linux-kernel - Re: [External] Re: [PATCH v18 6/9] mm: hugetlb: add a kernel parameter hugetlb_free

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Date:   Thu, 11 Mar 2021 14:36:03 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <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>,
        Alexander Viro <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>,
        Oscar Salvador <osalvador@...e.de>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Joao Martins <joao.m.martins@...cle.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>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [External] Re: [PATCH v18 6/9] mm: hugetlb: add a kernel
 parameter hugetlb_free_vmemmap

On Wed, Mar 10, 2021 at 11:37 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Mon 08-03-21 18:28:04, Muchun Song wrote:
> > Add a kernel parameter hugetlb_free_vmemmap to enable the feature of
> > freeing unused vmemmap pages associated with each hugetlb page on boot.
> >
> > We disables PMD mapping of vmemmap pages for x86-64 arch when this
> > feature is enabled. Because vmemmap_remap_free() depends on vmemmap
> > being base page mapped.
> >
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > Reviewed-by: Oscar Salvador <osalvador@...e.de>
> > Reviewed-by: Barry Song <song.bao.hua@...ilicon.com>
> > Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
> > Tested-by: Chen Huang <chenhuang5@...wei.com>
> > Tested-by: Bodeddula Balasubramaniam <bodeddub@...zon.com>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
> >  Documentation/admin-guide/mm/hugetlbpage.rst    |  3 +++
> >  arch/x86/mm/init_64.c                           |  8 ++++++--
> >  include/linux/hugetlb.h                         | 19 +++++++++++++++++++
> >  mm/hugetlb_vmemmap.c                            | 24 ++++++++++++++++++++++++
> >  5 files changed, 66 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 04545725f187..de91d54573c4 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -1557,6 +1557,20 @@
> >                       Documentation/admin-guide/mm/hugetlbpage.rst.
> >                       Format: size[KMG]
> >
> > +     hugetlb_free_vmemmap=
> > +                     [KNL] When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set,
> > +                     this controls freeing unused vmemmap pages associated
> > +                     with each HugeTLB page. When this option is enabled,
> > +                     we disable PMD/huge page mapping of vmemmap pages which
> > +                     increase page table pages. So if a user/sysadmin only
> > +                     uses a small number of HugeTLB pages (as a percentage
> > +                     of system memory), they could end up using more memory
> > +                     with hugetlb_free_vmemmap on as opposed to off.
> > +                     Format: { on | off (default) }
>
> Please note this is an admin guide and for those this seems overly low

OK.

> level. I would use something like the following
>                         [KNL] Reguires CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
>                         enabled.
>                         Allows heavy hugetlb users to free up some more
>                         memory (6 * PAGE_SIZE for each 2MB hugetlb
>                         page).
>                         This feauture is not free though. Large page
>                         tables are not use to back vmemmap pages which
>                         can lead to a performance degradation for some
>                         workloads. Also there will be memory allocation
>                         required when hugetlb pages are freed from the
>                         pool which can lead to corner cases under heavy
>                         memory pressure.

Very thanks. I will update this.

> > +
> > +                     on:  enable the feature
> > +                     off: disable the feature
> > +
> >       hung_task_panic=
> >                       [KNL] Should the hung task detector generate panics.
> >                       Format: 0 | 1
> --
> Michal Hocko
> SUSE Labs