Re: [External] Re: [PATCH v13 02/12] mm: hugetlb: introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Jan 25, 2021 at 12:09 PM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> On 1/24/21 8:06 PM, Muchun Song wrote:
> > 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.
>
> Yes, it could be written with '#' (or "comment") comment syntax instead of as help text.

Got it. I will update in the next version. Thanks.

>
> 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
> >>>
> >
>
>
> --
> ~Randy
>