On Mon, Jan 06, 2025 at 11:46:39AM -0800, Darrick J. Wong wrote: > That sounds brittle -- even if someday a FALLOC_FL_WRITE_ZEROES gets > merged into the kernel, if anything perturbs the file mapping (e.g. > background backup process reflinks the file) then you immediately become > vulnerable to these crash integrity problems without notice. > > (Unless you're actually getting leases on the file ranges and reacting > appropriately when the leases break...) They way I understood the description they have a user space program exposing the XFS file over the network. So if a change to the mapping happens (e.g. due to defragmentation) they would in the worst case pay the cost of an allocation transaction. That is if they are really going through the normal kernel file abstraction and don't try to bypass it by say abusing FIEMAP information, in which case all hope is lost and the scheme has no chance of reliably working, unless we add ioctls to expose the pNFS layouts to userspace and they use that instead of FIEMAP.
