[PATCH 0/2] Ext3 improvements to go to Linus

[PATCH 0/2] Ext3 improvements to go to Linus

[Jan Kara]

  Hi,

  I plan to send the two following patches to ext3 to Linus tomorrow - the
first one improves the description of the default journaling mode config option
(I'm pushing this mainly because the config option is new in this kernel so I
feel it's better to have a better description from the beginning). The second
one improves the error message when we refuse to change the journaling mode on
remount (see the changelog for why I think it's important).

									Honza

[PATCH] ext3: Update Kconfig description of EXT3_DEFAULTS_TO_ORDERED

[Jan Kara]

From: Theodore Ts'o <tytso@mit.edu>

The old description for this configuration option was perhaps not
completely balanced in terms of describing the tradeoffs of using a
default of data=writeback vs. data=ordered.  Despite the fact that old
description very strongly recomended disabling this feature, all of
the major distributions have elected to preserve the existing 'legacy'
default, which is a strong hint that it perhaps wasn't telling the
whole story.

This revised description has been vetted by a number of ext3
developers as being better at informing the user about the tradeoffs
of enabling or disabling this configuration feature.

Cc: linux-ext4@vger.kernel.org
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext3/Kconfig |   32 +++++++++++++++++---------------
 1 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/fs/ext3/Kconfig b/fs/ext3/Kconfig
index fb3c1a2..522b154 100644
--- a/fs/ext3/Kconfig
+++ b/fs/ext3/Kconfig
@@ -29,23 +29,25 @@ config EXT3_FS
 	  module will be called ext3.
 
 config EXT3_DEFAULTS_TO_ORDERED
-	bool "Default to 'data=ordered' in ext3 (legacy option)"
+	bool "Default to 'data=ordered' in ext3"
 	depends on EXT3_FS
 	help
-	  If a filesystem does not explicitly specify a data ordering
-	  mode, and the journal capability allowed it, ext3 used to
-	  historically default to 'data=ordered'.
-
-	  That was a rather unfortunate choice, because it leads to all
-	  kinds of latency problems, and the 'data=writeback' mode is more
-	  appropriate these days.
-
-	  You should probably always answer 'n' here, and if you really
-	  want to use 'data=ordered' mode, set it in the filesystem itself
-	  with 'tune2fs -o journal_data_ordered'.
-
-	  But if you really want to enable the legacy default, you can do
-	  so by answering 'y' to this question.
+	  The journal mode options for ext3 have different tradeoffs
+	  between when data is guaranteed to be on disk and
+	  performance.	The use of "data=writeback" can cause
+	  unwritten data to appear in files after an system crash or
+	  power failure, which can be a security issue.	 However,
+	  "data=ordered" mode can also result in major performance
+	  problems, including seconds-long delays before an fsync()
+	  call returns.	 For details, see:
+
+	  http://ext4.wiki.kernel.org/index.php/Ext3_data_mode_tradeoffs
+
+	  If you have been historically happy with ext3's performance,
+	  data=ordered mode will be a safe choice and you should
+	  answer 'y' here.  If you understand the reliability and data
+	  privacy issues of data=writeback and are willing to make
+	  that trade off, answer 'n'.
 
 config EXT3_FS_XATTR
 	bool "Ext3 extended attributes"
-- 
1.6.0.2

[PATCH] ext3: Improve error message that changing journaling mode on remount is not possible

[Jan Kara]

This patch makes the error message about changing journaling mode on remount
more descriptive. Some people are going to hit this error now due to commit
bbae8bcc49bc4d002221dab52c79a50a82e7cd1f if they configure a kernel to default
to data=writeback mode. The problem happens if they have data=ordered set for
the root filesystem in /etc/fstab but not in the kernel command line (and they
don't use initrd). Their filesystem then gets mounted as data=writeback by
kernel but then their boot fails because init scripts won't be able to remount
the filesystem rw. Better error message will hopefully make it easier for them
to find the error in their setup and bother us less with error reports :).

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext3/super.c |   40 +++++++++++++++++++++++++++-------------
 1 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/fs/ext3/super.c b/fs/ext3/super.c
index 524b349..a8d80a7 100644
--- a/fs/ext3/super.c
+++ b/fs/ext3/super.c
@@ -543,6 +543,19 @@ static inline void ext3_show_quota_options(struct seq_file *seq, struct super_bl
 #endif
 }
 
+static char *data_mode_string(unsigned long mode)
+{
+	switch (mode) {
+	case EXT3_MOUNT_JOURNAL_DATA:
+		return "journal";
+	case EXT3_MOUNT_ORDERED_DATA:
+		return "ordered";
+	case EXT3_MOUNT_WRITEBACK_DATA:
+		return "writeback";
+	}
+	return "unknown";
+}
+
 /*
  * Show an option if
  *  - it's set to a non-default value OR
@@ -616,13 +629,8 @@ static int ext3_show_options(struct seq_file *seq, struct vfsmount *vfs)
 	if (test_opt(sb, NOBH))
 		seq_puts(seq, ",nobh");
 
-	if (test_opt(sb, DATA_FLAGS) == EXT3_MOUNT_JOURNAL_DATA)
-		seq_puts(seq, ",data=journal");
-	else if (test_opt(sb, DATA_FLAGS) == EXT3_MOUNT_ORDERED_DATA)
-		seq_puts(seq, ",data=ordered");
-	else if (test_opt(sb, DATA_FLAGS) == EXT3_MOUNT_WRITEBACK_DATA)
-		seq_puts(seq, ",data=writeback");
-
+	seq_printf(seq, ",data=%s", data_mode_string(sbi->s_mount_opt &
+						     EXT3_MOUNT_DATA_FLAGS));
 	if (test_opt(sb, DATA_ERR_ABORT))
 		seq_puts(seq, ",data_err=abort");
 
@@ -1024,12 +1032,18 @@ static int parse_options (char *options, struct super_block *sb,
 		datacheck:
 			if (is_remount) {
 				if ((sbi->s_mount_opt & EXT3_MOUNT_DATA_FLAGS)
-						!= data_opt) {
-					printk(KERN_ERR
-						"EXT3-fs: cannot change data "
-						"mode on remount\n");
-					return 0;
-				}
+						== data_opt)
+					break;
+				printk(KERN_ERR
+					"EXT3-fs (device %s): Cannot change "
+					"data mode on remount. The filesystem "
+					"is mounted in data=%s mode and you "
+					"try to remount it in data=%s mode.\n",
+					sb->s_id,
+					data_mode_string(sbi->s_mount_opt &
+							EXT3_MOUNT_DATA_FLAGS),
+					data_mode_string(data_opt));
+				return 0;
 			} else {
 				sbi->s_mount_opt &= ~EXT3_MOUNT_DATA_FLAGS;
 				sbi->s_mount_opt |= data_opt;
-- 
1.6.0.2