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Re: [libvirt] [PATCH V2] Use loop-control to allocate loop device.



On Tue, Aug 27, 2013 at 11:51:29AM -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
> we fall 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
> 
> V2:
> - Modified to use a dedicated error return for loop-control allocation
>   function.
> - Only do fallback if /dev/loop-control does not exist, otherwise return
>   error.
> 
> Signed-off-by: Ian Main <imain redhat com>
> ---
>  configure.ac       | 12 +++++++++
>  src/util/virfile.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 84 insertions(+), 1 deletion(-)
> 
> diff --git a/configure.ac b/configure.ac
> index ac8cfa1..10cd872 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -913,6 +913,18 @@ if test "$with_lxc" = "yes" || test "$with_lxc" = "check"; then
>              AC_MSG_ERROR([Required kernel features for LXC were not found])
>          fi
>      ])
> +    AC_LINK_IFELSE([AC_LANG_PROGRAM(
> +    [[
> +        #include <sched.h>
> +        #include <linux/loop.h>
> +        #include <sys/epoll.h>
> +    ]], [[
> +        unshare(!(LOOP_CTL_GET_FREE));
> +    ]])], [
> +       AC_DEFINE([HAVE_DECL_LOOP_CTL_GET_FREE], [1],
> +         [Define to 1 if you have the declaration of `LOOP_CTL_GET_FREE',
> +         and to 0 if you don't.])
> +    ])
>  fi
>  if test "$with_lxc" = "yes" ; then
>      AC_DEFINE_UNQUOTED([WITH_LXC], 1, [whether LXC driver is enabled])
> diff --git a/src/util/virfile.c b/src/util/virfile.c
> index 2b07ac9..22a0db5 100644
> --- a/src/util/virfile.c
> +++ b/src/util/virfile.c
> @@ -528,7 +528,56 @@ int virFileUpdatePerm(const char *path,
>  
>  
>  #if defined(__linux__) && HAVE_DECL_LO_FLAGS_AUTOCLEAR
> -static int virFileLoopDeviceOpen(char **dev_name)
> +
> +#if HAVE_DECL_LOOP_CTL_GET_FREE
> +
> +/* virFileLoopDeviceOpenLoopCtl() returns -1 when a real failure has occured
> + * while in the process of allocating or opening the loop device.  On success
> + * we return 0 and modify the fd to the appropriate file descriptor.
> + * If /dev/loop-control does not exist, we return 0 and do not set fd. */
> +
> +static int virFileLoopDeviceOpenLoopCtl(char **dev_name, int *fd)
> +{
> +    int devnr;
> +    int ctl_fd;
> +    char *looppath = NULL;
> +
> +    VIR_DEBUG("Opening loop-control device");
> +    if ((ctl_fd = open("/dev/loop-control", O_RDWR)) < 0) {
> +        virReportSystemError(errno, "%s",
> +                             _("Unable to open /dev/loop-control"));
> +        if (errno == ENOENT) {
> +            return 0;
> +        }
> +        return -1;
> +    }
> +
> +    if ((devnr = ioctl(ctl_fd, LOOP_CTL_GET_FREE)) < 0) {
> +        virReportSystemError(errno, "%s",
> +                             _("Unable to get free loop device via ioctl"));
> +        close(ctl_fd);
> +        return -1;
> +    }
> +    close(ctl_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 0;
> +}
> +#endif /* HAVE_DECL_LOOP_CTL_GET_FREE */
> +
> +static int virFileLoopDeviceOpenSearch(char **dev_name)
>  {
>      int fd = -1;
>      DIR *dh = NULL;
> @@ -601,6 +650,28 @@ cleanup:
>      return fd;
>  }
>  
> +static int virFileLoopDeviceOpen(char **dev_name)
> +{
> +    int loop_fd = -1;
> +
> +#ifdef HAVE_DECL_LOOP_CTL_GET_FREE
> +    if (virFileLoopDeviceOpenLoopCtl(dev_name, &loop_fd) < 0)
> +        return -1;
> +
> +    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.");
> +    } else {
> +        return loop_fd;
> +    }

We don't really need to VIR_WARN here - it is an expected scenario
on old kernels. We're already logging the loop_fd value above, so
I'd change this if/else pair to be simply:

   if (loop_fd >= 0)
      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;
> +}
>  
>  int virFileLoopDeviceAssociate(const char *file,
>                                 char **dev)


Daniel
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