Re: [External] Re: [PATCH v3 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

[Muchun Song]

On Tue, Jan 12, 2021 at 7:17 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 12-01-21 18:13:02, Muchun Song wrote:
> > On Tue, Jan 12, 2021 at 6:02 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Sun 10-01-21 20:40:14, Muchun Song wrote:
> > > [...]
> > > > @@ -1770,6 +1788,14 @@ int dissolve_free_huge_page(struct page *page)
> > > >               int nid = page_to_nid(head);
> > > >               if (h->free_huge_pages - h->resv_huge_pages == 0)
> > > >                       goto out;
> > > > +
> > > > +             /*
> > > > +              * We should make sure that the page is already on the free list
> > > > +              * when it is dissolved.
> > > > +              */
> > > > +             if (unlikely(!PageHugeFreed(head)))
> > > > +                     goto out;
> > > > +
> > >
> > > Do you really want to report EBUSY in this case? This doesn't make much
> > > sense to me TBH. I believe you want to return 0 same as when you race
> > > and the page is no longer PageHuge.
> >
> > Return 0 is wrong. Because the page is not freed to the buddy allocator.
> > IIUC, dissolve_free_huge_page returns 0 when the page is already freed
> > to the buddy allocator. Right?
>
> 0 is return when the page is either dissolved or it doesn't need
> dissolving. If there is a race with somebody else freeing the page then
> there is nothing to dissolve. Under which condition it makes sense to
> report the failure and/or retry dissolving?

If there is a race with somebody else freeing the page, the page
can be freed to the hugepage pool not the buddy allocator. Do
you think that this page is dissolved?

> --
> Michal Hocko
> SUSE Labs