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Data Corrupting ext3 Bug In Latest Linux 2.4.20
An anonymous reader writes "Andrew Morton alerted readers of the Linux Kernel mailing list today that ext3 in the 2.4.20 kernel has a new bug that can easily cause file data corruption at unmount time. The bug will only affect people using ext3 in "data=journal" mode, which fortunately is not the default... Full details can be read on KernelTrap."
In 2.4.20-pre5 an optimisation was made to the ext3 fsync function
/* In writeback mode, we need to force out data buffers too. In /*
e read the FAQ at http://www.tux.org/lkml/
which can very easily cause file data corruption at unmount time. This
was first reported by Nick Piggin on November 29th (one day after 2.4.20 was
released, and three months after the bug was merged. Unfortunate timing)
This only affects filesystems which were mounted with the `data=journal'
option. Or files which are operating under `chattr -j'. So most people
are unaffected. The problem is not present in 2.5 kernels.
The symptoms are that any file data which was written within the thirty
seconds prior to the unmount may not make it to disk. A workaround is
to run `sync' before unmounting.
The optimisation was intended to avoid writing out and waiting on the
inode's buffers when the subsequent commit would do that anyway. This
optimisation was applied to both data=journal and data=ordered modes.
But it is only valid for data=ordered mode.
In data=journal mode the data is left dirty in memory and the unmount
will silently discard it.
The fix is to only apply the optimisation to inodes which are operating
under data=ordered.
--- linux-akpm/fs/ext3/fsync.c~ext3-fsync-fix Sat Nov 30 23:37:33 2002
+++ linux-akpm-akpm/fs/ext3/fsync.c Sat Nov 30 23:39:30 2002
@@ -63,10 +63,12 @@ int ext3_sync_file(struct file * file, s
*/
ret = fsync_inode_buffers(inode);
-
- * the other modes, ext3_force_commit takes care of forcing out
- * just the right data blocks. */
- if (test_opt(inode->i_sb, DATA_FLAGS) == EXT3_MOUNT_WRITEBACK_DATA)
+
+ * If the inode is under ordered-data writeback it is not necessary to
+ * sync its data buffers here - commit will do that, with potentially
+ * better IO merging
+ */
+ if (!ext3_should_order_data(inode))
ret |= fsync_inode_data_buffers(inode);
ext3_force_commit(inode->i_sb);
_
-
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Pleas
-- Note: If you don't agree with me, don't bother replying. I won't read it.
Unmounts happen at shutdown. You also need to unmount before scanning/fixing a filesystem. The whole bug here pertains to the fact that it isn't flushing ("syncing") the last 30 seconds of cached data to the disk beforehand. A cold reboot without unmounting could potentially cause all kinds of other data inconsistency problems to pop up.
/etc/rc.d/init.d/mountfs (or whatever it's called on your system) script right before the "umount" line.
The temporary fix seems to be to run sync manually. Stick "sync" in your
Liberty in your lifetime