Re: s_first_meta_bg treatment incompatibility between kernel and e2fsprogs

Damien Guibouret
2009-11-15T10:26:41Z

Theodore Tso wrote:
> On Wed, Nov 11, 2009 at 01:09:00PM +0100, Damien Guibouret wrote:
>
> Yup, you're right. Ouch. That is a kernel bug, and it means that if
> we resize a filesystem to the point where we need to use meta_bg
> (because we've run out of blocks to reserve), if there are
> uninitialized block bitmaps, kernels that don't have a fix will
> misbehave by reserving too many file system metadata blocks. This
> will waste bit of disk space, which fsck will fix.
>
> (s_first_meta_bg by definition is always less than or equal to
> s_gdb_count.)
>
> I think this patch should fix things up.
>
> - Ted
>
> commit b33c339814f97fc48a843f45f6068f84bc735141
> Author: Theodore Ts'o <tytso@mit.edu>
> Date: Sat Nov 14 23:20:30 2009 -0500
>
> ext4: Fix uninit block bitmap initialization when s_meta_first_bg is non-zero
>
> The number of old-style block group descriptor blocks is
> s_meta_first_bg when the meta_bg feature flag is set.
>
> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
>
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 1d04189..f3032c9 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -761,7 +761,13 @@ static unsigned long ext4_bg_num_gdb_meta(struct super_block *sb,
> static unsigned long ext4_bg_num_gdb_nometa(struct super_block *sb,
> ext4_group_t group)
> {
> - return ext4_bg_has_super(sb, group) ? EXT4_SB(sb)->s_gdb_count : 0;
> + if (!ext4_bg_has_super(sb, group))
> + return 0;
> +
> + if (EXT4_HAS_INCOMPAT_FEATURE(sb,EXT4_FEATURE_INCOMPAT_META_BG))
> + return le32_to_cpu(EXT4_SB(sb)->s_es->s_first_meta_bg);
> + else
> + return EXT4_SB(sb)->s_gdb_count;
> }
>
> /**
>
>

Hello,

I've open a kernel bug since:
http://bugzilla.kernel.org/show_bug.cgi?id=14601
with a proposed patch (little different from yours but it is matter of
taste :)
And I think there is some other places where kernel should be fixed when
it uses s_gdb_count (but here my knowledge of the sources are not deep
enough to be sure on what shall be performed).

Regards,

Damien