linux-kernel - Re: [External] Re: [PATCH v3 03/21] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE

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Date:   Wed, 11 Nov 2020 11:28:15 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, viro@...iv.linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v3 03/21] mm/hugetlb: Introduce a new
 config HUGETLB_PAGE_FREE_VMEMMAP

On Wed, Nov 11, 2020 at 3:31 AM Mike Kravetz <mike.kravetz@...cle.com> 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@...edance.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.
> > "
> >
> > 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?

Yeah, it already has a boot/kernel command line option named
"hugetlb_free_vmemmap". We can refer to

  [PATCH v3 18/21] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap

Thanks :)


>
> --
> Mike Kravetz



--
Yours,
Muchun