Re: [RFC PATCH 01/10] ftrfs: add on-disk format and in-memory data structures

Darrick J. Wong
2026-04-13T15:11:59Z

On Mon, Apr 13, 2026 at 04:23:47PM +0200, Aurelien DESBRIERES wrote:
> From: Aurélien DESBRIERES <aurelien@hackers.camp>
>
> Add the core header defining FTRFS on-disk layout and in-memory
> VFS structures.
>
> On-disk layout:
> Block 0 : superblock (magic 0x46545246, CRC32-protected)
> Block 1..N : inode table (128 bytes/inode, CRC32 per inode)
> Block N+1..end : data blocks (CRC32 per block, RS FEC planned)
>
> Structures:
> ftrfs_super_block : on-disk superblock
> ftrfs_inode : on-disk inode (12 direct + 1 indirect + 1 dindirect)
> ftrfs_dir_entry : on-disk directory entry (256-byte name)
> ftrfs_sb_info : in-memory superblock info (VFS sb->s_fs_info)
> ftrfs_inode_info : in-memory inode (embedded VFS inode)
>
> FTRFS targets POSIX-compatible block devices (MRAM, NOR flash, eMMC)
> for use in radiation-intensive environments (space applications).
>
> Signed-off-by: Aurélien DESBRIERES <aurelien@hackers.camp>
> ---
> fs/ftrfs/ftrfs.h | 168 +++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 168 insertions(+)
> create mode 100644 fs/ftrfs/ftrfs.h
>
> diff --git a/fs/ftrfs/ftrfs.h b/fs/ftrfs/ftrfs.h
> new file mode 100644
> index 000000000..82502c9fb
> --- /dev/null
> +++ b/fs/ftrfs/ftrfs.h
> @@ -0,0 +1,168 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * FTRFS — Fault-Tolerant Radiation-Robust Filesystem
> + * Based on: Fuchs, Langer, Trinitis — ARCS 2015
> + *
> + * Author: roastercode - Aurelien DESBRIERES <aurelien@hackers.camp>
> + */
> +
> +#ifndef _FTRFS_H
> +#define _FTRFS_H
> +
> +#include <linux/fs.h>
> +#include <linux/fs_context.h>
> +#include <linux/types.h>
> +
> +/* Magic number: 'FTRF' */
> +#define FTRFS_MAGIC 0x46545246
> +
> +/* Block size: 4096 bytes */
> +#define FTRFS_BLOCK_SIZE 4096
> +#define FTRFS_BLOCK_SHIFT 12
> +
> +/* RS FEC: 16 parity bytes per 239-byte subblock (RS(255,239)) */
> +#define FTRFS_RS_PARITY 16
> +#define FTRFS_SUBBLOCK_DATA 239
> +#define FTRFS_SUBBLOCK_TOTAL (FTRFS_SUBBLOCK_DATA + FTRFS_RS_PARITY)
> +
> +/* Filesystem limits */
> +#define FTRFS_MAX_FILENAME 255
> +#define FTRFS_DIRECT_BLOCKS 12
> +#define FTRFS_INDIRECT_BLOCKS 1
> +#define FTRFS_DINDIRECT_BLOCKS 1
> +
> +/*
> + * On-disk superblock — block 0
> + * Total size: fits in one 4096-byte block
> + */
> +struct ftrfs_super_block {
> + __le32 s_magic; /* FTRFS_MAGIC */
> + __le32 s_block_size; /* Block size in bytes */
> + __le64 s_block_count; /* Total blocks */
> + __le64 s_free_blocks; /* Free blocks */
> + __le64 s_inode_count; /* Total inodes */
> + __le64 s_free_inodes; /* Free inodes */
> + __le64 s_inode_table_blk; /* Block where inode table starts */
> + __le64 s_data_start_blk; /* First data block */
> + __le32 s_version; /* Filesystem version */
> + __le32 s_flags; /* Flags */
> + __le32 s_crc32; /* CRC32 of this superblock */
> + __u8 s_uuid[16]; /* UUID */
> + __u8 s_label[32]; /* Volume label */
> + __u8 s_pad[3948]; /* Padding to 4096 bytes */
> +} __packed;
> +
> +/*
> + * On-disk inode
> + * Size: 128 bytes
> + */
> +struct ftrfs_inode {
> + __le16 i_mode; /* File mode */
> + __le16 i_uid; /* Owner UID */
> + __le16 i_gid; /* Owner GID */
> + __le16 i_nlink; /* Hard link count */

https://www.tomshardware.com/service-providers/web-hosting/friends-gif-cripples-website-discussion-platform-backup-with-377-gigabytes-of-rachels-happy-dance-1-6mb-animation-was-replicated-246-173-times-breaking-filesystem-limit

