[libvirt] [PATCH] Use loop-control to allocate loopback device.

[Ian Main]

On Tue, Aug 27, 2013 at 03:28:07PM +0100, Daniel P. Berrange wrote:
> On Mon, Aug 26, 2013 at 12:30:39PM -0700, Ian Main wrote:
> > This patch changes virFileLoopDeviceOpen() to use the new loop-control
> > device to allocate a new loop device.  If this behavior is unsupported
> > or an error occurs while trying to do this it falls back to the previous
> > method of searching /dev for a free device.
> > 
> > With this patch you can start as many image based LXC domains as you
> > like (well almost).
> > 
> > Fixes bug https://bugzilla.redhat.com/show_bug.cgi?id=995543
> > 
> > Signed-off-by: Ian Main <imain at redhat.com>
> > ---
> >  configure.ac       | 12 +++++++++++
> >  src/util/virfile.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
> >  2 files changed, 72 insertions(+), 1 deletion(-)
> > 
> 
> 
> >  #if defined(__linux__) && HAVE_DECL_LO_FLAGS_AUTOCLEAR
> > -static int virFileLoopDeviceOpen(char **dev_name)
> > +
> > +#if HAVE_DECL_LOOP_CTL_GET_FREE
> > +static int virFileLoopDeviceOpenLoopCtl(char **dev_name)
> > +{
> > +    int fd = -1;
> > +    int devnr;
> > +    char *looppath = NULL;
> > +
> > +    VIR_DEBUG("Opening loop-control device");
> > +    if ((fd = open("/dev/loop-control", O_RDWR)) < 0) {
> > +        virReportSystemError(errno, "%s",
> > +                             _("Unable to open /dev/loop-control"));
> > +        return -1;
> 
> We need to distinguish ENOENT (which should be non-fatal) from
> other (fatal) errors here.
> 
> I'd suggest that we add an 'int *fd' parameter to this method,
> and use the return value to indicate success / failure only.
> 
> eg, 
> 
>   if (virFileLoopDeviceOpenLoopCtl(devname, &fd) < 0)
>     return -1;
>   if (fd == -1)
>     fd = virFileLoopDeviceOpenSearch(devname);
>   return fd;

You bet.  To be clear does this mean you don't want to fall back to the
search method in the case of an error while using loop-control
allocation?  My thinking was we'd just give it a go regardless of the
issue..

	Ian
 
> 
> > +    }
> > +
> > +    if ((devnr = ioctl(fd, LOOP_CTL_GET_FREE)) < 0) {
> > +        virReportSystemError(errno, "%s",
> > +                             _("Unable to get free loop device via ioctl"));
> > +        close(fd);
> > +        return -1;
> > +    }
> > +    close(fd);
> > +
> > +    VIR_DEBUG("Found free loop device number %i", devnr);
> > +
> > +    if (virAsprintf(&looppath, "/dev/loop%i", devnr) < 0)
> > +        return -1;
> > +
> > +    if ((fd = open(looppath, O_RDWR)) < 0) {
> > +        virReportSystemError(errno,
> > +                _("Unable to open %s"), looppath);
> > +        VIR_FREE(looppath);
> > +        return -1;
> > +    }
> > +
> > +    *dev_name = looppath;
> > +    return fd;
> > +}
> > +#endif /* HAVE_DECL_LOOP_CTL_GET_FREE */
> > +
> > +static int virFileLoopDeviceOpenSearch(char **dev_name)
> >  {
> >      int fd = -1;
> >      DIR *dh = NULL;
> > @@ -601,6 +641,25 @@ cleanup:
> >      return fd;
> >  }
> >  
> > +static int virFileLoopDeviceOpen(char **dev_name)
> > +{
> > +    int loop_fd;
> > +
> > +#ifdef HAVE_DECL_LOOP_CTL_GET_FREE
> > +    loop_fd = virFileLoopDeviceOpenLoopCtl(dev_name);
> > +    VIR_DEBUG("Return from loop-control got fd %d\n", loop_fd);
> > +    if (loop_fd < 0) {
> > +        VIR_WARN("loop-control allocation failed, trying search technique.");
> 
> See note earlier, about distinguishing fatal from non-fatal errors.
> 
> > +    } else {
> > +        return loop_fd;
> > +    }
> > +#endif /* HAVE_DECL_LOOP_CTL_GET_FREE */
> > +
> > +    /* Without the loop control device we just use the old technique. */
> > +    loop_fd = virFileLoopDeviceOpenSearch(dev_name);
> > +
> > +    return loop_fd;
> > +}
> >  
> 
> 
> Regards,
> Daniel
> -- 
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