linux-ext4 - [PATCH] e2fsprogs: Fix reserved blocks calculation for mke2fs -m 50%

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Date:	Thu, 30 Aug 2012 20:16:01 +0900
From:	Akira Fujita <a-fujita@...jp.nec.com>
To:	Theodore Tso <tytso@....edu>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: [PATCH] e2fsprogs: Fix reserved blocks calculation for mke2fs -m
 50%

mke2fs -m option can set reserved blocks ratio up to 50%.
But if the last BG is not big enough to support the necessary data
structures, reserved blocks ratio can exceed 50%
and we never succeed in e2fsck with that FS.
This bug happens on ext2, ext3 and ext4.

To fix this, the patch recalculates reserved blocks count
if FS blocks count is changed in mke2fs because of
the last BG's size.

Steps to reproduce:

1. Create a FS which has the overhead for the last BG
   and specify 50 % for reserved blocks ratio
 # mke2fs -m 50 -t ext4 DEV 1025M

 mke2fs 1.42.5 (29-Jul-2012)
 warning: 256 blocks unused.

 Filesystem label=
 OS type: Linux
 Block size=4096 (log=2)
 Fragment size=4096 (log=2)
 Stride=0 blocks, Stripe width=0 blocks
 656640 inodes, 2621440 blocks
 1310848 blocks (50.00%) reserved for the super user
 ~~~~~~~ <-- Reserved blocks exceed 50% of FS blocks count!

2. e2fsck outputs filesystem corruption
 # e2fsck DEV

 e2fsck 1.42.5 (29-Jul-2012)
 Corruption found in superblock.  (r_blocks_count = 1310848).

 The superblock could not be read or does not describe a correct ext2
 filesystem.  If the device is valid and it really contains an ext2
 filesystem (and not swap or ufs or something else), then the superblock
 is corrupt, and you might try running e2fsck with an alternate superblock:
     e2fsck -b 32768 <device>

Signed-off-by: Akira Fujita <a-fujita@...jp.ne.com>
---
 lib/ext2fs/initialize.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
diff --git a/lib/ext2fs/initialize.c b/lib/ext2fs/initialize.c
index 5a6f8ea..3567722 100644
--- a/lib/ext2fs/initialize.c
+++ b/lib/ext2fs/initialize.c
@@ -101,6 +101,7 @@ errcode_t ext2fs_initialize(const char *name, int flags,
 	unsigned	reserved_inos;
 	char		*buf = 0;
 	char		c;
+	double		reserved_ratio;

 	if (!param || !ext2fs_blocks_count(param))
 		return EXT2_ET_INVALID_ARGUMENT;
@@ -391,6 +392,14 @@ ipg_retry:
 	if (rem && (rem < overhead+50)) {
 		ext2fs_blocks_count_set(super, ext2fs_blocks_count(super) -
 					rem);
+		/*
+		 * If blocks count is changed, we need to recalculate
+		 * reserved blocks count not to exceed 50%.
+		 */
+		reserved_ratio = 100.0 * ext2fs_r_blocks_count(param) /
+			ext2fs_blocks_count(param);
+		ext2fs_r_blocks_count_set(super, reserved_ratio *
+			ext2fs_blocks_count(super) / 100.0);

 		goto retry;
 	}
--
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