linux-ext4 - Re: [PATCH] jbd_commit_transaction() races with journal_try_to_drop

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Date:	Wed, 14 May 2008 19:08:56 +0200
From:	Jan Kara <jack@...e.cz>
To:	Mingming Cao <cmm@...ibm.com>
Cc:	Jan Kara <jack@...e.cz>, Badari Pulavarty <pbadari@...ibm.com>,
	akpm@...ux-foundation.org, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] jbd_commit_transaction() races with
	journal_try_to_drop_buffers() causing DIO failures

On Tue 13-05-08 15:23:09, Mingming Cao wrote:
> On Tue, 2008-05-13 at 16:54 +0200, Jan Kara wrote:
> > On Mon 12-05-08 17:39:43, Mingming Cao wrote:
> > > On Mon, 2008-05-12 at 17:54 +0200, Jan Kara wrote:
> > > Does this match what you are thinking? It certainly slow down the DIO
> > > path, but the positive side is it doesn't disturb the other code path...
> > > thanks for your feedback!
> > > 
> > > --------------------------------------------
> > > 
> > > An unexpected EIO error gets returned when writing to a file
> > > using buffered writes and DIO writes at the same time.
> > > 
> > > We found there are a number of places where journal_try_to_free_buffers()
> > > could race with journal_commit_transaction(), the later still
> > > helds the reference to the buffers on the t_syncdata_list or t_locked_list
> > > , while journal_try_to_free_buffers() tries to free them, which resulting an EIO
> > > error returns back to the dio caller. 
> > > 
> > > The logic fix is to retry freeing if journal_try_to_free_buffers() to failed
> > > to free those data buffers while journal_commit_transaction() is still
> > > reference those buffers.
> > > This is done via implement ext3 launder_page() callback, instead of inside
> > > journal_try_to_free_buffers() itself, so that it doesn't affecting other code
> > > path calling journal_try_to_free_buffers and only dio path get affected.
> > > 
> > > Signed-off-by: Mingming Cao <cmm@...ibm.com>
> > > Index: linux-2.6.26-rc1/fs/ext3/inode.c
> > > ===================================================================
> > > --- linux-2.6.26-rc1.orig/fs/ext3/inode.c	2008-05-03 11:59:44.000000000 -0700
> > > +++ linux-2.6.26-rc1/fs/ext3/inode.c	2008-05-12 12:41:27.000000000 -0700
> > > @@ -1766,6 +1766,23 @@ static int ext3_journalled_set_page_dirt
> > >  	return __set_page_dirty_nobuffers(page);
> > >  }
> > >  
> > > +static int ext3_launder_page(struct page *page)
> > > +{
> > > +        int ret;
> > > +	int retry = 5;
> > > +
> > > +	while (retry --) {
> > > +		ret = ext3_releasepage(page, GFP_KERNEL);
> > > +		if (ret == 1)
> > > +			break;
> > > +		else
> > > +			schedule();
> > > +	}
> > > +
> > > +        return ret;
> > > +}
> > > +
> > > +
> >   Yes, I meant something like this. We could be more clever and do:
> > 
> > 	head = bh = page_buffers(page);
> > 	do {
> > 		wait_on_buffer(bh);
> > 		bh = bh->b_this_page;
> > 	} while (bh != head);
> > 	/*
> > 	 * Now commit code should have been able to proceed and release
> >          * those buffers
> > 	 */
> >         schedule();
> > 
> 
> Bummer, we can't free buffers in ext3_launder_page() before calling
> try_to_free_page, as later
> invalidate_complete_page2()->try_to_free_page() expecting the page
> buffers are still here, and will return EIO if it launder_page() has
> already freed those buffers.:(
  Are you sure? Because if bufferes are released in ext3_launder_page(),
PagePrivate() has been set to 0 and we should directly fall through to
releasing the page without ever calling try_to_release_page()... So I'd
want to find out why PagePrivate is still set in
invalidate_complete_page2().

> Doing wait_on_buffer() alone in launder_page() is not enough as it
> doesn't wait for buffer reference drop to 0.
  Yes, this would not be enough.

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