Re: [External] [PATCH v2 07/14] mm/hugetlb_vmemmap: move comment block to Documentation/vm

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Jun 18, 2021 at 2:46 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> In preparation for device-dax for using hugetlbfs compound page tail
> deduplication technique, move the comment block explanation into a
> common place in Documentation/vm.
>
> Cc: Muchun Song <songmuchun@bytedance.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Suggested-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
>  Documentation/vm/compound_pagemaps.rst | 170 +++++++++++++++++++++++++
>  Documentation/vm/index.rst             |   1 +
>  mm/hugetlb_vmemmap.c                   | 162 +----------------------
>  3 files changed, 172 insertions(+), 161 deletions(-)
>  create mode 100644 Documentation/vm/compound_pagemaps.rst

IMHO, how about the name of vmemmap_remap.rst? page_frags.rst seems
to tell people it's about the page mapping not its vmemmap mapping.

Thanks.

>
> diff --git a/Documentation/vm/compound_pagemaps.rst b/Documentation/vm/compound_pagemaps.rst
> new file mode 100644
> index 000000000000..6b1af50e8201
> --- /dev/null
> +++ b/Documentation/vm/compound_pagemaps.rst
> @@ -0,0 +1,170 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +.. _commpound_pagemaps:
> +
> +==================================
> +Free some vmemmap pages of HugeTLB
> +==================================
> +
> +The struct page structures (page structs) are used to describe a physical
> +page frame. By default, there is a one-to-one mapping from a page frame to
> +it's corresponding page struct.
> +
> +HugeTLB pages consist of multiple base page size pages and is supported by
> +many architectures. See hugetlbpage.rst in the Documentation directory for
> +more details. On the x86-64 architecture, HugeTLB pages of size 2MB and 1GB
> +are currently supported. Since the base page size on x86 is 4KB, a 2MB
> +HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
> +4096 base pages. For each base page, there is a corresponding page struct.
> +
> +Within the HugeTLB subsystem, only the first 4 page structs are used to
> +contain unique information about a HugeTLB page. __NR_USED_SUBPAGE provides
> +this upper limit. The only 'useful' information in the remaining page structs
> +is the compound_head field, and this field is the same for all tail pages.
> +
> +By removing redundant page structs for HugeTLB pages, memory can be returned
> +to the buddy allocator for other uses.
> +
> +Different architectures support different HugeTLB pages. For example, the
> +following table is the HugeTLB page size supported by x86 and arm64
> +architectures. Because arm64 supports 4k, 16k, and 64k base pages and
> +supports contiguous entries, so it supports many kinds of sizes of HugeTLB
> +page.
> +
> ++--------------+-----------+-----------------------------------------------+
> +| Architecture | Page Size |                HugeTLB Page Size              |
> ++--------------+-----------+-----------+-----------+-----------+-----------+
> +|    x86-64    |    4KB    |    2MB    |    1GB    |           |           |
> ++--------------+-----------+-----------+-----------+-----------+-----------+
> +|              |    4KB    |   64KB    |    2MB    |    32MB   |    1GB    |
> +|              +-----------+-----------+-----------+-----------+-----------+
> +|    arm64     |   16KB    |    2MB    |   32MB    |     1GB   |           |
> +|              +-----------+-----------+-----------+-----------+-----------+
> +|              |   64KB    |    2MB    |  512MB    |    16GB   |           |
> ++--------------+-----------+-----------+-----------+-----------+-----------+
> +
> +When the system boot up, every HugeTLB page has more than one struct page
> +structs which size is (unit: pages):
> +
> +   struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
> +
> +Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
> +of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
> +relationship.
> +
> +   HugeTLB_Size = n * PAGE_SIZE
> +
> +Then,
> +
> +   struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
> +               = n * sizeof(struct page) / PAGE_SIZE
> +
> +We can use huge mapping at the pud/pmd level for the HugeTLB page.
> +
> +For the HugeTLB page of the pmd level mapping, then
> +
> +   struct_size = n * sizeof(struct page) / PAGE_SIZE
> +               = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
> +               = sizeof(struct page) / sizeof(pte_t)
> +               = 64 / 8
> +               = 8 (pages)
> +
> +Where n is how many pte entries which one page can contains. So the value of
> +n is (PAGE_SIZE / sizeof(pte_t)).
> +
> +This optimization only supports 64-bit system, so the value of sizeof(pte_t)
> +is 8. And this optimization also applicable only when the size of struct page
> +is a power of two. In most cases, the size of struct page is 64 bytes (e.g.
> +x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
> +size of struct page structs of it is 8 page frames which size depends on the
> +size of the base page.
> +
> +For the HugeTLB page of the pud level mapping, then
> +
> +   struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
> +               = PAGE_SIZE / 8 * 8 (pages)
> +               = PAGE_SIZE (pages)
> +
> +Where the struct_size(pmd) is the size of the struct page structs of a
> +HugeTLB page of the pmd level mapping.
> +
> +E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
> +HugeTLB page consists in 4096.
> +
> +Next, we take the pmd level mapping of the HugeTLB page as an example to
> +show the internal implementation of this optimization. There are 8 pages
> +struct page structs associated with a HugeTLB page which is pmd mapped.
> +
> +Here is how things look before optimization.
> +
> +    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
> + +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
> + |           |                     |     0     | -------------> |     0     |
> + |           |                     +-----------+                +-----------+
> + |           |                     |     1     | -------------> |     1     |
> + |           |                     +-----------+                +-----------+
> + |           |                     |     2     | -------------> |     2     |
> + |           |                     +-----------+                +-----------+
> + |           |                     |     3     | -------------> |     3     |
> + |           |                     +-----------+                +-----------+
> + |           |                     |     4     | -------------> |     4     |
> + |    PMD    |                     +-----------+                +-----------+
> + |   level   |                     |     5     | -------------> |     5     |
> + |  mapping  |                     +-----------+                +-----------+
> + |           |                     |     6     | -------------> |     6     |
> + |           |                     +-----------+                +-----------+
> + |           |                     |     7     | -------------> |     7     |
> + |           |                     +-----------+                +-----------+
> + |           |
> + |           |
> + |           |
> + +-----------+
> +
> +The value of page->compound_head is the same for all tail pages. The first
> +page of page structs (page 0) associated with the HugeTLB page contains the 4
> +page structs necessary to describe the HugeTLB. The only use of the remaining
> +pages of page structs (page 1 to page 7) is to point to page->compound_head.
> +Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> +will be used for each HugeTLB page. This will allow us to free the remaining
> +6 pages to the buddy allocator.
> +
> +Here is how things look after remapping.
> +
> +    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
> + +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
> + |           |                     |     0     | -------------> |     0     |
> + |           |                     +-----------+                +-----------+
> + |           |                     |     1     | -------------> |     1     |
> + |           |                     +-----------+                +-----------+
> + |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
> + |           |                     +-----------+                   | | | | |
> + |           |                     |     3     | ------------------+ | | | |
> + |           |                     +-----------+                     | | | |
> + |           |                     |     4     | --------------------+ | | |
> + |    PMD    |                     +-----------+                       | | |
> + |   level   |                     |     5     | ----------------------+ | |
> + |  mapping  |                     +-----------+                         | |
> + |           |                     |     6     | ------------------------+ |
> + |           |                     +-----------+                           |
> + |           |                     |     7     | --------------------------+
> + |           |                     +-----------+
> + |           |
> + |           |
> + |           |
> + +-----------+
> +
> +When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> +vmemmap pages and restore the previous mapping relationship.
> +
> +For the HugeTLB page of the pud level mapping. It is similar to the former.
> +We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
> +
> +Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
> +(e.g. aarch64) provides a contiguous bit in the translation table entries
> +that hints to the MMU to indicate that it is one of a contiguous set of
> +entries that can be cached in a single TLB entry.
> +
> +The contiguous bit is used to increase the mapping size at the pmd and pte
> +(last) level. So this type of HugeTLB page can be optimized only when its
> +size of the struct page structs is greater than 2 pages.
> +
> diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst
> index eff5fbd492d0..19f981a73a54 100644
> --- a/Documentation/vm/index.rst
> +++ b/Documentation/vm/index.rst
> @@ -31,6 +31,7 @@ descriptions of data structures and algorithms.
>     active_mm
>     arch_pgtable_helpers
>     balance
> +   commpound_pagemaps
>     cleancache
>     free_page_reporting
>     frontswap
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index c540c21e26f5..69d1f0a90e02 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -6,167 +6,7 @@
>   *
>   *     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. __NR_USED_SUBPAGE provides
> - * this upper limit. The only 'useful' information in the remaining page structs
> - * is the compound_head field, and this field is the same for all tail pages.
> - *
> - * By removing redundant page structs for HugeTLB pages, memory can be returned
> - * to the buddy allocator for other uses.
> - *
> - * Different architectures support different HugeTLB pages. For example, the
> - * following table is the HugeTLB page size supported by x86 and arm64
> - * architectures. Because arm64 supports 4k, 16k, and 64k base pages and
> - * supports contiguous entries, so it supports many kinds of sizes of HugeTLB
> - * page.
> - *
> - * +--------------+-----------+-----------------------------------------------+
> - * | Architecture | Page Size |                HugeTLB Page Size              |
> - * +--------------+-----------+-----------+-----------+-----------+-----------+
> - * |    x86-64    |    4KB    |    2MB    |    1GB    |           |           |
> - * +--------------+-----------+-----------+-----------+-----------+-----------+
> - * |              |    4KB    |   64KB    |    2MB    |    32MB   |    1GB    |
> - * |              +-----------+-----------+-----------+-----------+-----------+
> - * |    arm64     |   16KB    |    2MB    |   32MB    |     1GB   |           |
> - * |              +-----------+-----------+-----------+-----------+-----------+
> - * |              |   64KB    |    2MB    |  512MB    |    16GB   |           |
> - * +--------------+-----------+-----------+-----------+-----------+-----------+
> - *
> - * When the system boot up, every HugeTLB page has more than one struct page
> - * structs which size is (unit: pages):
> - *
> - *    struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
> - *
> - * Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
> - * of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
> - * relationship.
