linux-ext4 - [PATCH 1/3] ext4: handle unwritten or delalloc buffers before enabling per-file data journaling

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Date:	Wed, 18 Nov 2015 10:34:32 +0900
From:	Daeho Jeong <daeho.jeong@...sung.com>
To:	tytso@....edu, linux-ext4@...r.kernel.org, daeho.jeong@...sung.com
Subject: [PATCH 1/3] ext4: handle unwritten or delalloc buffers before enabling
 per-file data journaling

We already allocate delalloc blocks before changing the inode mode into
"per-file data journal" mode to prevent delalloc blocks from remaining
not allocated, but another issue concerned with "BH_Unwritten" status
still exists. For example, by fallocate(), several buffers' status
change into "BH_Unwritten", but these buffers cannot be processed by
ext4_alloc_da_blocks(). So, they still remain in unwritten status after
per-file data journaling is enabled and they cannot be changed into
written status any more and, if they are journaled and eventually
checkpointed, these unwritten buffer will cause a kernel panic by the
below BUG_ON() function of submit_bh_wbc() when they are submitted
during checkpointing.

static int submit_bh_wbc(int rw, struct buffer_head *bh,...
{
        ...
        BUG_ON(buffer_unwritten(bh));

Moreover, when "dioread_nolock" option is enabled, the status of a
buffer is changed into "BH_Unwritten" after write_begin() completes and
the "BH_Unwritten" status will be cleared after I/O is done. Therefore,
if a buffer's status is changed into unwrutten but the buffer's I/O is
not submitted and completed, it can cause the same problem after
enabling per-file data journaling. You can easily generate this bug by
executing the following command.

./kvm-xfstests -C 10000 -m nodelalloc,dioread_nolock generic/269

To resolve these problems and define a boundary between the previous
mode and per-file data journaling mode, we need to flush and wait all
the I/O of buffers of a file before enabling per-file data journaling
of the file.

Signed-off-by: Daeho Jeong <daeho.jeong@...sung.com>
---
 fs/ext4/inode.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 612fbcf..1f9458e 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5168,9 +5168,14 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
 	 * be allocated any more. even more truncate on delalloc blocks
 	 * could trigger BUG by flushing delalloc blocks in journal.
 	 * There is no delalloc block in non-journal data mode.
+	 * We also have to handle unwritten buffers generated by
+	 * fallocate() and dioread_nolock option. Once per-file data
+	 * journaling is enabled, unwritten buffers will remain in
+	 * unwritten status forever and they will be the seeds of
+	 * kernel panic when they are checkpointed.
 	 */
-	if (val && test_opt(inode->i_sb, DELALLOC)) {
-		err = ext4_alloc_da_blocks(inode);
+	if (val) {
+		err = filemap_write_and_wait(inode->i_mapping);
 		if (err < 0)
 			return err;
 	}
-- 
1.7.9.5

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