Re: [PATCH v2 3/3] mm: hugetlb: add hugetlb_free_vmemmap sysctl

[Muchun Song <songmuchun@bytedance.com>]

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.

Thanks Luis.