linux-ext4 - Re: [PATCH] Make non-journal fsync work properly.

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Date:	Tue, 8 Sep 2009 07:57:02 -0700
From:	Curt Wohlgemuth <curtw@...gle.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Frank Mayhar <fmayhar@...gle.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] Make non-journal fsync work properly.

Hi Ted:

On Mon, Sep 7, 2009 at 10:06 PM, Theodore Tso<tytso@....edu> wrote:
> On Fri, Sep 04, 2009 at 07:55:00PM -0700, Frank Mayhar wrote:
>> Teach ext4_write_inode() and ext4_do_update_inode() about non-journal
>> mode:  If we're not using a journal, ext4_write_inode() now calls
>> ext4_do_update_inode() (after getting the iloc via ext4_get_inode_loc())
>> with a new "do_sync" parameter.  If that parameter is nonzero
>> ext4_do_update_inode() calls sync_dirty_buffer() instead of
>> ext4_handle_dirty_metadata().
>
> Hi Frank,
>
> The problem with this patch is that it's only safe to call
> sync_dirty_buffer() if we are not journalling.  If we are using the
> journal, we must *not* call sync_dirty_buffer(), but instead must use
> jbd2_journal_dirty_metadata().
>
> The problem is that there are paths where ext4_do_update_inode() can
> get called with do_sync==1, even when journalling is enabled.
> Specifically, if ext4_write_inode() is called with wait==1, wait is
> passed to ext4_do_update_inode() as do_sync, and then when a journal
> is present, we will end up calling sync_dirty_buffer(), which means we
> will be writing out the modified metadata *before* the transaction has
> committed.
>
> If you try using your patch with journalling enabled, and you try
> doing some power fail testing, my code inspection leads me to believe
> with 99% certainty that the filesystem will be corrupted as a result.
>
> I think what you need to do instead is to add an extra parameter
> do_sync to ext4_handle_dirty_metadata(), and continue to call
> ext4_handle_dirty_metadata.  However in code paths where we will later
> force a commit to guarantee that the metadata has been written out
> (i.e., in the fsync() code path), ext4_handle_dirty_metadata() should
> be called with the new do_sync parameter set to 1.
>
> Does that make sense?

I think we can take a look at this, but there are a lot of calls to
ext4_handle_dirty_metadata(), and it's not clear on a quick inspection
that we'd be able to determine which would need to be called with
do_sync = 1...

On the other hand, this would take care of a similar problem that I
was going to be sending a patch for this week: where removing an
extent block without a journal requires a sync_dirty_buffer() in order
to avoid writeback of the extent header in the block, *after* the
block is marked free in the bitmap.

There are probably other cases where, without a journal, an explicit
sync_dirty_buffer() is needed for metadata.  Handling this in
ext4_handle_dirty_metadata() may be the best way to solve this.

Thanks,
Curt

>
>                                                - Ted
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