Re: [PATCH v2 0/9] Add support for external environment block on Btrfs

[Neal Gompa <ngompa13@gmail.com>]

On Mon, Sep 15, 2025 at 5:08 AM Michael Chang <mchang@suse.com> wrote:
>
> This patch series adds support for storing the GRUB environment block in
> a reserved area of the Btrfs header. On copy on write filesystems such
> as Btrfs, the normal file based envblk cannot be updated safely at
> runtime because block addresses are not stable. The reserved area
> provides a fixed location that GRUB can write directly, allowing
> commands such as grub-reboot and savedefault to work on Btrfs volumes.
>
> The series proceeds in small chunks to keep each change buildable and
> easier to review. The first patches add new data structures and helpers
> for creating, opening, and writing an environment block in the reserved
> area. Later patches update set_variables, unset_variables, and
> list_variables so they can use the external block when it is present. An
> entry is added to the Btrfs header to reserve space at 256 KiB for the
> environment block. Finally, grub.cfg is modified so that load_env and
> save_env use the external block automatically when env_block is defined.
>
> v2:
> - Define ENV_BTRFS_OFFSET as 256*1024
> - Do not conflate type and variable definitions
> - Align typedef with struct declaration to follow coding style
> - Use bool as the return type of is_abstraction()
> - Add "if (dev->disk != NULL)" check in is_abstraction()
> - Refine the loop logic in is_abstraction() tests
> - Remove extra indentation and redundant lines in read_envblk_fs
> - Use off_t and size_t for offset and size variables, fix similar cases
>   throughout
> - Use explicit check "(fp == NULL)" instead of "(! fp)", fix similar
>   cases throughout
> - Use bool for the "found" field in var_lookup_ctx
> - Add documentation describing the Btrfs environment block and special
>   environment block variables
>
> Michael Chang (9):
>   util/grub-editenv: add basic structures and probe call for external
>     envblk
>   util/grub-editenv: add fs_envblk open helper
>   util/grub-editenv: add fs_envblk write helper
>   util/grub-editenv: wire set_variables to optional fs_envblk
>   util/grub-editenv: wire unset_variables to optional fs_envblk
>   util/grub-editenv: wire list_variables to optional fs_envblk
>   btrfs: add environment block to reserved header area
>   00_header.in: wire grub.cfg to use env_block when present
>   docs: add Btrfs env block and special env vars
>
>  docs/grub.texi           |  60 ++++++
>  grub-core/fs/btrfs.c     |   3 +-
>  include/grub/fs.h        |   2 +
>  util/grub-editenv.c      | 393 ++++++++++++++++++++++++++++++++++++++-
>  util/grub.d/00_header.in |  26 ++-
>  5 files changed, 477 insertions(+), 7 deletions(-)
>
> --
> 2.51.0
>

The patches look reasonable to me and things still seem to work. It is
much easier to understand all the pieces with this series breakout. :)

Reviewed-by: Neal Gompa <ngompa13@gmail.com>


-- 
真実はいつも一つ！/ Always, there's only one truth!

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