[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <YGLe31q/4kAoLmmr@dhcp22.suse.cz>
Date: Tue, 30 Mar 2021 10:18:39 +0200
From: Michal Hocko <mhocko@...e.com>
To: Muchun Song <songmuchun@...edance.com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>,
Linux Memory Management List <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>,
Roman Gushchin <guro@...com>,
Shakeel Butt <shakeelb@...gle.com>,
Oscar Salvador <osalvador@...e.de>,
David Hildenbrand <david@...hat.com>,
David Rientjes <rientjes@...gle.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Peter Zijlstra <peterz@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
HORIGUCHI NAOYA <naoya.horiguchi@....com>,
"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
Waiman Long <longman@...hat.com>, Peter Xu <peterx@...hat.com>,
Mina Almasry <almasrymina@...gle.com>,
Hillf Danton <hdanton@...a.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Barry Song <song.bao.hua@...ilicon.com>,
Will Deacon <will@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [External] Re: [PATCH v2 1/8] mm/cma: change cma mutex to irq
safe spinlock
On Tue 30-03-21 16:08:36, Muchun Song wrote:
> On Tue, Mar 30, 2021 at 4:01 PM Michal Hocko <mhocko@...e.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
--
Michal Hocko
SUSE Labs
Powered by blists - more mailing lists