Re: [External] Re: [PATCH v2 3/3] mm: hugetlb: introduce CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Jun 16, 2021 at 7:01 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
>
>
> On 6/12/21 10:45 AM, Muchun Song wrote:
> > When using HUGETLB_PAGE_FREE_VMEMMAP, the freeing unused vmemmap pages
> > associated with each HugeTLB page is default off. Now the vmemmap is PMD
> > mapped. So there is no side effect when this feature is enabled with no
> > HugeTLB pages in the system. Someone may want to enable this feature in
> > the compiler time instead of using boot command line. So add a config to
> > make it default on when someone do not want to enable it via command line.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt |  3 +++
> >  fs/Kconfig                                      | 10 ++++++++++
> >  mm/hugetlb_vmemmap.c                            |  6 ++++--
> >  3 files changed, 17 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index a01aadafee38..8eee439d943c 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -1604,6 +1604,9 @@
> >                       on:  enable the feature
> >                       off: disable the feature
> >
> > +                     Built with CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON=y,
> > +                     the default is on.
> > +
> >                       This is not compatible with memory_hotplug.memmap_on_memory.
> >                       If both parameters are enabled, hugetlb_free_vmemmap takes
> >                       precedence over memory_hotplug.memmap_on_memory.
> > diff --git a/fs/Kconfig b/fs/Kconfig
> > index f40b5b98f7ba..e78bc5daf7b0 100644
> > --- a/fs/Kconfig
> > +++ b/fs/Kconfig
> > @@ -245,6 +245,16 @@ config HUGETLB_PAGE_FREE_VMEMMAP
> >       depends on X86_64
> >       depends on SPARSEMEM_VMEMMAP
> >
> Now that you have no longer have the directmap in basepages limitation, I suppose you no
> longer need explicit arch support for HUGETLB_PAGE_FREE_VMEMMAP right?
>
> If so, I suppose you might be able to remove the 'depends on X86_64' part and "gain"
> ARM64, PPC, etc support.

You are right. This is the next step I want to do. Also, include IA64 and RISCV.

>
>         Joao