Re: [External] Re: [PATCH v5 13/21] mm/hugetlb: Use PG_slab to indicate split pmd

[Muchun Song]

On Mon, Nov 23, 2020 at 3:48 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Fri 20-11-20 17:30:27, Muchun Song wrote:
> > On Fri, Nov 20, 2020 at 4:16 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Fri 20-11-20 14:43:17, Muchun Song wrote:
> > > > When we allocate hugetlb page from buddy, we may need split huge pmd
> > > > to pte. When we free the hugetlb page, we can merge pte to pmd. So
> > > > we need to distinguish whether the previous pmd has been split. The
> > > > page table is not allocated from slab. So we can reuse the PG_slab
> > > > to indicate that the pmd has been split.
> > >
> > > PageSlab is used outside of the slab allocator proper and that code
> > > might get confused by this AFAICS.
> >
> > I got your concerns. Maybe we can use PG_private instead of the
> > PG_slab.
>
> Reusing a page flag arbitrarily is not that easy. Hugetlb pages have a
> lot of spare room in struct page so I would rather use something else.

This page is the PMD page table of vmemmap, not the vmemmap page
of HugeTLB. And the page table does not use PG_private. Maybe it is
enough. Thanks.

> --
> Michal Hocko
> SUSE Labs



-- 
Yours,
Muchun