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Date:   Fri, 20 Nov 2020 20:40:46 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, viro@...iv.linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oscar Salvador <osalvador@...e.de>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v5 00/21] Free some vmemmap pages of
 hugetlb page

On Fri, Nov 20, 2020 at 4:42 PM Michal Hocko <mhocko@...e.com> wrote:
>
> 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.

Hi Michal,

I have replied to you about those questions on the other mail thread.

Thanks.

>
> 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?

This feature depends on the CONFIG_SPARSEMEM_VMEMMAP,
in this case, the pfn_to_page can work. The return value of the
pfn_to_page is actually the address of it's struct page struct.
I can not figure out where the problem is. Can you describe the
problem in detail please? Thanks.

>
> 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.

Yeah, I agree with you. This can be an optimization. And we can
add it to the todo list and implement it in the future. Now the patch
series is already huge.

>
> >  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



-- 
Yours,
Muchun

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