| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ========================================= |
| A vmemmap diet for HugeTLB and Device DAX |
| ========================================= |
| |
| HugeTLB |
| ======= |
| |
| This section is to explain how HugeTLB Vmemmap Optimization (HVO) works. |
| |
| The ``struct page`` structures 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 |
| ``struct page``. |
| |
| HugeTLB pages consist of multiple base page size pages and is supported by many |
| architectures. See Documentation/admin-guide/mm/hugetlbpage.rst 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 ``struct page``. |
| |
| Within the HugeTLB subsystem, only the first 4 ``struct page`` are used to |
| contain unique information about a HugeTLB page. ``__NR_USED_SUBPAGE`` provides |
| this upper limit. The only 'useful' information in the remaining ``struct page`` |
| is the compound_head field, and this field is the same for all tail pages. |
| |
| By removing redundant ``struct page`` 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 ``struct page`` (page 0) associated with the HugeTLB page contains the 4 |
| ``struct page`` necessary to describe the HugeTLB. The only use of the remaining |
| pages of ``struct page`` (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 ``struct page`` |
| 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. There is only **one** head ``struct page``, the tail |
| ``struct page`` with ``PG_head`` are fake head ``struct page``. We need an |
| approach to distinguish between those two different types of ``struct page`` so |
| that ``compound_head()`` can return the real head ``struct page`` when the |
| parameter is the tail ``struct page`` but with ``PG_head``. The following code |
| snippet describes how to distinguish between real and fake head ``struct page``. |
| |
| .. code-block:: c |
| |
| if (test_bit(PG_head, &page->flags)) { |
| unsigned long head = READ_ONCE(page[1].compound_head); |
| |
| if (head & 1) { |
| if (head == (unsigned long)page + 1) |
| /* head struct page */ |
| else |
| /* tail struct page */ |
| } else { |
| /* head struct page */ |
| } |
| } |
| |
| We can safely access the field of the **page[1]** with ``PG_head`` because the |
| page is a compound page composed with at least two contiguous pages. |
| The implementation refers to ``page_fixed_fake_head()``. |
| |
| Device DAX |
| ========== |
| |
| The device-dax interface uses the same tail deduplication technique explained |
| in the previous chapter, except when used with the vmemmap in |
| the device (altmap). |
| |
| The following page sizes are supported in DAX: PAGE_SIZE (4K on x86_64), |
| PMD_SIZE (2M on x86_64) and PUD_SIZE (1G on x86_64). |
| |
| The differences with HugeTLB are relatively minor. |
| |
| It only use 3 ``struct page`` for storing all information as opposed |
| to 4 on HugeTLB pages. |
| |
| There's no remapping of vmemmap given that device-dax memory is not part of |
| System RAM ranges initialized at boot. Thus the tail page deduplication |
| happens at a later stage when we populate the sections. HugeTLB reuses the |
| the head vmemmap page representing, whereas device-dax reuses the tail |
| vmemmap page. This results in only half of the savings compared to HugeTLB. |
| |
| Deduplicated tail pages are not mapped read-only. |
| |
| Here's how things look like on device-dax after the sections are populated:: |
| |
| +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ |
| | | | 0 | -------------> | 0 | |
| | | +-----------+ +-----------+ |
| | | | 1 | -------------> | 1 | |
| | | +-----------+ +-----------+ |
| | | | 2 | ----------------^ ^ ^ ^ ^ ^ |
| | | +-----------+ | | | | | |
| | | | 3 | ------------------+ | | | | |
| | | +-----------+ | | | | |
| | | | 4 | --------------------+ | | | |
| | PMD | +-----------+ | | | |
| | level | | 5 | ----------------------+ | | |
| | mapping | +-----------+ | | |
| | | | 6 | ------------------------+ | |
| | | +-----------+ | |
| | | | 7 | --------------------------+ |
| | | +-----------+ |
| | | |
| | | |
| | | |
| +-----------+ |
