linux-kernel - Re: DEFINE_IDA causing memory leaks? (was Re: [PATCH 1/2] virtio: fix memory leak of virtio ida cache layers)

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Date:	Thu, 17 Sep 2015 09:48:37 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Suman Anna <s-anna@...com>, Ohad Ben-Cohen <ohad@...ery.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	linux-scsi@...r.kernel.org, open-iscsi@...glegroups.com,
	Tejun Heo <htejun@...il.com>
Subject: Re: DEFINE_IDA causing memory leaks? (was  Re: [PATCH 1/2] virtio:
 fix memory leak of virtio ida cache layers)

On Thu, 2015-09-17 at 19:06 +0300, Michael S. Tsirkin wrote:
> On Thu, Sep 17, 2015 at 07:15:44AM -0700, James Bottomley wrote:
> > On Thu, 2015-09-17 at 08:33 +0300, Michael S. Tsirkin wrote:
> > > On Wed, Sep 16, 2015 at 07:29:17PM -0500, Suman Anna wrote:
> > > > The virtio core uses a static ida named virtio_index_ida for
> > > > assigning index numbers to virtio devices during registration.
> > > > The ida core may allocate some internal idr cache layers and
> > > > an ida bitmap upon any ida allocation, and all these layers are
> > > > truely freed only upon the ida destruction. The virtio_index_ida
> > > > is not destroyed at present, leading to a memory leak when using
> > > > the virtio core as a module and atleast one virtio device is
> > > > registered and unregistered.
> > > > 
> > > > Fix this by invoking ida_destroy() in the virtio core module
> > > > exit.
> > > > 
> > > > Cc: "Michael S. Tsirkin" <mst@...hat.com>
> > > > Signed-off-by: Suman Anna <s-anna@...com>
> > > 
> > > Interesting.
> > > Will the same apply to e.g. sd_index_ida in drivers/scsi/sd.c
> > > or iscsi_sess_ida in drivers/scsi/scsi_transport_iscsi.c?
> > > 
> > > If no, why not?
> > > 
> > > One doesn't generally expect to have to free global variables.
> > > Maybe we should forbid DEFINE_IDA in modules?
> > > 
> > > James, could you comment on this please?
> > 
> > ida is Tejun's baby (cc'd).  However, it does look like without
> > ida_destroy() you will leave a cached ida->bitmap dangling because we're
> > trying to be a bit clever in ida_remove() so we cache the bitmap to
> > relieve ida_pre_get() of the burden if we would otherwise free it.
> > 
> > I don't understand why you'd want to forbid DEFINE_IDA ... all it does
> > is pre-initialise a usually static ida structure.  The initialised
> > structure will have a NULL bitmap cache that's allocated in the first
> > ida_pre_get() ... that all seems to work as expected and no different
> > from a dynamically allocated struct ida.  Or are you thinking because
> > ida_destory() doesn't set bitmap to NULL, it damages the reuse?  In
> > which case I'm not sure there's much benefit to making it reusable, but
> > I suppose we could by adding a memset into ida_destroy().
> > 
> > James
> 
> It's just unusual to have  a descructor without a constructor.
> I bet more drivers misuse this AI because of this.


Well, there's an easy fix for that.  We could have ida_remove() actually
free the bitmap and not cache it if it's the last layer.  That way ida
would naturally empty and we wouldn't need a destructor.   Tejun, would
that work?

James

> > > > ---
> > > >  drivers/virtio/virtio.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > > > index b1877d73fa56..7062bb0975a5 100644
> > > > --- a/drivers/virtio/virtio.c
> > > > +++ b/drivers/virtio/virtio.c
> > > > @@ -412,6 +412,7 @@ static int virtio_init(void)
> > > >  static void __exit virtio_exit(void)
> > > >  {
> > > >  	bus_unregister(&virtio_bus);
> > > > +	ida_destroy(&virtio_index_ida);
> > > >  }
> > > >  core_initcall(virtio_init);
> > > >  module_exit(virtio_exit);
> > > > -- 
> > > > 2.5.0
> > > --
> > > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
> > > the body of a message to majordomo@...r.kernel.org
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> > > 
> > 
> > 
> > 
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