On Thu, Apr 14, 2016 at 09:57:07AM +0200, Florian Margaine wrote:
> This lets userland get the filesystem freezing status, aka whether the
> filesystem is frozen or not. This is so that an application can know if
> it should freeze the filesystem or if it isn't necessary when taking a
> snapshot.
freezing nests, so there is no reason for avoiding a freeze when
doing a snapshot. Indeed, if you don't wrap freeze/thaw around a
snapshot, then if the fs is thawed while the snapshot is in progress
then you are going to get a corrupt snapshot....
And, besides, polling for frozenness from userspace is inherently
racy - by the time the syscall returns, the information may be
incorrect, so you can't rely on it for decision making purposes in
userspace.
> +static int ioctl_fsgetfrozen(struct file *filp)
> +{
> + struct super_block *sb = file_inode(filp)->i_sb;
> +
> + if (!capable(CAP_SYS_ADMIN))
> + return -EPERM;
> +
> + return sb->s_writers.frozen;
This makes the internal freeze implementation states part of the
userspace ABI. This needs an API that is separate from the internal
implementation...
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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