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Date:   Thu, 3 Mar 2022 06:59:22 -0800
From:   Luis Chamberlain <mcgrof@kernel.org>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Xiongchun duan <duanxiongchun@bytedance.com>,
        Muchun Song <smuchun@gmail.com>,
        Adam Manzanares <a.manzanares@samsung.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v2 3/3] mm: hugetlb: add hugetlb_free_vmemmap sysctl
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On Thu, Mar 03, 2022 at 07:15:05PM +0800, Muchun Song wrote:
> On Thu, Mar 3, 2022 at 5:25 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
> >
> > On Wed, Mar 02, 2022 at 04:37:58PM +0800, Muchun Song wrote:
> > > We must add "hugetlb_free_vmemmap=on" to boot cmdline and reboot the
> > > server to enable the feature of freeing vmemmap pages of HugeTLB
> > > pages. Rebooting usually taske a long time. Add a sysctl to enable
> > > the feature at runtime and do not need to reboot.
> > >
> > > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > > ---
> > >  Documentation/admin-guide/sysctl/vm.rst | 13 ++++++++++
> > >  include/linux/memory_hotplug.h          |  9 +++++++
> > >  mm/hugetlb_vmemmap.c                    | 42 ++++++++++++++++++++++++++++-----
> > >  mm/hugetlb_vmemmap.h                    |  4 +++-
> > >  mm/memory_hotplug.c                     |  5 ++++
> > >  5 files changed, 66 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> > > index f4804ce37c58..01f18e6cc227 100644
> > > --- a/Documentation/admin-guide/sysctl/vm.rst
> > > +++ b/Documentation/admin-guide/sysctl/vm.rst
> > > @@ -561,6 +561,19 @@ Change the minimum size of the hugepage pool.
> > >  See Documentation/admin-guide/mm/hugetlbpage.rst
> > >
> > >
> > > +hugetlb_free_vmemmap
> > > +====================
> > > +
> > > +A toggle value indicating if vmemmap pages are allowed to be optimized.
> > > +If it is off (0), then it can be set true (1).  Once true, the vmemmap
> > > +pages associated with each HugeTLB page will be optimized, and the toggle
> > > +cannot be set back to false.  It only optimizes the subsequent allocation
> > > +of HugeTLB pages from buddy system, while already allocated HugeTLB pages
> > > +will not be optimized.
> >
> > The commit log or documentation does not descrie why its safe to toggle
> > one way and not the other?
> >
> 
> I thought it was easy to handle the transition from disable to enable
> (code is simple).  I might be wrong. I'll try to handle the other side in
> the next version if it is not hard to handle.

You should do the homework and explain why something is not possible.
And if you are enabling to disable something why is it safe to do so
at runtime?

  Luis