linux-ext4 - Re: what happened with dccaf33fa37 "ext4: flush any pending end

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Date:	Mon, 19 Mar 2012 17:42:56 +0100
From:	Jan Kara <jack@...e.cz>
To:	gregkh@...uxfoundation.org
Cc:	stable@...nel.org, linux-ext4@...r.kernel.org,
	Jiaying Zhang <jiayingz@...gle.com>,
	Theodore Ts'o <tytso@....edu>, Michael Tokarev <mjt@....msk.ru>
Subject: Re: what happened with dccaf33fa37 "ext4: flush any pending end_io
 requests before DIO" for 3.0?

On Sat 17-03-12 13:31:30, Michael Tokarev wrote:
> Ping?
> 
> Maybe just a one-line reply isn't THAT difficult?
> 
> We've a data corruption bug in current longterm stable kernel
> series which is known and has a fix and tagged for -stable for
> over half a year already...
  Greg, any idea why this patch was not included?

							Honza

> On 28.02.2012 15:42, Michael Tokarev wrote:
> > Is there something wrong with my question?  I asked it 1.5 months ago...
> > 
> > Meanwhile, we're using this patch on our database server since
> > Aug-2011, and it appears to work correctly - direct and buffered
> > I/O works together without surprizes.  Without this patch, I see
> > unexpected results.
> > 
> > Thanks,
> > 
> > /mjt
> > 
> > On 01.12.2011 00:38, Michael Tokarev wrote:
> >> Hello.
> >>
> >> Back in August 2011, a commit has been tagged to be included
> >> into stable, this one:
> >>
> >> commit dccaf33fa37a1bc5d651baeb3bfeb6becb86597b
> >> Author: Jiaying Zhang <jiayingz@...gle.com>
> >> Date:   Fri Aug 19 19:13:32 2011 -0400
> >>
> >>     ext4: flush any pending end_io requests before DIO reads w/dioread_nolock
> >>
> >>     There is a race between ext4 buffer write and direct_IO read with
> >>     dioread_nolock mount option enabled. The problem is that we clear
> >>     PageWriteback flag during end_io time but will do
> >>     uninitialized-to-initialized extent conversion later with dioread_nolock.
> >>     If an O_direct read request comes in during this period, ext4 will return
> >>     zero instead of the recently written data.
> >>
> >>     This patch checks whether there are any pending uninitialized-to-initialized
> >>     extent conversion requests before doing O_direct read to close the race.
> >>     Note that this is just a bandaid fix. The fundamental issue is that we
> >>     clear PageWriteback flag before we really complete an IO, which is
> >>     problem-prone. To fix the fundamental issue, we may need to implement an
> >>     extent tree cache that we can use to look up pending to-be-converted extents.
> >>
> >>     Signed-off-by: Jiaying Zhang <jiayingz@...gle.com>
> >>     Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> >>     Cc: stable@...nel.org
> >>
> >>
> >> There was one more ext4 commit at that time, which made its way into
> >> stable but this one did not.
> >>
> >> I wonder if the reason for that was the fact that it needed a small
> >> "backport" for 3.0, since in 3.1+ the code has been moved into another
> >> file, and the context is slightly different.  In that case, attached
> >> is the "backport" which we use with 3.0.x since that time.
> >>
> >> Thanks!
> >>
> >> /mjt
> > 
> 
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