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



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 redhat com>
---
 configure.ac       | 12 +++++++++++
 src/util/virfile.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 72 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..f70f9fe 100644
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -528,7 +528,47 @@ 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
+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;
+    }
+
+    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.");
+    } 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;
+}
 
 int virFileLoopDeviceAssociate(const char *file,
                                char **dev)
-- 
1.8.1.4


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