Re: [PATCH v3] vfs: add FILE_DEDUPE_RANGE_REPORT_PROGRESS flag to FIDEDUPERANGE

[Christoph Hellwig]

On Mon, Aug 17, 2026 at 07:56:09PM +0800, Matthias Goergens wrote:
> On success FIDEDUPERANGE reports the requested length in bytes_deduped
> even when the filesystem shortens a destination range and deduplicates
> fewer bytes.  This predates the VFS hoisting of the ioctl (the btrfs
> ioctl behaved the same way), and changing the default would change an
> ABI that deployed consumers such as duperemove depend on: they advance
> their offsets by bytes_deduped and expect the historical semantics.
> 
> Add a flag to opt into the truthful behaviour.  With

What is "truthful"?  It just is different.  And you completely fail
to explain why it is useful here, instead spewing a weird AI-like
monologue just duplicating the patch content.

Start with why you care, i.e. what application or type of application
cares how much actually was deduplicated, and how you define the
user visible behavior of that having happened.