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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Mon, 25 Jan 2021 12:06:09 +0800
Message-ID: <CAMZfGtUGT6UP3aBEGmMvahOu5akvqoVoiXQqQvAdY82P6VGiTg@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v13 02/12] mm: hugetlb: introduce a new
 config HUGETLB_PAGE_FREE_VMEMMAP
To:     David Rientjes <rientjes@google.com>,
        Randy Dunlap <rdunlap@infradead.org>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>, mingo@redhat.com,
        bp@alien8.de, x86@kernel.org, hpa@zytor.com,
        dave.hansen@linux.intel.com, luto@kernel.org,
        Peter Zijlstra <peterz@infradead.org>, viro@zeniv.linux.org.uk,
        Andrew Morton <akpm@linux-foundation.org>, paulmck@kernel.org,
        mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com,
        oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de,
        Mina Almasry <almasrymina@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
        David Hildenbrand <david@redhat.com>,
        =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= 
        <naoya.horiguchi@nec.com>,
        Xiongchun duan <duanxiongchun@bytedance.com>,
        linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Mon, Jan 25, 2021 at 7:58 AM David Rientjes <rientjes@google.com> wrote:
>
>
> On Sun, 17 Jan 2021, Muchun Song wrote:
>
> > The HUGETLB_PAGE_FREE_VMEMMAP option is used to enable the freeing
> > of unnecessary vmemmap associated with HugeTLB pages. The config
> > option is introduced early so that supporting code can be written
> > to depend on the option. The initial version of the code only
> > provides support for x86-64.
> >
> > Like other code which frees vmemmap, this config option depends on
> > HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
> > used to register bootmem info. Therefore, make sure
> > register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
> > is defined.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > Reviewed-by: Oscar Salvador <osalvador@suse.de>
> > Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
> > ---
> >  arch/x86/mm/init_64.c |  2 +-
> >  fs/Kconfig            | 18 ++++++++++++++++++
> >  2 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> > index 0a45f062826e..0435bee2e172 100644
> > --- a/arch/x86/mm/init_64.c
> > +++ b/arch/x86/mm/init_64.c
> > @@ -1225,7 +1225,7 @@ static struct kcore_list kcore_vsyscall;
> >
> >  static void __init register_page_bootmem_info(void)
> >  {
> > -#ifdef CONFIG_NUMA
> > +#if defined(CONFIG_NUMA) || defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
> >       int i;
> >
> >       for_each_online_node(i)
> > diff --git a/fs/Kconfig b/fs/Kconfig
> > index 976e8b9033c4..e7c4c2a79311 100644
> > --- a/fs/Kconfig
> > +++ b/fs/Kconfig
> > @@ -245,6 +245,24 @@ config HUGETLBFS
> >  config HUGETLB_PAGE
> >       def_bool HUGETLBFS
> >
> > +config HUGETLB_PAGE_FREE_VMEMMAP
> > +     def_bool HUGETLB_PAGE
>
> I'm not sure I understand the rationale for providing this help text if
> this is def_bool depending on CONFIG_HUGETLB_PAGE.  Are you intending that
> this is actually configurable and we want to provide guidance to the admin
> on when to disable it (which it currently doesn't)?  If not, why have the
> help text?

This is __not__ configurable. Seems like a comment to help others
understand this option. Like Randy said.

Thanks.

>
> > +     depends on X86_64
> > +     depends on SPARSEMEM_VMEMMAP
> > +     depends on HAVE_BOOTMEM_INFO_NODE
> > +     help
> > +       The option HUGETLB_PAGE_FREE_VMEMMAP allows for the freeing of
> > +       some vmemmap pages associated with pre-allocated HugeTLB pages.
> > +       For example, on X86_64 6 vmemmap pages of size 4KB each can be
> > +       saved for each 2MB HugeTLB page.  4094 vmemmap pages of size 4KB
> > +       each can be saved for each 1GB HugeTLB page.
> > +
> > +       When a HugeTLB page is allocated or freed, the vmemmap array
> > +       representing the range associated with the page will need to be
> > +       remapped.  When a page is allocated, vmemmap pages are freed
> > +       after remapping.  When a page is freed, previously discarded
> > +       vmemmap pages must be allocated before remapping.
> > +
> >  config MEMFD_CREATE
> >       def_bool TMPFS || HUGETLBFS
> >