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

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Message-ID: <4F4CBDA1.1080302@msgid.tls.msk.ru>
Date:	Tue, 28 Feb 2012 15:42:25 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	stable@...nel.org
CC:	linux-ext4@...r.kernel.org
Subject: Re: what happened with dccaf33fa37 "ext4: flush any pending end_io
 requests before DIO" for 3.0?

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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