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

[Peter Krempa]

On Mon, Jul 20, 2020 at 18:33:19 +0100, Daniel Berrange wrote:
> 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) {

Can't we handle the _ABSENT case before even attempting to open the
file?