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

[Muchun Song]

On Wed, Nov 11, 2020 at 3:50 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Tue, Nov 10, 2020 at 11:31:31AM -0800, Mike Kravetz wrote:
> > On 11/9/20 5:52 AM, Oscar Salvador 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@xxxxxxxxxxxxx>
> > >> ---
> > >>  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.
> > > "
> > >
> > > 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.
> >
> > Yes, this is somewhat a trade-off.
> >
> > As a config option, this is something that would likely be decided by
> > distros.  I almost hate to suggest this, but is it something that an
> > end user would want to decide?  Is this something that perhaps should
> > be a boot/kernel command line option?
>
> I don't like config options.  I like boot options even less.  I don't
> know how to describe to an end-user whether they should select this
> or not.  Is there a way to make this not a tradeoff?  Or make the
> tradeoff so minimal as to be not worth describing?  (do we have numbers
> for the worst possible situation when enabling this option?)
>
> I haven't read through these patches in detail, so maybe we do this
> already, but when we free the pages to the buddy allocator, do we retain
> the third page to use for the PTEs (and free pages 3-7), or do we allocate
> a separate page for the PTES and free pages 2-7?

Sorry for missing this reply. It is a good idea. I will start an investigation
and implement this. Thanks Matthew.



-- 
Yours,
Muchun