> - *
> - *    HugeTLB_Size = n * PAGE_SIZE
> - *
> - * Then,
> - *
> - *    struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
> - *                = n * sizeof(struct page) / PAGE_SIZE
> - *
> - * We can use huge mapping at the pud/pmd level for the HugeTLB page.
> - *
> - * For the HugeTLB page of the pmd level mapping, then
> - *
> - *    struct_size = n * sizeof(struct page) / PAGE_SIZE
> - *                = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
> - *                = sizeof(struct page) / sizeof(pte_t)
> - *                = 64 / 8
> - *                = 8 (pages)
> - *
> - * Where n is how many pte entries which one page can contains. So the value of
> - * n is (PAGE_SIZE / sizeof(pte_t)).
> - *
> - * This optimization only supports 64-bit system, so the value of sizeof(pte_t)
> - * is 8. And this optimization also applicable only when the size of struct page
> - * is a power of two. In most cases, the size of struct page is 64 bytes (e.g.
> - * x86-64 and arm64). So if we use pmd level mapping for a HugeTLB page, the
> - * size of struct page structs of it is 8 page frames which size depends on the
> - * size of the base page.
> - *
> - * For the HugeTLB page of the pud level mapping, then
> - *
> - *    struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
> - *                = PAGE_SIZE / 8 * 8 (pages)
> - *                = PAGE_SIZE (pages)
> - *
> - * Where the struct_size(pmd) is the size of the struct page structs of a
> - * HugeTLB page of the pmd level mapping.
> - *
> - * E.g.: A 2MB HugeTLB page on x86_64 consists in 8 page frames while 1GB
> - * HugeTLB page consists in 4096.
> - *
> - * Next, we take the pmd level mapping of the HugeTLB page as an example to
> - * show the internal implementation of this optimization. There are 8 pages
> - * struct page structs associated with a HugeTLB page which is pmd mapped.
> - *
> - * Here is how things look before optimization.
> - *
> - *    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
> - * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
> - * |           |                     |     0     | -------------> |     0     |
> - * |           |                     +-----------+                +-----------+
> - * |           |                     |     1     | -------------> |     1     |
> - * |           |                     +-----------+                +-----------+
> - * |           |                     |     2     | -------------> |     2     |
> - * |           |                     +-----------+                +-----------+
> - * |           |                     |     3     | -------------> |     3     |
> - * |           |                     +-----------+                +-----------+
> - * |           |                     |     4     | -------------> |     4     |
> - * |    PMD    |                     +-----------+                +-----------+
> - * |   level   |                     |     5     | -------------> |     5     |
> - * |  mapping  |                     +-----------+                +-----------+
> - * |           |                     |     6     | -------------> |     6     |
> - * |           |                     +-----------+                +-----------+
> - * |           |                     |     7     | -------------> |     7     |
> - * |           |                     +-----------+                +-----------+
> - * |           |
> - * |           |
> - * |           |
> - * +-----------+
> - *
> - * The value of page->compound_head is the same for all tail pages. The first
> - * page of page structs (page 0) associated with the HugeTLB page contains the 4
> - * page structs necessary to describe the HugeTLB. The only use of the remaining
> - * pages of page structs (page 1 to page 7) is to point to page->compound_head.
> - * Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> - * will be used for each HugeTLB page. This will allow us to free the remaining
> - * 6 pages to the buddy allocator.
> - *
> - * Here is how things look after remapping.
> - *
> - *    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
> - * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
> - * |           |                     |     0     | -------------> |     0     |
> - * |           |                     +-----------+                +-----------+
> - * |           |                     |     1     | -------------> |     1     |
> - * |           |                     +-----------+                +-----------+
> - * |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
> - * |           |                     +-----------+                   | | | | |
> - * |           |                     |     3     | ------------------+ | | | |
> - * |           |                     +-----------+                     | | | |
> - * |           |                     |     4     | --------------------+ | | |
> - * |    PMD    |                     +-----------+                       | | |
> - * |   level   |                     |     5     | ----------------------+ | |
> - * |  mapping  |                     +-----------+                         | |
> - * |           |                     |     6     | ------------------------+ |
> - * |           |                     +-----------+                           |
> - * |           |                     |     7     | --------------------------+
> - * |           |                     +-----------+
> - * |           |
> - * |           |
> - * |           |
> - * +-----------+
> - *
> - * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> - * vmemmap pages and restore the previous mapping relationship.
> - *
> - * For the HugeTLB page of the pud level mapping. It is similar to the former.
> - * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
> - *
> - * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
> - * (e.g. aarch64) provides a contiguous bit in the translation table entries
> - * that hints to the MMU to indicate that it is one of a contiguous set of
> - * entries that can be cached in a single TLB entry.
> - *
> - * The contiguous bit is used to increase the mapping size at the pmd and pte
> - * (last) level. So this type of HugeTLB page can be optimized only when its
> - * size of the struct page structs is greater than 2 pages.
> + * See Documentation/vm/compound_pagemaps.rst
>   */
>  #define pr_fmt(fmt)    "HugeTLB: " fmt
>
> --
> 2.17.1
>