Subject: [PATCH v3 0/8] gen_init_cpio: add copy_file_range / reflink support

From: David Disseldorp <[email-masked]>
Date: Tue, 19 Aug 2025 13:05:43 +1000

This patchset adds copy_file_range() support to gen_init_cpio. When
combined with data segment alignment, large-file archiving performance
is improved on Btrfs and XFS due to reflinks (see patch 7 benchmarks).

cpio data segment alignment is provided by "bending" the newc spec
to zero-pad the filename field. GNU cpio and Linux initramfs
extractors handle this fine as long as PATH_MAX isn't exceeded. A
kernel initramfs extraction unit test for this is provided.

Changes since v2
- add Nicolas' Reviewed-by tag to patches 1-7
- add patch 8 test for extracting a cpio archive with filename padding
- use trailing '||' for multi-line if conditions instead of prefix
- refer to -o output_file in usage instead of output_path
- define _GNU_SOURCE alongside O_LARGEFILE use, instead of later

Changes since v1 RFC
- add alignment patches 6-7
- slightly rework commit and error messages
- rename l->len to avoid 1/i confusion