[PATCH v12 00/13] Free some vmemmap pages of HugeTLB page

Muchun Song
2021-01-06T14:20:43Z

Hi all,

This patch series will free some vmemmap pages(struct page structures)
associated with each hugetlbpage when preallocated to save memory.

In order to reduce the difficulty of the first version of code review.
From this version, we disable PMD/huge page mapping of vmemmap if this
feature was enabled. This accutualy eliminate a bunch of the complex code
doing page table manipulation. When this patch series is solid, we cam add
the code of vmemmap page table manipulation in the future.

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.

The 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 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. HUGETLB_CGROUP_MIN_ORDER
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 returned to
the buddy allocator for other uses.

When the system boot up, every 2M HugeTLB has 512 struct page structs which
size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).

HugeTLB struct pages(8 pages) page frame(8 pages)
+-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+
| | | 0 | -------------> | 0 |
| | +-----------+ +-----------+
| | | 1 | -------------> | 1 |
| | +-----------+ +-----------+
| | | 2 | -------------> | 2 |
| | +-----------+ +-----------+
| | | 3 | -------------> | 3 |
| | +-----------+ +-----------+
| | | 4 | -------------> | 4 |
| 2MB | +-----------+ +-----------+
| | | 5 | -------------> | 5 |
| | +-----------+ +-----------+
| | | 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 | --------------------+ | | |
| 2MB | +-----------+ | | |
| | | 5 | ----------------------+ | |
| | +-----------+ | |
| | | 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.

Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
pages.

In this case, for the 1GB HugeTLB page, we can save 4094 pages. This is a
very substantial gain. On our server, run some SPDK/QEMU applications which
will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB
(1G hugepage)/~12GB (2MB hugepage) memory.

Because there are vmemmap page tables reconstruction on the freeing/allocating
path, it increases some overhead. Here are some overhead analysis.

1) Allocating 10240 2MB hugetlb pages.

a) With this patch series applied:
# time echo 10240 > /proc/sys/vm/nr_hugepages

real 0m0.166s
user 0m0.000s
sys 0m0.166s

# bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
Attaching 2 probes...

@latency:
[8K, 16K) 8360 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[16K, 32K) 1868 |@@@@@@@@@@@ |
[32K, 64K) 10 | |
[64K, 128K) 2 | |

b) Without this patch series:
# time echo 10240 > /proc/sys/vm/nr_hugepages

real 0m0.066s
user 0m0.000s
sys 0m0.066s

# bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
Attaching 2 probes...

@latency:
[4K, 8K) 10176 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[8K, 16K) 62 | |
[16K, 32K) 2 | |

Summarize: this feature is about ~2x slower than before.

2) Freeing 10240 2MB hugetlb pages.

a) With this patch series applied:
# time echo 0 > /proc/sys/vm/nr_hugepages

real 0m0.004s
user 0m0.000s
sys 0m0.002s

# bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
Attaching 2 probes...

@latency:
[16K, 32K) 10240 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|

b) Without this patch series:
# time echo 0 > /proc/sys/vm/nr_hugepages

real 0m0.077s
user 0m0.001s
sys 0m0.075s

# bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
Attaching 2 probes...

@latency:
[4K, 8K) 9950 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[8K, 16K) 287 |@ |
[16K, 32K) 3 | |

Summarize: The overhead of __free_hugepage is about ~2-4x slower than before.
But according to the allocation test above, I think that here is
also ~2x slower than before.

But why the 'real' time of patched is smaller than before? Because
In this patch series, the freeing hugetlb is asynchronous(through
kwoker).

Although the overhead has increased, the overhead is not significant. Like Mike
said, "However, remember that the majority of use cases create hugetlb pages at
or shortly after boot time and add them to the pool. So, additional overhead is
at pool creation time. There is no change to 'normal run time' operations of
getting a page from or returning a page to the pool (think page fault/unmap)".

Todo:
- Free all of the tail vmemmap pages
Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. we really can
free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page
structures has beed set PG_head flag. If we can adjust compound_head()
slightly and make compound_head() return the real head struct page when
the parameter is the tail struct page but with PG_head flag set.

In order to make the code evolution route clearer. This feature can can be
a separate patch after this patchset is solid.

- Support for other architectures (e.g. aarch64).
- Enable PMD/huge page mapping of vmemmap even if this feature was enabled.

Changelog in v11 -> v12:
- Move VM_WARN_ON_PAGE to a separate patch.
- Call __free_hugepage() with hugetlb_lock (See patch #5.) to serialize
with dissolve_free_huge_page(). It is to prepare for patch #9.
- Introduce PageHugeInflight. See patch #9.

