[libvirt PATCH 1/4] util: add a helper method for controlling the COW flag on btrfs

[Daniel P. Berrangé]

btrfs defaults to performing copy-on-write for files. This is often
undesirable for VM images, so we need to be able to control whether this
behaviour is used.

The virFileSetCOW() will allow for this. We use a tristate, since out of
the box, we want the default behaviour attempt to disable cow, but only
on btrfs, silently do nothing on non-btrfs. If someone explicitly asks
to disable/enable cow, then we want to raise a hard error on non-btrfs.

Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
---
 src/libvirt_private.syms |  1 +
 src/util/virfile.c       | 76 ++++++++++++++++++++++++++++++++++++++++
 src/util/virfile.h       |  3 ++
 3 files changed, 80 insertions(+)

diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index 73b72c9e10..eea31a736d 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -2125,6 +2125,7 @@ virFileRewrite;
 virFileRewriteStr;
 virFileSanitizePath;
 virFileSetACLs;
+virFileSetCOW;
 virFileSetupDev;
 virFileSetXAttr;
 virFileTouch;
diff --git a/src/util/virfile.c b/src/util/virfile.c
index 213acdbcaa..5b169b3d11 100644
--- a/src/util/virfile.c
+++ b/src/util/virfile.c
@@ -71,6 +71,7 @@
 # endif
 # include <sys/ioctl.h>
 # include <linux/cdrom.h>
+# include <linux/fs.h>
 #endif
 
 #if HAVE_LIBATTR
@@ -4504,3 +4505,78 @@ virFileDataSync(int fd)
     return fdatasync(fd);
 #endif
 }
+
+
+int
+virFileSetCOW(const char *path,
+              virTristateBool state)
+{
+#if __linux__
+    int val = 0;
+    struct statfs buf;
+    VIR_AUTOCLOSE fd = -1;
+
+    VIR_DEBUG("Setting COW flag on '%s' to '%s'",
+              path, virTristateBoolTypeToString(state));
+
+    fd = open(path, O_RDONLY|O_NONBLOCK|O_LARGEFILE);
+    if (fd < 0) {
+        virReportSystemError(errno, _("unable to open '%s'"),
+                             path);
+        return -1;
+    }
+
+    if (fstatfs(fd, &buf) < 0)  {
+        virReportSystemError(errno, _("unable query filesystem type on '%s'"),
+                             path);
+        return -1;
+    }
+
+    if (buf.f_type != BTRFS_SUPER_MAGIC) {
+        if (state == VIR_TRISTATE_BOOL_ABSENT) {
+            virReportSystemError(ENOSYS,
+                                 _("unable to control COW flag on '%s', not btrfs"),
+                                 path);
+            return -1;
+        }
+        return 0;
+    }
+
+    if (ioctl(fd, FS_IOC_GETFLAGS, &val) < 0) {
+        virReportSystemError(errno, _("unable get directory flags on '%s'"),
+                             path);
+        return -1;
+    }
+
+    VIR_DEBUG("Current flags on '%s' are 0x%x", path, val);
+    if (state == VIR_TRISTATE_BOOL_YES) {
+        val &= ~FS_NOCOW_FL;
+    } else {
+        val |= FS_NOCOW_FL;
+    }
+
+    VIR_DEBUG("New flags on '%s' will be 0x%x", path, val);
+    if (ioctl(fd, FS_IOC_SETFLAGS, &val) < 0) {
+        int saved_err = errno;
+        VIR_DEBUG("Failed to set flags on '%s': %s", path, g_strerror(saved_err));
+        if (state != VIR_TRISTATE_BOOL_ABSENT) {
+            virReportSystemError(saved_err,
+                                 _("unable control COW flag on '%s'"),
+                                 path);
+            return -1;
+        } else {
+            VIR_DEBUG("Ignoring failure to set COW");
+        }
+    }
+
+    return 0;
+#else /* ! __linux__ */
+    if (state != VIR_TRISTATE_BOOL_ABSENT) {
+        virReportSystemError(ENOSYS,
+                             _("Unable to set copy-on-write state on '%s' to '%s'"),
+                             path, virTristateBoolTypeToString(state));
+        return -1;
+    }
+    return 0;
+#endif /* ! __linux__ */
+}
diff --git a/src/util/virfile.h b/src/util/virfile.h
index 7a92364a5c..cb0e586a7d 100644
--- a/src/util/virfile.h
+++ b/src/util/virfile.h
@@ -374,3 +374,6 @@ int virFileRemoveXAttr(const char *path,
     G_GNUC_NO_INLINE;
 
 int virFileDataSync(int fd);
+
+int virFileSetCOW(const char *path,
+                  virTristateBool state);
-- 
2.26.2