Re: [PATCH] docs/power: note Btrfs limitation with FIBMAP for swap file offset

[Christoph Hellwig]

On Tue, May 12, 2026 at 03:12:44PM +0530, Arun Rao wrote:
> Understood. My intent was mainly to document the current behaviour
> observed by users on Btrfs systems where filefrag/FIBMAP-based
> guidance fails, and to point users toward the existing btrfs-progs
> tooling.
> 
> That said, I understand the concern about encouraging users to rely on
> resume_offset-based workflows in general given the underlying
> fragility of file block mappings on moving or CoW filesystems.

Yes.  Can you maybe extend the text a bit to generally warn about this?

I also realized that filefrag uses FIEMAP by default these days and not
just FIBMAP, which is what makes it so dangerous on btrfs.  btrfs
smartly does not implement ->bmap, but it does implement FIEMAP in
this weird way mapping to the logical address space.