Re: [PATCH] btrfs-progs: support free space tree in mkfs

[David Sterba <dsterba@suse.cz>]

On Wed, Sep 02, 2020 at 11:50:49AM -0700, Boris Burkov wrote:
> Add a runtime feature (-R) flag for the free space tree. A filesystem
> that is mkfs'd with -R free-space-tree then mounted with no options has
> the same contents as one mkfs'd without the option, then mounted with
> '-o space_cache=v2'.
> 
> The only tricky thing is in exactly how to call the tree creation code.
> Using btrfs_create_free_space_tree as is did not quite work, because an
> extra reference to the eb (root->commit_root) is leaked, which mkfs
> complains about with a warning. I opted to follow how the uuid tree is
> created by adding it to the dirty roots list for cleanup by
> commit_tree_roots in commit_transaction. As a result,
> btrfs_create_free_space_tree no longer exactly matches the version in
> the kernel sources.
> 
> Signed-off-by: Boris Burkov <boris@bur.io>

Thanks.

> ---
>  common/fsfeatures.c             | 3 +++
>  common/fsfeatures.h             | 3 ++-
>  kernel-shared/disk-io.c         | 5 +++++
>  kernel-shared/free-space-tree.c | 1 +
>  mkfs/main.c                     | 8 ++++++++
>  5 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/common/fsfeatures.c b/common/fsfeatures.c
> index 48ab37ca..3bebc97f 100644
> --- a/common/fsfeatures.c
> +++ b/common/fsfeatures.c
> @@ -107,6 +107,9 @@ static const struct btrfs_feature runtime_features[] = {
>  	{ "quota", BTRFS_RUNTIME_FEATURE_QUOTA, NULL,
>  		VERSION_TO_STRING2(3, 4), NULL, 0, NULL, 0,
>  		"quota support (qgroups)" },
> +	{ "free-space-tree", BTRFS_RUNTIME_FEATURE_FREE_SPACE_TREE, NULL,
> +		VERSION_TO_STRING2(4, 5), NULL, 0, NULL, 0,
> +		"free space tree (space_cache=v2)" },

The unfilled items are: sysfs file (we have that, free_space_tree), the
safe version is 4.9 (that's where the FREE_SPACE_TREE_VALID bit comes
from).

I've run the mkfs tests with the -R flag and all passed, so I'll enable
it in the CI script, with the fixes mentioned above.

We should still have a separate test, similar to what
mkfs/021-rfeatures-quota-rootdir does but this time the presence of the
feature should be checked at least after mkfs (by inspect+dump-super)
and conditionally by mount in case the kernel supports it.

Last but not least the documentation should be updated for mkfs in the
section 'RUNTIME FEATURES'. Thanks.