Re: [PATCH v2 2/7] iomap: Add zero unwritten mappings dio support

[Christoph Hellwig <hch@lst.de>]

On Tue, Jan 07, 2025 at 05:26:36PM -0800, Darrick J. Wong wrote:
> "I think we should wire it up as a new FALLOC_FL_WRITE_ZEROES mode,
> document very vigorously that it exists to facilitate pure overwrites
> (specifically that it returns EOPNOTSUPP for always-cow files), and not
> add more ioctls."
> 
> If we added this new fallocate mode to set up written mappings, would it
> be enough to write in the programming manuals that applications should
> use it to prepare a file for block-untorn writes?  Perhaps we should
> change the errno code to EMEDIUMTYPE for the mixed mappings case.
> 
> Alternately, maybe we /should/ let programs open a lease-fd on a file
> range, do their untorn writes through the lease fd, and if another
> thread does something to break the lease, then the lease fd returns EIO
> until you close it.

This still violates the "no unexpected errors" paradigm.  The whole
FALLOC_FL_WRITE_ZEROES (I hate that name btw) model would only work
if we had a software fallback that make the operations slower but
still work in case of an unexpected change to the extent mapping.