> - What is unsafe with VOP_BALLOC? A file system supporting files with holes
>   should export a VOP to allocate these blocks.

lack of locking.

i agree about the necessity of the API.
however, the current form of VOP_BALLOC (and VOP_BMAP) is not
usable for the code outside of the filesystem itsself.

> - Shall I change the userland (fssconfig(8) and dump(8)) on branch 'yamt-vop'
>   to make fss work again?

how the userland is related?
(the branch currently covers only src/sys/.)

YAMAMOTO Takashi