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

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Date:	Fri, 06 May 2016 06:01:12 +0000 (GMT)
From:	Daeho Jeong <daeho.jeong@...sung.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	jack@...e.cz,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	이기태 <kitae87.lee@...sung.com>
Subject: Re: [PATCH] ext4: guarantee already started handles to successfully
 finish while ro remounting

> Hmm, I'm not really comfortable with putting this hack in, since this
> is papering over the real problem, which is that Android is trying to
> use the emergency remount read-only sysrq option and this is
> fundamentally unsafe.  I'm not sure what else could break if it is
> situation normal that there is active processes busily writing to the
> file system and sysrq-u followed by reboot is the normal way the
> Android kernel does a reboot.

> A much better solution would be to change the Android userspace to
> call the FIFREEZE ioctl on each mounted file system, and then call for
> a reboot.

I agree with you. I know that current Android shutdown procedure is
not a safe way. But, without this patch, "even not in Android system",
when we trigger the emergency read-only remount while evicting inodes,
i_size of the inode becomes zero and the inode is not in orphan list,
but blocks of the inode are still allocated to the inode, because
ext4_truncate() will fail while stating the handle which was already started
by ext4_evict_inode(). This causes the filesystem inconsistency and
we will encounter an ext4 kernel panic in the next boot by this problem.

I think that this kind of filesystem crash can occur anywhere that
the same journal handle is repeatly used. During an atomic filesystem
operation, a part of the operation will succeed and the other one will fail.