Re: [PATCH] btrfs: Add comment for BTRFS_ROOT_REF_COWS

[David Sterba <dsterba@suse.cz>]

On Wed, Feb 12, 2020 at 03:46:51PM +0800, Qu Wenruo wrote:
> This bit is being used in too many locations while there is still no
> good enough explaination for how this bit is used.
> 
> Not to mention its name really doesn't make much sense.
> 
> So this patch will add my explanation on this bit, considering only
> subvolume trees, along with its reloc trees have this bit, to me it
> looks like this bit shows whether tree blocks of a root can be shared.

I think there's more tan just sharing, it should say something about
reference counted sharing. See eg. btrfs_block_can_be_shared:

 864         /*
 865          * Tree blocks not in reference counted trees and tree roots
 866          * are never shared. If a block was allocated after the last
 867          * snapshot and the block was not allocated by tree relocation,
 868          * we know the block is not shared.
 869          */

And there can be more specialities found when grepping for REF_COWS. The
comment explaination should be complete or at least mention what's not
documenting. The I find the suggested version insufficient but don't
have a concrete suggestions for improvement. By reading the comment and
going through code I don't feel any wiser.