linux-kernel - Re: [PATCH 3/4] Do not allow umounting of frozen filesystems

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Date:	Wed, 23 Sep 2009 01:41:09 +0900
From:	Fernando Luis Vazquez Cao <fernando@....ntt.co.jp>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Christoph Hellwig <hch@....de>, t-sato@...jp.nec.com,
	m-hamaguchi@...jp.nec.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Eric Sandeen <sandeen@...hat.com>
Subject: Re: [PATCH 3/4] Do not allow umounting of frozen filesystems

Al Viro さんは書きました:
> On Thu, Aug 27, 2009 at 11:06:07PM +0900, Fernando Luis V?zquez Cao wrote:
>   
>> Instead of making umount users wait until the filesystem is
>> unfreezed return EBUSY, which is very convenient in HA
>> configurations.
>>
>> This could have been implemented at a lower level but it would
>> require considerable plumbing in functions such as release_mounts
>> which do not return errors.
>>     
>
>   
>> +	if (sb->s_bdev != NULL) {
>> +		mutex_lock(&sb->s_bdev->bd_fsfreeze_mutex);
>> +		if (sb->s_frozen != SB_UNFROZEN) {
>> +			mutex_unlock(&sb->s_bdev->bd_fsfreeze_mutex);
>> +			return -EBUSY;
>> +		}
>> +	}
>>     
>
> NAK.  First of all, it _partially_ breaks umount -l for no good reason.
> If the first fs on the mountpoint is frozen, we fail; if it's deeper
> we succeed just fine (and delay actual fs shutdown until the thaw).
>
> As far as I can see, the real problem is that fsthaw ioctl has braindead
> API; it takes some opened file on fs in question.  Why not do a bdev
> ioctl instead?  Then we could let umount go ahead just fine, leaving
> fs frozen (and not shut down until it thaws).  And whoever does thaw
> (via bdev ioctl) will automatically trigger the actual fs shutdown.
> Just with Christoph's pair of patches...
>   

I basically agree with you. The current API creates a lot of locking 
issues that could be tackled
more cleanly with the bdev ioctls you suggest.

> IOW, I'd rather add two new ioctls (check if frozen/thaw), both by
> bdev.  On top of the first two patches in this set.
>   

I am happy to see you would welcome a check ioctl.

If there is consensus on the bdev ioctl approach I could send patches.

Thanks,

Fernando
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