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

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Date:	Fri, 16 Jan 2009 13:46:44 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Randy Dunlap <randy.dunlap@...cle.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

Randy Dunlap wrote:
> 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)" ?

Sure, that's good.

> 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

Oops, was going to be unfreeZe 'til I realized peterz took that already!

-Eric

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

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