Re: [External] Re: [PATCH v9 02/11] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Muchun Song]

On Wed, Dec 16, 2020 at 9:04 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 12/13/20 7:45 AM, Muchun Song wrote:
> > The purpose of introducing HUGETLB_PAGE_FREE_VMEMMAP is to configure
> > whether to enable the feature of freeing unused vmemmap associated with
> > HugeTLB pages. And this is just for dependency check. Now only support
> > x86-64.
> >
> > Because this config depends on HAVE_BOOTMEM_INFO_NODE. And the function
> > of the register_page_bootmem_info() is aimed to register bootmem info.
> > So we should register bootmem info when this config is enabled.
>
> Suggested commit message rewording?
>
> 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.

Thank Mike. Will update.

>
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > ---
> >  arch/x86/mm/init_64.c |  2 +-
> >  fs/Kconfig            | 15 +++++++++++++++
> >  2 files changed, 16 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..4c3a9c614983 100644
> > --- a/fs/Kconfig
> > +++ b/fs/Kconfig
> > @@ -245,6 +245,21 @@ config HUGETLBFS
> >  config HUGETLB_PAGE
> >       def_bool HUGETLBFS
> >
> > +config HUGETLB_PAGE_FREE_VMEMMAP
> > +     def_bool HUGETLB_PAGE
> > +     depends on X86_64
> > +     depends on SPARSEMEM_VMEMMAP
> > +     depends on HAVE_BOOTMEM_INFO_NODE
> > +     help
> > +       When using HUGETLB_PAGE_FREE_VMEMMAP, the system can save up some
> > +       memory from pre-allocated HugeTLB pages when they are not used.
> > +       6 pages per HugeTLB page of the pmd level mapping and (PAGE_SIZE - 2)
> > +       pages per HugeTLB page of the pud level mapping.
> > +
> > +       When the pages are going to be used or freed up, the vmemmap array
> > +       representing that range needs to be remapped again and the pages
> > +       we discarded earlier need to be rellocated again.
>
> I see the previous discussion with David about wording here.  How about
> leaving the functionality description general, and provide a specific
> example for x86_64?  As mentioned we can always update when new arch support
> is added.  Suggested text?

Good suggestion. Thanks.

>
>         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 before remapping.
>
> --
> Mike Kravetz
>
> > +
> >  config MEMFD_CREATE
> >       def_bool TMPFS || HUGETLBFS
> >
> >



-- 
Yours,
Muchun