Changelog in v10 -> v11:
- Fix compiler error when !CONFIG_HUGETLB_PAGE_FREE_VMEMMAP.
- Rework some comments and commit changes.
- Rework vmemmap_remap_free() to 3 parameters.

Thanks to Oscar and Mike's suggestions and review.

Changelog in v9 -> v10:
- Fix a bug in patch #11. Thanks to Oscar for pointing that out.
- Rework some commit log or comments. Thanks Mike and Oscar for the suggestions.
- Drop VMEMMAP_TAIL_PAGE_REUSE in the patch #3.

Thank you very much Mike and Oscar for reviewing the code.

Changelog in v8 -> v9:
- Rework some code. Very thanks to Oscar.
- Put all the non-hugetlb vmemmap functions under sparsemem-vmemmap.c.

Changelog in v7 -> v8:
- Adjust the order of patches.

Very thanks to David and Oscar. Your suggestions are very valuable.

Changelog in v6 -> v7:
- Rebase to linux-next 20201130
- Do not use basepage mapping for vmemmap when this feature is disabled.
- Rework some patchs.
[PATCH v6 08/16] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page
[PATCH v6 10/16] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page

Thanks to Oscar and Barry.

Changelog in v5 -> v6:
- Disable PMD/huge page mapping of vmemmap if this feature was enabled.
- Simplify the first version code.

Changelog in v4 -> v5:
- Rework somme comments and code in the [PATCH v4 04/21] and [PATCH v4 05/21].

Thanks to Mike and Oscar's suggestions.

Changelog in v3 -> v4:
- Move all the vmemmap functions to hugetlb_vmemmap.c.
- Make the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP default to y, if we want to
disable this feature, we should disable it by a boot/kernel command line.
- Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions.
- Initialize page table lock for vmemmap through core_initcall mechanism.

Thanks for Mike and Oscar's suggestions.

Changelog in v2 -> v3:
- Rename some helps function name. Thanks Mike.
- Rework some code. Thanks Mike and Oscar.
- Remap the tail vmemmap page with PAGE_KERNEL_RO instead of PAGE_KERNEL.
Thanks Matthew.
- Add some overhead analysis in the cover letter.
- Use vmemap pmd table lock instead of a hugetlb specific global lock.

Changelog in v1 -> v2:
- Fix do not call dissolve_compound_page in alloc_huge_page_vmemmap().
- Fix some typo and code style problems.
- Remove unused handle_vmemmap_fault().
- Merge some commits to one commit suggested by Mike.

Muchun Song (13):
mm/memory_hotplug: Factor out bootmem core functions to bootmem_info.c
mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP
mm: Introduce VM_WARN_ON_PAGE macro
mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page
mm/hugetlb: Defer freeing of HugeTLB pages
mm/hugetlb: Allocate the vmemmap pages associated with each HugeTLB
page
mm/hugetlb: Set the PageHWPoison to the raw error page
mm/hugetlb: Flush work when dissolving a HugeTLB page
mm/hugetlb: Introduce PageHugeInflight
mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap
mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate
mm/hugetlb: Gather discrete indexes of tail page
mm/hugetlb: Optimize the code with the help of the compiler

Documentation/admin-guide/kernel-parameters.txt | 14 ++
Documentation/admin-guide/mm/hugetlbpage.rst | 3 +
arch/x86/mm/init_64.c | 13 +-
fs/Kconfig | 18 ++
include/linux/bootmem_info.h | 65 ++++++
include/linux/hugetlb.h | 37 ++++
include/linux/hugetlb_cgroup.h | 15 +-
include/linux/memory_hotplug.h | 27 ---
include/linux/mm.h | 5 +
include/linux/mmdebug.h | 8 +
mm/Makefile | 2 +
mm/bootmem_info.c | 124 +++++++++++
mm/hugetlb.c | 218 +++++++++++++++++--
mm/hugetlb_vmemmap.c | 278 ++++++++++++++++++++++++
mm/hugetlb_vmemmap.h | 45 ++++
mm/memory_hotplug.c | 116 ----------
mm/sparse-vmemmap.c | 256 ++++++++++++++++++++++
mm/sparse.c | 1 +
18 files changed, 1073 insertions(+), 172 deletions(-)
create mode 100644 include/linux/bootmem_info.h
create mode 100644 mm/bootmem_info.c
create mode 100644 mm/hugetlb_vmemmap.c
create mode 100644 mm/hugetlb_vmemmap.h

--
2.11.0