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@xxxxxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
Documentation/vm/index.rst | 1 +
Documentation/vm/vmemmap_dedup.rst | 175 +++++++++++++++++++++++++++++
mm/hugetlb_vmemmap.c | 168 +--------------------------
3 files changed, 177 insertions(+), 167 deletions(-)
create mode 100644 Documentation/vm/vmemmap_dedup.rst
diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst
index 44365c4574a3..2fb612bb72c9 100644
--- a/Documentation/vm/index.rst
+++ b/Documentation/vm/index.rst
@@ -37,5 +37,6 @@ algorithms. If you are looking for advice on simply allocating memory, see the
transhuge
unevictable-lru
vmalloced-kernel-stacks
+ vmemmap_dedup
z3fold
zsmalloc
diff --git a/Documentation/vm/vmemmap_dedup.rst b/Documentation/vm/vmemmap_dedup.rst
new file mode 100644
index 000000000000..8143b2ce414d
--- /dev/null
+++ b/Documentation/vm/vmemmap_dedup.rst
@@ -0,0 +1,175 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _vmemmap_dedup:
+
+==================================
+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 1 to 7 to page 0. Only 1 page of page structs
+will be used for each HugeTLB page. This will allow us to free the remaining
+7 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 | ---------------^ ^ ^ ^ ^ ^ ^
+ | | +-----------+ | | | | | |
+ | | | 2 | -----------------+ | | | | |
+ | | +-----------+ | | | | |
+ | | | 3 | -------------------+ | | | |
+ | | +-----------+ | | | |
+ | | | 4 | ---------------------+ | | |
+ | PMD | +-----------+ | | |
+ | level | | 5 | -----------------------+ | |
+ | mapping | +-----------+ | |
+ | | | 6 | -------------------------+ |
+ | | +-----------+ |
+ | | | 7 | ---------------------------+
+ | | +-----------+
+ | |
+ | |
+ | |
+ +-----------+
+
+When a HugeTLB is freed to the buddy system, we should allocate 7 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 - 1) 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 1 page.
+
+Notice: The head vmemmap page is not freed to the buddy allocator and all
+tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
+more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page)
+associated with each HugeTLB page. The compound_head() can handle this
+correctly (more details refer to the comment above compound_head()).
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 791626983c2e..dbaa837b19c6 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -6,173 +6,7 @@
*
* 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.
- *
- * 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 1 to 7 to page 0. Only 1 page of page structs
- * will be used for each HugeTLB page. This will allow us to free the remaining
- * 7 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 | ---------------^ ^ ^ ^ ^ ^ ^
- * | | +-----------+ | | | | | |
- * | | | 2 | -----------------+ | | | | |
- * | | +-----------+ | | | | |
- * | | | 3 | -------------------+ | | | |
- * | | +-----------+ | | | |
- * | | | 4 | ---------------------+ | | |
- * | PMD | +-----------+ | | |
- * | level | | 5 | -----------------------+ | |
- * | mapping | +-----------+ | |
- * | | | 6 | -------------------------+ |
- * | | +-----------+ |
- * | | | 7 | ---------------------------+
- * | | +-----------+
- * | |
- * | |
- * | |
- * +-----------+
- *
- * When a HugeTLB is freed to the buddy system, we should allocate 7 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 - 1) 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 1 page.
- *
- * Notice: The head vmemmap page is not freed to the buddy allocator and all
- * tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
- * more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page)
- * associated with each HugeTLB page. The compound_head() can handle this
- * correctly (more details refer to the comment above compound_head()).
+ * See Documentation/vm/vmemmap_dedup.rst
*/
#define pr_fmt(fmt) "HugeTLB: " fmt
--
2.17.2
