Re: [RFC PATCH 2/3] btrfs-progs: kernel based default features for mkfs

[Jeff Mahoney <jeffm@suse.com>]

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On 10/21/15 4:45 AM, Anand Jain wrote:
> mkfs from latest btrfs-progs will enable latest default features, 
> and if the kernel is down-rev and does not support a latest
> default feature then mount fails, as expected.
> 
> This patch disables default features based on the running kernel.

I like the idea generally based on comments further in the thread, but
what I don't like is:

1) It's silent.
2) There's no way to override it.

If we're going to change the defaults at runtime, we should tell the
user what has changed and why.  Otherwise, an identical mkfs.btrfs
binary will behave differently on different systems without feedback
and that violates the principle of least surprise.  If they want to do
what Qu suggests later in the thread, where the device is being
prepared for use on a newer kernel, it should be possible to force it.
 The normal -f should be fine there.

- -Jeff

> Signed-off-by: Anand Jain <anand.jain@oracle.com> --- mkfs.c | 5
> ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mkfs.c b/mkfs.c index a5802f7..2b9d734 100644 ---
> a/mkfs.c +++ b/mkfs.c @@ -1357,10 +1357,13 @@ int main(int ac, char
> **av) int dev_cnt = 0; int saved_optind; char
> fs_uuid[BTRFS_UUID_UNPARSED_SIZE] = { 0 }; -	u64 features =
> BTRFS_MKFS_DEFAULT_FEATURES; +	u64 features; struct mkfs_allocation
> allocation = { 0 }; struct btrfs_mkfs_config mkfs_cfg;
> 
> +	features = btrfs_features_allowed_by_kernel(); +	features &=
> BTRFS_MKFS_DEFAULT_FEATURES; + while(1) { int c; static const
> struct option long_options[] = {
> 


- -- 
Jeff Mahoney
SUSE Labs
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