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

[Michal Hocko]

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?
-- 
Michal Hocko
SUSE Labs