mirror of
https://github.com/torvalds/linux.git
synced 2024-11-26 06:02:05 +00:00
4dc7d37370
Mostly rewording, but remove entirely the copy of page_fixed_fake_head() in the documentation; we can refer people to the actual source if necessary. Link: https://lkml.kernel.org/r/20240326171045.410737-10-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
232 lines
12 KiB
ReStructuredText
232 lines
12 KiB
ReStructuredText
|
|
.. 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 its 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 262144 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``.
|
|
|
|
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).
|
|
For powerpc equivalent details see Documentation/arch/powerpc/vmemmap_dedup.rst
|
|
|
|
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 | --------------------------+
|
|
| | +-----------+
|
|
| |
|
|
| |
|
|
| |
|
|
+-----------+
|