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

Muchun Song
2021-01-12T15:06:38Z

On Tue, Jan 12, 2021 at 8:37 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 12-01-21 19:43:21, Muchun Song wrote:
> > On Tue, Jan 12, 2021 at 7:17 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Tue 12-01-21 18:13:02, Muchun Song wrote:
> > > > 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?
> > >
> > > 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?
>
> OK, I see what you mean. Effectively the page would be in a limbo, not
> yet in the pool nor in the allocator but it can find its way to the
> either of the two. But I still dislike returning a failure because that
> would mean e.g. memory hotplug to fail. Can you simply retry inside this
> code path (drop the lock, cond_resched and retry)?

Yeah. This is what I want to do (making the memory hotplug as
successful as possible). So I send the patch:

[PATCH v3 4/6] mm: hugetlb: add return -EAGAIN for dissolve_free_huge_page

Adding a simple retry inside this function when hitting this race is
also fine to me. I can do that.

> --
> Michal Hocko
> SUSE Labs