linux-kernel - Re: [PATCH] fs: add the FIGETFROZEN ioctl call

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Message-ID: <20160415021737.GZ567@dastard>
Date:	Fri, 15 Apr 2016 12:17:37 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Florian Margaine <florian@...gaine.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: add the FIGETFROZEN ioctl call

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@...morbit.com