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

From: Muchun Song <songmuchun@bytedance.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>,
	Linux Memory Management List <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>, Roman Gushchin <guro@fb.com>,
	Shakeel Butt <shakeelb@google.com>,
	Oscar Salvador <osalvador@suse.de>,
	David Hildenbrand <david@redhat.com>,
	David Rientjes <rientjes@google.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Matthew Wilcox <willy@infradead.org>,
	HORIGUCHI NAOYA <naoya.horiguchi@nec.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
	Waiman Long <longman@redhat.com>, Peter Xu <peterx@redhat.com>,
	Mina Almasry <almasrymina@google.com>,
	Hillf Danton <hdanton@sina.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Barry Song <song.bao.hua@hisilicon.com>,
	Will Deacon <will@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [External] Re: [PATCH v2 1/8] mm/cma: change cma mutex to irq safe spinlock
Date: Tue, 30 Mar 2021 16:21:45 +0800	[thread overview]
Message-ID: <CAMZfGtWOY+gxSkfVNdW-GfKfZXKCgjwYGo=Y6ABk5L=HDsxhVg@mail.gmail.com> (raw)
In-Reply-To: <YGLe31q/4kAoLmmr@dhcp22.suse.cz>

On Tue, Mar 30, 2021 at 4:18 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 30-03-21 16:08:36, Muchun Song wrote:
> > On Tue, Mar 30, 2021 at 4:01 PM Michal Hocko <mhocko@suse.com> 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