linux-ext4 - Re: Possible race between direct IO and JBD?

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Message-ID: <20080428122626.GC17054@duck.suse.cz>
Date:	Mon, 28 Apr 2008 14:26:26 +0200
From:	Jan Kara <jack@...e.cz>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	akpm@...ux-foundation.org, pbadari@...ibm.com,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Possible race between direct IO and JBD?

  Hi,

On Fri 25-04-08 16:38:23, Mingming Cao wrote:
> While looking at a bug related to direct IO returns to EIO, after
> looking at the code, I found there is a window that
> try_to_free_buffers() from direct IO could race with JBD, which holds
> the reference to the data buffers before journal_commit_transaction()
> ensures the data buffers has reached to the disk.
> 
> A little more detail: to prepare for direct IO, generic_file_direct_IO()
> calls invalidate_inode_pages2_range() to invalidate the pages in the
> cache before performaning direct IO.  invalidate_inode_pages2_range()
> tries to free the buffers via try_to free_buffers(), but sometimes it
> can't, due to the buffers is possible still on some transaction's
> t_sync_datalist or t_locked_list waiting for
> journal_commit_transaction() to process it. 
> 
> Currently Direct IO simply returns EIO if try_to_free_buffers() finds
> the buffer is busy, as it has no clue that JBD is referencing it.
> 
> Is this a known issue and expected behavior? Any thoughts?
  Are you seeing this in data=ordered mode? As Andrew pointed out we do
filemap_write_and_wait() so all the relevant data buffers of the inode
should be already on disk. In __journal_try_to_free_buffer() we check
whether the buffer is already-written-out data buffer and unfile and free
it in that case. It shouldn't happen that a data buffer has
b_next_transaction set so really the only idea why try_to_free_buffers()
could fail is that somebody manages to write to a page via mmap before
invalidate_inode_pages2_range() gets to it. Under which kind of load do you
observe the problem? Do you know exactly because of which condition does
journal_try_to_free_buffers() fail?

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