Re: [External] Re: [PATCH v20 4/9] mm: hugetlb: free the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Sat, Apr 17, 2021 at 5:10 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 4/15/21 1:40 AM, Muchun Song wrote:
> > Every HugeTLB has more than one struct page structure. We __know__ that
> > we only use the first 4 (__NR_USED_SUBPAGE) struct page structures
> > to store metadata associated with each HugeTLB.
> >
> > There are a lot of struct page structures associated with each HugeTLB
> > page. For tail pages, the value of compound_head is the same. So we can
> > reuse first page of tail page structures. We map the virtual addresses
> > of the remaining pages of tail page structures to the first tail page
> > struct, and then free these page frames. Therefore, we need to reserve
> > two pages as vmemmap areas.
> >
> > When we allocate a HugeTLB page from the buddy, we can free some vmemmap
> > pages associated with each HugeTLB page. It is more appropriate to do it
> > in the prep_new_huge_page().
> >
> > The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
> > pages associated with a HugeTLB page can be freed, returns zero for
> > now, which means the feature is disabled. We will enable it once all
> > the infrastructure is there.
> >
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> > Tested-by: Chen Huang <chenhuang5@xxxxxxxxxx>
> > Tested-by: Bodeddula Balasubramaniam <bodeddub@xxxxxxxxxx>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> There may need to be some trivial rebasing due to Oscar's changes
> when they go in.

Yeah, thanks for your reminder.

>
> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> --
> Mike Kravetz