Re: [External] Re: [PATCH v2 3/6] mm: hugetlb: fix a race between freeing and dissolving the page

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Jan 8, 2021 at 5:31 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Fri 08-01-21 17:01:03, Muchun Song wrote:
> > On Fri, Jan 8, 2021 at 4:43 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Thu 07-01-21 23:11:22, Muchun Song wrote:
> [..]
> > > > But I find a tricky problem to solve. See free_huge_page().
> > > > If we are in non-task context, we should schedule a work
> > > > to free the page. We reuse the page->mapping. If the page
> > > > is already freed by the dissolve path. We should not touch
> > > > the page->mapping. So we need to check PageHuge().
> > > > The check and llist_add() should be protected by
> > > > hugetlb_lock. But we cannot do that. Right? If dissolve
> > > > happens after it is linked to the list. We also should
> > > > remove it from the list (hpage_freelist). It seems to make
> > > > the thing more complex.
> > >
> > > I am not sure I follow you here but yes PageHuge under hugetlb_lock
> > > should be the reliable way to check for the race. I am not sure why we
> > > really need to care about mapping or other state.
> >
> > CPU0:                               CPU1:
> > free_huge_page(page)
> >   if (PageHuge(page))
> >                                     dissolve_free_huge_page(page)
> >                                       spin_lock(&hugetlb_lock)
> >                                       update_and_free_page(page)
> >                                       spin_unlock(&hugetlb_lock)
> >     llist_add(page->mapping)
> >     // the mapping is corrupted
> >
> > The PageHuge(page) and llist_add() should be protected by
> > hugetlb_lock. Right? If so, we cannot hold hugetlb_lock
> > in free_huge_page() path.
>
> OK, I see. I completely forgot about this snowflake. I thought that
> free_huge_page was a typo missing initial __. Anyway you are right that
> this path needs a check as well. But I don't see why we couldn't use the
> lock here. The lock can be held only inside the !in_task branch.

Because we hold the hugetlb_lock without disable irq. So if an interrupt
occurs after we hold the lock. And we also free a HugeTLB page. Then
it leads to deadlock.

task context:                             interrupt context:

put_page(page)
  spin_lock(&hugetlb_lock)

                                                put_page(page)
                                                  spin_lock(&hugetlb_lock)
                                                  // deadlock

  spin_unlock(&hugetlb_lock)

> Although it would be much more nicer if the lock was held at this layer
> rather than both free_huge_page and __free_huge_page. But that clean up
> can be done on top.
>
> --
> Michal Hocko
> SUSE Labs