linux-ext4 - Re: [PATCH] fs/jbd2: t_updates should increase when start_this_handle() failed in jbd2__journal

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Message-ID: <20130629132229.GJ13405@ZenIV.linux.org.uk>
Date:	Sat, 29 Jun 2013 14:22:29 +0100
From:	Joel Becker <jlbec@...lplan.org>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Younger Liu <younger.liu@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org,
	Ocfs2-Devel <ocfs2-devel@....oracle.com>,
	Li Zefan <lizefan@...wei.com>, jack@...e.cz
Subject: Re: [PATCH] fs/jbd2: t_updates should increase when
 start_this_handle() failed in jbd2__journal_restart()

On Sun, Jun 23, 2013 at 01:36:28PM -0400, Theodore Ts'o wrote:
> On Fri, Jun 21, 2013 at 09:29:31PM +0800, Younger Liu wrote:
> > 
> > This bug was triggered by the following scenario:
> > In ocfs2 file system, allocate a very large disk space for a small file
> > with ocfs2_fallocate(), while the journal file size is 32M. 
> > 
> > Because there are much many journal blocks needed by jbd2_journal_restart(), 
> > so that nblocks is greater than journal->j_max_transaction_buffers 
> > in start_this_handle(), and then return -ENOSPC.
> 
> Ah, I see.  I have a patch that should prevent the kernel from
> crashing in this situation, and which adds some additional checks to
> make sure no one tries to use the handle after jbd2_journal_restart()
> fails in this circumstance.
> 
> However, you may want to further pursue a fix in ocfs2 so you don't
> actually return ENOSPC to userspace, since it is a very misleading
> error message --- it's not that the file system is out of space, but
> that the journal is too small for the amount of space that you are
> trying to allocate using fallocate().
> 
> I would think a better way of handling this situation would be to log
> a warning message that the journal is probably too small, and then to
> break up the fallocate into smaller chunks, so that it can
> successfully complete despite the fact that the journal was
> unfortunately missized.

	Yes, this solution is a good one.

Joel

> 
> I'll be sending the proposed fix in a moment; could you check and see
> if the patch prevents ocfs2/jbd2 from tripping over the assertion
> given your test case?
> 
> Thanks,
> 
> 					- Ted
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