On Fri, Jun 18, 2021 at 2:46 AM Joao Martins <joao.m.martins@xxxxxxxxxx> 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@xxxxxxxxxxxxx> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> > Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx> > Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx> > --- > 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@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 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 >
