linux-kernel - Re: [PATCH V2] Allow SysRq emergency thaw to thaw frozen filesystems

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Date:	Fri, 16 Jan 2009 11:38:12 -0800
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Takashi Sato <t-sato@...jp.nec.com>
Subject: Re: [PATCH V2] Allow SysRq emergency thaw to thaw frozen filesystems

Eric Sandeen wrote:
> Now that the filesystem freeze operation has been elevated
> to the VFS, and is just an ioctl away, some sort of safety net
> for unintentionally frozen root filesystems may be in order.
> 
> The timeout thaw originally proposed did not get merged, but
> perhaps something like this would be useful in emergencies.
> 
> For example, freeze /path/to/mountpoint may freeze your
> root filesystem if you forgot that you had that unmounted.
> 
> I chose 'j' as the last remaining character other than 'h'
> which is sort of reserved for help (because help is generated
> on any unknown character).
> 
> I've tested this on a non-root fs with multiple (nested) freezers,
> as well as on a system rendered unresponsive due to a frozen
> root fs.
> 
> Thanks,
> -Eric
> 
> Signed-off-by: Eric Sandeen <sandeen@...hat.com>
> ---
> 
> Index: linux-2.6/drivers/char/sysrq.c
> ===================================================================
> --- linux-2.6.orig/drivers/char/sysrq.c	2009-01-16 13:24:14.688575212 -0600
> +++ linux-2.6/drivers/char/sysrq.c	2009-01-16 13:26:35.232575643 -0600
> @@ -346,6 +346,17 @@ static struct sysrq_key_op sysrq_moom_op
>  	.enable_mask	= SYSRQ_ENABLE_SIGNAL,
>  };
>  
> +static void sysrq_handle_thaw(int key, struct tty_struct *tty)
> +{
> +	emergency_thaw_all();
> +}
> +static struct sysrq_key_op sysrq_thaw_op = {
> +	.handler	= sysrq_handle_thaw,
> +	.help_msg	= "Thaw(J)",

Can the help text be less terse, e.g.:  "thaw-filesystems(J)" ?

Not capital-T Thaw because that was used to mean "use sysrq-T" to invoke
this sysrq.


> +	.action_msg	= "Emergency Thaw of all frozen filesystems",
> +	.enable_mask	= SYSRQ_ENABLE_SIGNAL,
> +};
> +
>  static void sysrq_handle_kill(int key, struct tty_struct *tty)
>  {
>  	send_sig_all(SIGKILL);
> @@ -396,9 +407,9 @@ static struct sysrq_key_op *sysrq_key_ta
>  	&sysrq_moom_op,			/* f */
>  	/* g: May be registered by ppc for kgdb */
>  	NULL,				/* g */
> -	NULL,				/* h */
> +	NULL,				/* h - reserved for help */

Yes, thanks.

>  	&sysrq_kill_op,			/* i */
> -	NULL,				/* j */
> +	&sysrq_thaw_op,			/* j */
>  	&sysrq_SAK_op,			/* k */
>  #ifdef CONFIG_SMP
>  	&sysrq_showallcpus_op,		/* l */
> Index: linux-2.6/fs/buffer.c
> ===================================================================
> --- linux-2.6.orig/fs/buffer.c	2009-01-16 13:24:15.564575078 -0600
> +++ linux-2.6/fs/buffer.c	2009-01-16 13:26:35.257575540 -0600
> @@ -258,6 +258,29 @@ struct super_block *freeze_bdev(struct b
>  }
>  EXPORT_SYMBOL(freeze_bdev);
>  
> +void do_thaw_all(unsigned long unused)
> +{
> +	struct super_block *sb;
> +	char b[BDEVNAME_SIZE];
> +
> +	list_for_each_entry(sb, &super_blocks, s_list) {
> +		while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
> +			printk(KERN_WARNING "Emergency Thaw on %s\n",
> +			       bdevname(sb->s_bdev, b));
> +	}
> +	printk(KERN_WARNING "Emergency Thaw complete\n");
> +}
> +
> +/**
> + * emergency_thaw_all -- forcibly thaw every frozen filesystem
> + *
> + * Used for emergency unfreeze of all filesystems via SysRq-z

                                                              -j

> + */
> +void emergency_thaw_all(void)
> +{
> +	pdflush_operation(do_thaw_all, 0);
> +}
> +
>  /**
>   * thaw_bdev  -- unlock filesystem
>   * @bdev:	blockdevice to unlock


-- 
~Randy
--
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