Re: [PATCH] ext4: fix the free blocks calculation for ext3 file systems w/ uninit_bg

[Sander Eikelenboom <linux@eikelenboom.it>]

Thursday, June 7, 2012, 3:52:33 PM, you wrote:

> Ext3 filesystems that are converted to use as many ext4 file system
> features as possible will enable uninit_bg to speed up e2fsck times.
> These file systems will have a native ext3 layout of inode tables and
> block allocation bitmaps (as opposed to ext4's flex_bg layout).
> Unfortunately, in these cases, when first allocating a block in an
> uninitialized block group, ext4 would incorrectly calculate the number
> of free blocks in that block group, and then errorneously report that
> the file system was corrupt:

> EXT4-fs error (device vdd): ext4_mb_generate_buddy:741: group 30, 32254 clusters in bitmap, 32258 in gd

> This problem can be reproduced via:

>     mke2fs -q -t ext4 -O ^flex_bg /dev/vdd 5g
>     mount -t ext4 /dev/vdd /mnt
>     fallocate -l 4600m /mnt/test

> The problem was caused by a bone headed mistake in the check to see if a
> particular metadata block was part of the block group.

> Many thanks to Kees Cook for finding and bisecting the buggy commit
> which introduced this bug (commit fd034a84e1, present since v3.2).

> Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
> Reported-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> Cc: stable@kernel.org
> Cc: Sander Eikelenboom <linux@eikelenboom.it>
> Cc: Kees Cook <keescook@chromium.org>
> ---

> Note: I send to push this to Linus soon, preferably before 3.5-rc2, so
> I'd appreciate any comments ASAP.  Thanks!!

Compiled it, turned uninit_bg back on, now copying some GB's worth of data to the FS.
No problems so far...

BTW: Is there any (real) advantage to recreate the FS using the ext4's flex_bg instead of converting the ext3 to ext4 and using ext3 uninit_bg ?

--
Sander


>  fs/ext4/balloc.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 99b6324..cee7812 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -90,8 +90,8 @@ unsigned ext4_num_overhead_clusters(struct super_block *sb,
>          * unusual file system layouts.
>          */
>         if (ext4_block_in_group(sb, ext4_block_bitmap(sb, gdp), block_group)) {
> -               block_cluster = EXT4_B2C(sbi, (start -
> -                                              ext4_block_bitmap(sb, gdp)));
> +               block_cluster = EXT4_B2C(sbi,
> +                                        ext4_block_bitmap(sb, gdp) - start);
>                 if (block_cluster < num_clusters)
>                         block_cluster = -1;
>                 else if (block_cluster == num_clusters) {
> @@ -102,7 +102,7 @@ unsigned ext4_num_overhead_clusters(struct super_block *sb,
>  
>         if (ext4_block_in_group(sb, ext4_inode_bitmap(sb, gdp), block_group)) {
>                 inode_cluster = EXT4_B2C(sbi,
> -                                        start - ext4_inode_bitmap(sb, gdp));
> +                                        ext4_inode_bitmap(sb, gdp) - start);
>                 if (inode_cluster < num_clusters)
>                         inode_cluster = -1;
>                 else if (inode_cluster == num_clusters) {
> @@ -114,7 +114,7 @@ unsigned ext4_num_overhead_clusters(struct super_block *sb,
>         itbl_blk = ext4_inode_table(sb, gdp);
>         for (i = 0; i < sbi->s_itb_per_group; i++) {
>                 if (ext4_block_in_group(sb, itbl_blk + i, block_group)) {
> -                       c = EXT4_B2C(sbi, start - itbl_blk + i);
> +                       c = EXT4_B2C(sbi, itbl_blk + i - start);
>                         if ((c < num_clusters) || (c == inode_cluster) ||
>                             (c == block_cluster) || (c == itbl_cluster))
>                                 continue;