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    SubjectRe: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page
    On Fri 20-11-20 14:43:04, Muchun Song wrote:
    [...]

    Thanks for improving the cover letter and providing some numbers. I have
    only glanced through the patchset because I didn't really have more time
    to dive depply into them.

    Overall it looks promissing. To summarize. I would prefer to not have
    the feature enablement controlled by compile time option and the kernel
    command line option should be opt-in. I also do not like that freeing
    the pool can trigger the oom killer or even shut the system down if no
    oom victim is eligible.

    One thing that I didn't really get to think hard about is what is the
    effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be
    invalid when racing with the split. How do we enforce that this won't
    blow up?

    I have also asked in a previous version whether the vmemmap manipulation
    should be really unconditional. E.g. shortlived hugetlb pages allocated
    from the buddy allocator directly rather than for a pool. Maybe it
    should be restricted for the pool allocation as those are considered
    long term and therefore the overhead will be amortized and freeing path
    restrictions better understandable.

    > Documentation/admin-guide/kernel-parameters.txt | 9 +
    > Documentation/admin-guide/mm/hugetlbpage.rst | 3 +
    > arch/x86/include/asm/hugetlb.h | 17 +
    > arch/x86/include/asm/pgtable_64_types.h | 8 +
    > arch/x86/mm/init_64.c | 7 +-
    > fs/Kconfig | 14 +
    > include/linux/bootmem_info.h | 78 +++
    > include/linux/hugetlb.h | 19 +
    > include/linux/hugetlb_cgroup.h | 15 +-
    > include/linux/memory_hotplug.h | 27 -
    > mm/Makefile | 2 +
    > mm/bootmem_info.c | 124 ++++
    > mm/hugetlb.c | 163 ++++-
    > mm/hugetlb_vmemmap.c | 765 ++++++++++++++++++++++++
    > mm/hugetlb_vmemmap.h | 103 ++++

    I will need to look closer but I suspect that a non-trivial part of the
    vmemmap manipulation really belongs to mm/sparse-vmemmap.c because the
    split and remapping shouldn't really be hugetlb specific. Sure hugetlb
    knows how to split but all the splitting should be implemented in
    vmemmap proper.

    > mm/memory_hotplug.c | 116 ----
    > mm/sparse.c | 5 +-
    > 17 files changed, 1295 insertions(+), 180 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

    Thanks!
    --
    Michal Hocko
    SUSE Labs

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