linux-kernel - Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page

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Date:   Fri, 20 Nov 2020 09:42:02 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     corbet@....net, mike.kravetz@...cle.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, song.bao.hua@...ilicon.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [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