linux-ext4 - Re: [PATCH] ext4: guarantee already started handles to successfully finish while ro remounting

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Date:	Thu, 5 May 2016 15:45:11 +0200
From:	Jan Kara <jack@...e.cz>
To:	Daeho Jeong <daeho.jeong@...sung.com>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org,
	Kitae Lee <kitae87.lee@...sung.com>
Subject: Re: [PATCH] ext4: guarantee already started handles to successfully
 finish while ro remounting

On Mon 02-05-16 09:50:37, Daeho Jeong wrote:
> We check whether a new handle can be started through
> ext4_journal_check_start() and the function refuses to start the handle
> when the filesystem is mounted with read-only. But now, when we remount
> the filesystem with read-only option, already started handles are
> allowed to be written on disk, but the subsequent metadata modification
> using the handles are refused by ext4_journal_check_start().
> 
> As an example, in ext4_evict_inode(), i_size can be set to 0 using
> a successfully started handle, but, when we remount the filesystem
> with read-only option at that time, the subsequent ext4_truncate()
> will be failed and the filesystem integrity will be damaged.
> 
> Therefore, we need to permit the metadata modification using already
> started handles to be proceeded, even if s_flags of the filesystem is
> set to MS_RDONLY.
> 
> Kitae found the problem and suggested the solution.

So can you share a reproducer for this issue? Because my initial thinking
is that checks during remount should fail the remount with EBUSY if there
is any modification outstanding... If they don't we have a racy remount and
fs-freezing code, which is a bug.

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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