linux-kernel - Re: [PATCH] virtio: suppress kmemleak false positive

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Date:	Fri, 11 Jan 2013 14:47:19 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Daniel Baluta <daniel.baluta@...il.com>
Cc:	Alexandru Copot <alex.mihai.c@...il.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"mst@...hat.com" <mst@...hat.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virtio: suppress kmemleak false positive

On Fri, Jan 11, 2013 at 02:32:12PM +0000, Daniel Baluta wrote:
> On Fri, Jan 11, 2013 at 4:02 PM, Catalin Marinas <catalin.marinas@....com>
> wrote:
> > On Fri, Jan 11, 2013 at 01:51:43PM +0000, Alexandru Copot wrote:
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -23,6 +23,7 @@
> > >  #include <linux/slab.h>
> > >  #include <linux/module.h>
> > >  #include <linux/hrtimer.h>
> > > +#include <linux/kmemleak.h>
> > >
> > >  /* virtio guest is communicating with a virtual "device" that actually runs on
> > >   * a host processor.  Memory barriers are used to control SMP effects. */
> > > @@ -140,6 +141,7 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
> > >       desc = kmalloc((out + in) * sizeof(struct vring_desc), gfp);
> > >       if (!desc)
> > >               return -ENOMEM;
> > > +     kmemleak_not_leak(desc);
> >
> > Please add a comment above this call in case people later wonder why
> > this annotation is needed.
> 
> So, kememleak cannot handle this kind of pointer aliases?

No. Basically it stores the allocated object start/end in an rb-tree and
cannot cope with overlapping blocks. If we allow aliases, we could have
some overlapping.

I think there was a patch in the past to add a separate rb-tree for
aliases but I didn't particularly like it because it affected the
performance.

> Also, I wonder if phys_to_virt(virt_to_phys(x)) == x holds true all the time.

It should as long as x is a valid kernel virtual address in the logical
(linear) mapping.

-- 
Catalin
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