linux-kernel - [ 081/108] ext4: flush any pending end_io requests before DIO reads w/dioread

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Message-Id: <20120330195733.789810217@linuxfoundation.org>
Date:	Fri, 30 Mar 2012 12:58:43 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Jiaying Zhang <jiayingz@...gle.com>,
	"Theodore Tso" <tytso@....edu>, Jan Kara <jack@...e.cz>,
	Michael Tokarev <mjt@....msk.ru>
Subject: [ 081/108] ext4: flush any pending end_io requests before DIO reads w/dioread_nolock

3.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jiaying Zhang <jiayingz@...gle.com>

commit dccaf33fa37a1bc5d651baeb3bfeb6becb86597b upstream.

(backported to 3.0 by mjt)

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: Jan Kara <jack@...e.cz>
Signed-off-by: Michael Tokarev <mjt@....msk.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/ext4/inode.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3510,12 +3510,17 @@ static ssize_t ext4_ind_direct_IO(int rw
 	}

 retry:
-	if (rw == READ && ext4_should_dioread_nolock(inode))
+	if (rw == READ && ext4_should_dioread_nolock(inode)) {
+		if (unlikely(!list_empty(&ei->i_completed_io_list))) {
+			mutex_lock(&inode->i_mutex);
+			ext4_flush_completed_IO(inode);
+			mutex_unlock(&inode->i_mutex);
+		}
 		ret = __blockdev_direct_IO(rw, iocb, inode,
 				 inode->i_sb->s_bdev, iov,
 				 offset, nr_segs,
 				 ext4_get_block, NULL, NULL, 0);
-	else {
+	} else {
 		ret = blockdev_direct_IO(rw, iocb, inode,
 				 inode->i_sb->s_bdev, iov,
 				 offset, nr_segs,

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