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 8:04 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Fri 08-01-21 19:52:54, Muchun Song wrote:
> > On Fri, Jan 8, 2021 at 7:44 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Fri 08-01-21 18:08:57, Muchun Song wrote:
> > > > On Fri, Jan 8, 2021 at 5:31 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > >
> > > > > On Fri 08-01-21 17:01:03, Muchun Song wrote:
> > > > > > On Fri, Jan 8, 2021 at 4:43 PM Michal Hocko <mhocko@xxxxxxxx> 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.
> > >
> > > There is nothing really to prevent making hugetlb_lock irq safe, isn't
> > > it?
> >
> > Yeah. We can make the hugetlb_lock irq safe. But why have we not
> > done this? Maybe the commit changelog can provide more information.
> >
> > See https://github.com/torvalds/linux/commit/c77c0a8ac4c522638a8242fcb9de9496e3cdbb2d
>
> Dang! Maybe it is the time to finally stack one workaround on top of the
> other and put this code into the shape. The amount of hackery and subtle
> details has just grown beyond healthy!

>From readability point of view, maybe making the hugetlb_lock irq safe
is an improvement (in the feature).

The details are always messy. :)

> --
> Michal Hocko
> SUSE Labs