Re: [External] Re: [PATCH v3 03/21] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Muchun Song <songmuchun@bytedance.com>]

On Mon, Nov 9, 2020 at 9:52 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Sun, Nov 08, 2020 at 10:10:55PM +0800, 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. Now only support x86.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  arch/x86/mm/init_64.c |  2 +-
> >  fs/Kconfig            | 16 ++++++++++++++++
> >  mm/bootmem_info.c     |  3 +--
> >  3 files changed, 18 insertions(+), 3 deletions(-)
> >
> > 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..21b8d39a9715 100644
> > --- a/fs/Kconfig
> > +++ b/fs/Kconfig
> > @@ -245,6 +245,22 @@ config HUGETLBFS
> >  config HUGETLB_PAGE
> >       def_bool HUGETLBFS
> >
> > +config HUGETLB_PAGE_FREE_VMEMMAP
> > +     bool "Free unused vmemmap associated with HugeTLB pages"
> > +     default y
> > +     depends on X86
> > +     depends on HUGETLB_PAGE
> > +     depends on SPARSEMEM_VMEMMAP
> > +     depends on HAVE_BOOTMEM_INFO_NODE
> > +     help
> > +       There are many struct page structures associated with each HugeTLB
> > +       page. But we only use a few struct page structures. In this case,
> > +       it wastes some memory. It is better to free the unused struct page
> > +       structures to buddy system which can save some memory. For
> > +       architectures that support it, say Y here.
> > +
> > +       If unsure, say N.
>
> I am not sure the above is useful for someone who needs to decide
> whether he needs/wants to enable this or not.
> I think the above fits better in a Documentation part.
>
> I suck at this, but what about the following, or something along those
> lines?
>
> "
> When using SPARSEMEM_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 4095 per 1GB HugeTLB page.
> 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.
> Therefore, this is a trade-off between saving memory and
> increasing time in allocation/free path.
> "

Will do. Thanks for your suggestions.

>
> It would be also great to point out that this might be a
> trade-off between saving up memory and increasing the cost
> of certain operations on allocation/free path.
> That is why I mentioned it there.

OK, I will add this to the Documentation part, thanks.

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun