linux-kernel - Re: [RFC v2 6/6] fs: add automatic kernel fs freeze / thaw and remove kthread freezing

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Message-ID: <20210420125903.GC3604224@infradead.org>
Date:   Tue, 20 Apr 2021 13:59:03 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     rafael@...nel.org, gregkh@...uxfoundation.org,
        viro@...iv.linux.org.uk, jack@...e.cz, bvanassche@....org,
        jeyu@...nel.org, ebiederm@...ssion.com, mchehab@...nel.org,
        keescook@...omium.org, linux-fsdevel@...r.kernel.org,
        kernel@...force.de, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 6/6] fs: add automatic kernel fs freeze / thaw and
 remove kthread freezing

> This also removes all the superflous freezer calls on all filesystems
> as they are no longer needed as the VFS now performs filesystem
> freezing/thaw if the filesystem has support for it. The filesystem
> therefore is in charge of properly dealing with quiescing of the
> filesystem through its callbacks.

Can you split that out from the main logic change?  Maybe even into one
patch per file system?

> +#ifdef CONFIG_PM_SLEEP
> +static bool super_should_freeze(struct super_block *sb)
> +{
> +	if (!sb->s_root)
> +		return false;
> +	if (!(sb->s_flags & MS_BORN))
> +		return false;

This is already done in the iterate_supers_excl and
iterate_supers_reverse_excl helpers that this helper is always called
through.

> +	/*
> +	 * We don't freeze virtual filesystems, we skip those filesystems with
> +	 * no backing device.
> +	 */
> +	if (sb->s_bdi == &noop_backing_dev_info)
> +		return false;

Why?

> +	/* No need to freeze read-only filesystems */
> +	if (sb_rdonly(sb))
> +		return false;

freeze_super/thaw_super already takes care of read-only file systems,
and IMHO in a better way.

> +	int error = 0;
> +
> +	spin_lock(&sb_lock);
> +	if (!super_should_freeze(sb))
> +		goto out;
> +
> +	pr_info("%s (%s): freezing\n", sb->s_type->name, sb->s_id);
> +
> +	spin_unlock(&sb_lock);

I don't see how super_should_freeze needs sb_lock.  But if it does
the lock should be taken in the function.

> +	atomic_inc(&sb->s_active);

Doesn't this need a atomic_inc_not_zero if we're racing with a delayed
unmount?

> +	error = freeze_locked_super(sb, false);
> +	if (error)
> +		atomic_dec(&sb->s_active);

And this really needs something like deactivate_locked_super.

> +	spin_lock(&sb_lock);
> +	if (error && error != -EBUSY)
> +		pr_notice("%s (%s): Unable to freeze, error=%d",
> +			  sb->s_type->name, sb->s_id, error);
> +
> +out:
> +	spin_unlock(&sb_lock);

Huh, what is the point of sb_lock here?

> +int fs_suspend_freeze(void)
> +{
> +	return iterate_supers_reverse_excl(fs_suspend_freeze_sb, NULL);
> +}

I'd just fold this helper into its only caller.

> +	error = __thaw_super_locked(sb, false);
> +	if (!error)
> +		atomic_dec(&sb->s_active);
> +
> +	spin_lock(&sb_lock);
> +	if (error && error != -EBUSY)
> +		pr_notice("%s (%s): Unable to unfreeze, error=%d",
> +			  sb->s_type->name, sb->s_id, error);
> +
> +out:
> +	spin_unlock(&sb_lock);
> +	return error;
> +}
> +
> +int fs_resume_unfreeze(void)
> +{
> +	return iterate_supers_excl(fs_suspend_thaw_sb, NULL);
> +}

Same comments as on the freeze side.