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

[Muchun Song]

On Fri, Jan 8, 2021 at 9:08 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 1/7/21 7:11 AM, Muchun Song wrote:
> > On Thu, Jan 7, 2021 at 10:11 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >>
> >> On Thu 07-01-21 20:59:33, Muchun Song wrote:
> >>> On Thu, Jan 7, 2021 at 8:38 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >> [...]
> >>>> Right. Can we simply back off in the dissolving path when ref count is
> >>>> 0 && PageHuge() if list_empty(page->lru)? Is there any other scenario
> >>>> when the all above is true and the page is not being freed?
> >>>
> >>> The list_empty(&page->lru) may always return false.
> >>> The page before freeing is on the active list
> >>> (hstate->hugepage_activelist).Then it is on the free list
> >>> after freeing. So list_empty(&page->lru) is always false.
> >>
> >> The point I was trying to make is that the page has to be enqueued when
> >> it is dissolved and freed. If the page is not enqueued then something
> >> racing. But then I have realized that this is not a great check to
> >> detect the race because pages are going to be released to buddy
> >> allocator and that will reuse page->lru again. So scratch that and sorry
> >> for the detour.
> >>
> >> But that made me think some more and one way to reliably detect the race
> >> should be PageHuge() check in the freeing path. This is what dissolve
> >> path does already. PageHuge becomes false during update_and_free_page()
> >> while holding the hugetlb_lock. So can we use that?
> >
> > It may make the thing complex. Apart from freeing it to the
> > buddy allocator, free_huge_page also does something else for
> > us. If we detect the race in the freeing path, if it is not a HugeTLB
> > page, the freeing path just returns. We also should move those
> > things to the dissolve path. Right?
> >
> > 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.
>
> You are correct.  This is also an issue/potential problem with this
> race.  It seems that adding the state information might be the least
> complex way to address issue.

Yeah, I agree with you. Adding a state is a simple solution.


>
> --
> Mike Kravetz