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    Subject[PATCH v7 00/15] Free some vmemmap pages of hugetlb page
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    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 4088 pages(There are
    4096 pages for struct page structs, we reserve 2 pages for vmemmap and 8
    pages for page tables. So we can save 4088 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)/~11GB
    (2MB hugepage, the worst case is 10GB while the best is 12GB) 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:
    1. 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.
    2. Support for other architectures (e.g. aarch64).
    3. Enable PMD/huge page mapping of vmemmap even if this feature was enabled.

    Changelog in v7:
    1. Rebase to linux-next 20201130
    2. Do not use basepage mapping for vmemmap when this feature is disabled.
    Thanks to Oscar and Barry.
    3. 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

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

    Changelog in v5:
    1. 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 v4:
    1. Move all the vmemmap functions to hugetlb_vmemmap.c.
    2. 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.
    3. Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions.
    4. Initialize page table lock for vmemmap through core_initcall mechanism.

    Thanks for Mike and Oscar's suggestions.

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

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

    Muchun Song (15):
    mm/memory_hotplug: Move bootmem info registration API to
    bootmem_info.c
    mm/memory_hotplug: Move {get,put}_page_bootmem() to bootmem_info.c
    mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP
    mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate
    mm/bootmem_info: Introduce {free,prepare}_vmemmap_page()
    mm/hugetlb: Disable freeing vmemmap if struct page size is not power
    of two
    x86/mm/64: Disable PMD page mapping of vmemmap
    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 hugetlb page
    mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap
    mm/hugetlb: Gather discrete indexes of tail page
    mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct
    page

    Documentation/admin-guide/kernel-parameters.txt | 9 +
    Documentation/admin-guide/mm/hugetlbpage.rst | 3 +
    arch/x86/mm/init_64.c | 13 +-
    fs/Kconfig | 14 +
    include/linux/bootmem_info.h | 64 +++++
    include/linux/hugetlb.h | 35 +++
    include/linux/hugetlb_cgroup.h | 15 +-
    include/linux/memory_hotplug.h | 27 --
    mm/Makefile | 2 +
    mm/bootmem_info.c | 124 ++++++++
    mm/hugetlb.c | 144 ++++++++--
    mm/hugetlb_vmemmap.c | 367 ++++++++++++++++++++++++
    mm/hugetlb_vmemmap.h | 79 +++++
    mm/memory_hotplug.c | 116 --------
    mm/sparse.c | 1 +
    15 files changed, 834 insertions(+), 179 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

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