vmemmap pages associated
 with each HugeTLB page
To:     Muchun Song <songmuchun@bytedance.com>, corbet@lwn.net,
        mike.kravetz@oracle.com, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, x86@kernel.org, hpa@zytor.com,
        dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org,
        viro@zeniv.linux.org.uk, akpm@linux-foundation.org,
        paulmck@kernel.org, mchehab+huawei@kernel.org,
        pawan.kumar.gupta@linux.intel.com, rdunlap@infradead.org,
        oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
        almasrymina@google.com, rientjes@google.com, willy@infradead.org,
        osalvador@suse.de, mhocko@suse.com, song.bao.hua@hisilicon.com,
        david@redhat.com, naoya.horiguchi@nec.com,
        joao.m.martins@oracle.com
Cc:     duanxiongchun@bytedance.com, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org
References: <20210225132130.26451-1-songmuchun@bytedance.com>
 <20210225132130.26451-4-songmuchun@bytedance.com>
From:   "Singh, Balbir" <bsingharora@gmail.com>
Message-ID: <6a4a4fd8-d272-c034-f88c-cf372d8825a0@gmail.com>
Date:   Fri, 5 Mar 2021 10:50:37 +1100
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On 26/2/21 12:21 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@bytedance.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> ---
>  include/linux/bootmem_info.h |  27 +++++-
>  include/linux/mm.h           |   3 +
>  mm/Makefile                  |   1 +
>  mm/hugetlb.c                 |   3 +
>  mm/hugetlb_vmemmap.c         | 219 +++++++++++++++++++++++++++++++++++++++++++
>  mm/hugetlb_vmemmap.h         |  20 ++++
>  mm/sparse-vmemmap.c          | 207 ++++++++++++++++++++++++++++++++++++++++
>  7 files changed, 479 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 77e64e3eac80..4ddfc31f21c6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2971,6 +2971,9 @@ 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 daabf86d7da8..3d7d57e3b55b 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 c232cb67dda2..43fed6785322 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;
> @@ -1463,6 +1464,8 @@ 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..0209b736e0b4
> --- /dev/null
> +++ b/mm/hugetlb_vmemmap.c
> @@ -0,0 +1,219 @@
> +// 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. 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. 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)
> +
> +/*
> + * 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;
> +
> +	/*
> +	 * 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);
> +}
> 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@bytedance.com>
> + */
> +#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..d3076a7a3783 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -27,8 +27,215 @@
>  #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 lowest-level entry (PTE).
> + * @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_page) {
> +		BUG_ON(pte_none(*pte));
> +		BUG_ON(walk->reuse_addr != addr);
> +
> +		walk->reuse_page = pte_page(*pte++);

The concurrency semantics of this code are not clear, do we need READ_ONCE()/
WRITE_ONCE() semantics if this page walk is lockless? Can we run this code
in parallel on the same section? I presume not

> +		/*
> +		 * Because 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) || pmd_leaf(*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 only change the mapping of the vmemmap virtual address range
> +	 * [@start + PAGE_SIZE, end), so we only need to flush the TLB which
> +	 * belongs to the range.
> +	 */
> +	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 to, then free vmemmap
> + *			which the range are mapped to.
> + * @start:	start address of the vmemmap virtual address range that we want
> + *		to remap.
> + * @end:	end address of the vmemmap virtual address range that we want to
> + *		remap.
> + * @reuse:	reuse address.
> + *
> + * Note: This function depends on vmemmap being base page mapped. Please make
> + * sure that we disable PMD mapping of vmemmap pages when calling this function.

This is something that the walking code enforces via BUG_ON's right?

> + */
> +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 range [@start, @end) and the range [@reuse, @reuse + PAGE_SIZE)
> +	 *   should be continuous.
> +	 * - The @reuse address is part of the range [@reuse, @end) that we are
> +	 *   walking which is passed to vmemmap_remap_range().
> +	 * - 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);

Why even take a reuse arg then, just set reuse = start - PAGE_SIZE? If we do that
we can rename the function to reflect that the second page is reused or keep this
function and create an inline wrapper with reuse set to start - PAGE_SIZE and use
that for this use case and remove this BUG_ON

> +
> +	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
> 


Balbir