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

[Joao Martins <joao.m.martins@oracle.com>]

On 6/21/21 2:12 PM, Muchun Song wrote:
> 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.
> 

Good point.

FWIW, I wanted to avoid the use of the word 'remap' solely because that might be
implementation specific e.g. hugetlbfs remaps struct pages, whereas device-dax will
populate struct pages already with the tail dedup.

Me using 'compound_pagemaps' was short of 'compound struct page map' or 'compound vmemmap'.

Maybe one other alternative is 'tail_dedup.rst' or 'metadata_dedup.rst' ? That's probably
more generic to what really is being done.

Regardless, I am also good with 'vmemmap_remap.rst' if that's what folks prefer.


>>
>> 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
>>
>