[RFC PATCH v4 07/11] fs: statx add write zeroes unmap attribute

[Darrick J. Wong]

On Tue, May 06, 2025 at 07:02:39AM +0200, Christoph Hellwig wrote:
> On Mon, May 05, 2025 at 07:29:45AM -0700, Darrick J. Wong wrote:
> > attributes_mask contains attribute flags known to the filesystem,
> > whereas attributes contains flags actually set on the file.
> > "known_attributes" would have been a better name, but that's water under
> > the bridge. :P
> 
> Oooh.  I think I was very confused at what this patch does, and what
> it does seems confused as well.
> 
> The patch adds a new flag to the STATX_ATTR_* namespace, which
> historically was used for persistent on-disk flags like immutable,
> not the STATX_* namespace where I assumed it, and which has no
> support mask.  Which seems really odd for a pure kernel feature.
> Then again it seems to follow STATX_ATTR_WRITE_ATOMIC which seems
> just as wrongly place unless I'm missing something?

I think STATX_* (i.e. not STATX_ATTR_*) flags have two purposes: 1) to
declare that specific fields in struct statx actually have meaning, most
notably in scenarios where zeroes are valid field contents; and 2) if
filling out the field is expensive, userspace can elect not to have it
filled by leaving the bit unset.  I don't know how userspace is supposed
to figure out which fields are expensive.

STATX_ATTR_* are supposed to be reflect persistent inode state.  I think
STATX_ATTR_WRITE_ATOMIC is a (now unremovable) artifact of the era when
we were going to have a new iflag and feature bit for all the new
forcealign functionality.  For XFS it's not necessary anymore because we
always have software fallback and the statx::atomic_write_* fields being
nonzero is sufficient to detect the functionality.

(I'm confused about the whole premise of /this/ patch -- it's a "fast
zeroing" fallocate flag that causes the *device* to unmap, so that the
filesystem can preallocate and avoid unwritten extent conversions?
What happens if the block device is thinp and it runs out of space?
That seems antithetical to fallocate...)

--D