linux-kernel - Re: [PATCH 1/3] Implement generic freeze feature

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Message-ID: <85FB1C9FF587411A99FF66E76008D2A6@nsl.ad.nec.co.jp>
Date:	Thu, 11 Sep 2008 19:58:08 +0900
From:	"Takashi Sato" <t-sato@...jp.nec.com>
To:	"Eric Sandeen" <sandeen@...hat.com>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	<linux-fsdevel@...r.kernel.org>, <dm-devel@...hat.com>,
	<viro@...IV.linux.org.uk>, <linux-ext4@...r.kernel.org>,
	<xfs@....sgi.com>, "Christoph Hellwig" <hch@...radead.org>,
	<axboe@...nel.dk>, <mtk.manpages@...glemail.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Implement generic freeze feature

Hi,

Eric Sandeen:
>> +static int ioctl_freeze(struct file *filp)
>> +{
>> +     struct super_block *sb = filp->f_path.dentry->d_inode->i_sb;
>> +
>> +     if (!capable(CAP_SYS_ADMIN))
>> +         return -EPERM;
>> +
>> +     /* If filesystem doesn't support freeze feature, return. */
>> +     if (sb->s_op->write_super_lockfs == NULL)
>> +         return -EOPNOTSUPP;
>> +
>> +     /* If a regular file or a directory isn't specified, return. */
>> +     if (sb->s_bdev == NULL)
>> +         return -EINVAL;
>> +
>> +     /* Freeze */
>> +     sb = freeze_bdev(sb->s_bdev);
>> +     if (IS_ERR(sb))
>> +         return PTR_ERR(sb);
>> +     return 0;
>> +}
>
> Not a problem with your patch exactly, but I was just wondering; you
> check here whether the sb returned from freeze_bdev is an ERR_PTR (as
> does lock_fs()) - but, freeze_bdev never returns an error, does it?
> ->write_super_lockfs is a void...
>
> It really seems that at least we should be able to handle IO errors on
> the freeze request, and tell the user "No, your filesystem was not
> frozen..."?
>
> Maybe I'll whip up a patch to see about propagating freeze errors up
> from the filesystems that implement it, unless I'm missing some reason
> not to do so...?

Right.
We should handle an IO error which occurs in write_super_lockfs.
I will change the write_super_lockfs's type to "int" so that it can return an error.
And I will consider returning an error of ext3_write_super_lockfs because
journal_flush() in ext3_write_super_lockfs() doesn't handle an IO error.

> Also, should this be checking for a NULL returned from freeze_bdev as
> well?  I guess this should never happen if we have a file open on which
> we are calling the ioctl ...

I think ioctl_freeze doesn't need to check NULL because it never happen
as you said.

Cheers, Takashi
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