Re: [External] Re: [PATCH v7 03/15] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Muchun Song]

On Mon, Dec 7, 2020 at 8:47 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 07.12.20 13:42, Muchun Song wrote:
> > On Mon, Dec 7, 2020 at 8:19 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 30.11.20 16:18, 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.
> >>
> >> x86 - i386 and x86-64? (I assume the latter only ;) )
> >
> > Yeah, you are right. Only the latter support SPARSEMEM_VMEMMAP.
> >
> >>
> >>>
> >>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> >>> ---
> >>>  arch/x86/mm/init_64.c |  2 +-
> >>>  fs/Kconfig            | 14 ++++++++++++++
> >>>  2 files changed, 15 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;
> >>>
> >>
> >> Why does this hunk belong into this patch? Looks like this should go
> >> into another patch.
> >
> > Of course can. But Mike suggests that it is better to use it when
> > introducing a new config. Because this config depends on
> > HAVE_BOOTMEM_INFO_NODE. And register_page_bootmem_info
> > is aimed to register bootmem info. So maybe it is reasonable from
> > this point of view. What is your opinion?
> >
>
> Ah, I see. Maybe mention in the patch description, because the
> "Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP" part left me
> clueless. Stumbling over this change only left me rather clueless.

OK, I will improve the commit log. Thanks.

>
> >>
> >>>       for_each_online_node(i)
> >>> diff --git a/fs/Kconfig b/fs/Kconfig
> >>> index 976e8b9033c4..4961dd488444 100644
> >>> --- a/fs/Kconfig
> >>> +++ b/fs/Kconfig
> >>> @@ -245,6 +245,20 @@ config HUGETLBFS
> >>>  config HUGETLB_PAGE
> >>>       def_bool HUGETLBFS
> >>>
> >>> +config HUGETLB_PAGE_FREE_VMEMMAP
> >>> +     def_bool HUGETLB_PAGE
> >>> +     depends on X86
> >>> +     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 2MB HugeTLB page and 4094 per 1GB HugeTLB page.
> >>
> >> Calculations only apply to 4k base pages, no?
> >
> > No, if the base page is not 4k, we also can free 6 pages.
> >
> > For example:
> >
> > If the base page size is 64k, the PMD huge page size is 512MB. We also
>
> Note that 2MB huge pages on arm64 with 64k base pages are possible as
> well. Also, I think powerpc always has 16MB huge pages, independent of
> base page sizes.

I see now. Now only support x86-64, you are right, I should point out the base
page size. When supporting more architectures in the future. We can update
the message here. :)

Thanks.

>
>
> --
> Thanks,
>
> David / dhildenb
>


-- 
Yours,
Muchun