Re: [PATCH v2 1/3] mm: hugetlb: disable freeing vmemmap pages when struct page crosses page boundaries

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Mar 3, 2022 at 8:25 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 3/2/22 00:37, Muchun Song wrote:
> > If CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON is enabled and the size
> > of "struct page" is not power of two, we cannot optimize vmemmap pages
> > of HugeTLB pages. We should disable this feature in this case.
>
> I'll let you reply to the question from Luis, but IIUC there is no issue
> today as "struct page" is certainly a power of two.  This is more future
> looking.  Correct?

Partly right. The size of "struct page" is not the power of two if
!CONFIG_MEMCG && CONFIG_SLAB on x86_64.  But it is not
a conventional configuration nowadays.  So it is not a critical
problem. I am not sure if a Fixes tag is necessary.

>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  mm/hugetlb_vmemmap.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index b3118dba0518..836d1117f08b 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -121,6 +121,17 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
> >       if (!hugetlb_free_vmemmap_enabled())
> >               return;
> >
> > +     if (IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON) &&
> > +         !is_power_of_2(sizeof(struct page))) {
> > +             /*
> > +              * The hugetlb_free_vmemmap_enabled_key can be enabled when
> > +              * CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON. It should
> > +              * be disabled if "struct page" crosses page boundaries.
> > +              */
> > +             static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
>
> Should we possibly print a warning here as in the routine early_hugetlb_free_vmemmap_param?  This is called once per hstate, so
> perhaps pr_warn_once.

Good point. Will do.

Thanks.