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

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Jan 12, 2021 at 6:02 PM Michal Hocko <mhocko@suse.com> 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?

>
> >               /*
> >                * Move PageHWPoison flag from head page to the raw error page,
> >                * which makes any subpages rather than the error page reusable.
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs