Re: [PATCH v4 7/8] hugetlb: make free_huge_page irq safe

[Michal Hocko]

On Wed 07-04-21 11:12:37, Oscar Salvador wrote:
> On Mon, Apr 05, 2021 at 04:00:42PM -0700, Mike Kravetz wrote:
> > Commit c77c0a8ac4c5 ("mm/hugetlb: defer freeing of huge pages if in
> > non-task context") was added to address the issue of free_huge_page
> > being called from irq context.  That commit hands off free_huge_page
> > processing to a workqueue if !in_task.  However, this doesn't cover
> > all the cases as pointed out by 0day bot lockdep report [1].
> > 
> > :  Possible interrupt unsafe locking scenario:
> > :
> > :        CPU0                    CPU1
> > :        ----                    ----
> > :   lock(hugetlb_lock);
> > :                                local_irq_disable();
> > :                                lock(slock-AF_INET);
> > :                                lock(hugetlb_lock);
> > :   <Interrupt>
> > :     lock(slock-AF_INET);
> > 
> > Shakeel has later explained that this is very likely TCP TX zerocopy
> > from hugetlb pages scenario when the networking code drops a last
> > reference to hugetlb page while having IRQ disabled. Hugetlb freeing
> > path doesn't disable IRQ while holding hugetlb_lock so a lock dependency
> > chain can lead to a deadlock.
> > 
> > This commit addresses the issue by doing the following:
> > - Make hugetlb_lock irq safe.  This is mostly a simple process of
> >   changing spin_*lock calls to spin_*lock_irq* calls.
> > - Make subpool lock irq safe in a similar manner.
> > - Revert the !in_task check and workqueue handoff.
> > 
> > [1] https://lore.kernel.org/linux-mm/000000000000f1c03b05bc43aadc@xxxxxxxxxx/
> > 
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> 
> So, irq_lock_irqsave/spin_unlock_irqrestore is to be used in places
> that might have been called from an IRQ context?

Yes. spin_unlock_irq will enable interrupts unconditionally which is
certainly not what we want if the path is called with IRQ disabled by
the caller.
-- 
Michal Hocko
SUSE Labs