Re: [PATCH v2] xfs: zero newly allocated btree root space

[Christoph Hellwig]

On Tue, Jun 30, 2026 at 01:39:35PM -0700, Yousef Alhouseen wrote:
> Yes. The KMSAN report traces the allocation through
> xfs_bmap_extents_to_btree(), which calls xfs_bmap_broot_realloc(...,
> 1). With no existing root, that reaches krealloc(NULL, new_size). The
> conversion initializes the block header, first key, and end-anchored
> pointer, but does not initialize the layout/alignment gap within the
> full if_broot_bytes allocation. XFS_ILOG_DBROOT later copies that
> complete byte count into the log.
> 
> So the observed bytes are root-layout slack from the extents-to-btree
> conversion, rather than record storage that a later insertion should
> have filled.

This almost sounds like we should explicitly zero this case just for that
particular case.  Although Darrick was kinda heading in a different
direction and I don't want to start a fight here.