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

[Muchun Song]

On Tue, Mar 30, 2021 at 4:18 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> 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

Got it. Thanks.

> --
> Michal Hocko
> SUSE Labs