Re: [External] Re: [PATCH v2 1/8] mm/cma: change cma mutex to irq safe spinlock

[Michal Hocko]

On Tue 30-03-21 16:08:36, Muchun Song wrote:
> On Tue, Mar 30, 2021 at 4:01 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Mon 29-03-21 16:23:55, Mike Kravetz wrote:
> > > Ideally, cma_release could be called from any context.  However, that is
> > > not possible because a mutex is used to protect the per-area bitmap.
> > > Change the bitmap to an irq safe spinlock.
> >
> > I would phrase the changelog slightly differerent
> > "
> > cma_release is currently a sleepable operatation because the bitmap
> > manipulation is protected by cma->lock mutex. Hugetlb code which relies
> > on cma_release for CMA backed (giga) hugetlb pages, however, needs to be
> > irq safe.
> >
> > The lock doesn't protect any sleepable operation so it can be changed to
> > a (irq aware) spin lock. The bitmap processing should be quite fast in
> > typical case but if cma sizes grow to TB then we will likely need to
> > replace the lock by a more optimized bitmap implementation.
> > "
> >
> > it seems that you are overusing irqsave variants even from context which
> > are never called from the IRQ context so they do not need storing flags.
> >
> > [...]
> > > @@ -391,8 +391,9 @@ static void cma_debug_show_areas(struct cma *cma)
> > >       unsigned long start = 0;
> > >       unsigned long nr_part, nr_total = 0;
> > >       unsigned long nbits = cma_bitmap_maxno(cma);
> > > +     unsigned long flags;
> > >
> > > -     mutex_lock(&cma->lock);
> > > +     spin_lock_irqsave(&cma->lock, flags);
> >
> > spin_lock_irq should be sufficient. This is only called from the
> > allocation context and that is never called from IRQ context.
> 
> This makes me think more. I think that spin_lock should be
> sufficient. Right?

Nope. Think of the following scenario
	spin_lock(cma->lock);
	<IRQ>
	put_page
	  __free_huge_page
	    cma_release
	      spin_lock_irqsave() DEADLOCK
-- 
Michal Hocko
SUSE Labs