> + __le64 i_size; /* File size in bytes */
> + __le64 i_atime; /* Access time (ns) */
> + __le64 i_mtime; /* Modification time (ns) */
> + __le64 i_ctime; /* Change time (ns) */
> + __le32 i_blocks; /* Block count */
> + __le32 i_flags; /* Inode flags */
> + __le64 i_direct[FTRFS_DIRECT_BLOCKS]; /* Direct block pointers */
> + __le64 i_indirect; /* Single indirect */
> + __le64 i_dindirect; /* Double indirect */

I don't see a block header for indirect blocks, so I guess this
filesystem only supports mapping ... ($block_size / 4) ^ 2 +
($block_size / 4) + 12 blocks? IOWs, 1049612 of 4KiB blocks, or about
4GB of data? Why is isize 64-bit then?

--D

> + __le32 i_crc32; /* CRC32 of inode */
> + __u8 i_pad[2]; /* Padding to 128 bytes */
> +} __packed;
> +
> +/* Inode flags */
> +#define FTRFS_INODE_FL_RS_ENABLED 0x0001 /* RS FEC enabled */
> +#define FTRFS_INODE_FL_VERIFIED 0x0002 /* Integrity verified */
> +
> +/*
> + * On-disk directory entry
> + */
> +struct ftrfs_dir_entry {
> + __le64 d_ino; /* Inode number */
> + __le16 d_rec_len; /* Record length */
> + __u8 d_name_len; /* Name length */
> + __u8 d_file_type; /* File type */
> + char d_name[FTRFS_MAX_FILENAME + 1]; /* Filename */
> +} __packed;
> +
> +/*
> + * In-memory superblock info (stored in sb->s_fs_info)
> + */
> +struct ftrfs_sb_info {
> + /* Block allocator */
> + unsigned long *s_block_bitmap; /* In-memory free block bitmap */
> + unsigned long s_nblocks; /* Number of data blocks */
> + unsigned long s_data_start; /* First data block number */
> + struct ftrfs_super_block *s_ftrfs_sb; /* On-disk superblock copy */
> + struct buffer_head *s_sbh; /* Buffer head for superblock */
> + spinlock_t s_lock; /* Superblock lock */
> + unsigned long s_free_blocks;
> + unsigned long s_free_inodes;
> +};
> +
> +/*
> + * In-memory inode info (embedded in VFS inode via container_of)
> + */
> +struct ftrfs_inode_info {
> + __le64 i_direct[FTRFS_DIRECT_BLOCKS];
> + __le64 i_indirect;
> + __le64 i_dindirect;
> + __u32 i_flags;
> + struct inode vfs_inode; /* Must be last */
> +};
> +
> +static inline struct ftrfs_inode_info *FTRFS_I(struct inode *inode)
> +{
> + return container_of(inode, struct ftrfs_inode_info, vfs_inode);
> +}
> +
> +static inline struct ftrfs_sb_info *FTRFS_SB(struct super_block *sb)
> +{
> + return sb->s_fs_info;
> +}
> +
> +/* Function prototypes */
> +/* super.c */
> +int ftrfs_fill_super(struct super_block *sb, struct fs_context *fc);
> +
> +/* inode.c */
> +struct inode *ftrfs_iget(struct super_block *sb, unsigned long ino);
> +struct inode *ftrfs_new_inode(struct inode *dir, umode_t mode);
> +
> +/* dir.c */
> +extern const struct file_operations ftrfs_dir_operations;
> +extern const struct inode_operations ftrfs_dir_inode_operations;
> +
> +/* file.c */
> +extern const struct file_operations ftrfs_file_operations;
> +extern const struct inode_operations ftrfs_file_inode_operations;
> +
> +/* edac.c */
> +__u32 ftrfs_crc32(const void *buf, size_t len);
> +int ftrfs_rs_encode(uint8_t *data, uint8_t *parity);
> +int ftrfs_rs_decode(uint8_t *data, uint8_t *parity);
> +
> +/* block.c */
> +
> +#endif /* _FTRFS_H */
> +
> +/*
> + */
> +
> +/* alloc.c */
> +int ftrfs_setup_bitmap(struct super_block *sb);
> +void ftrfs_destroy_bitmap(struct super_block *sb);
> +u64 ftrfs_alloc_block(struct super_block *sb);
> +void ftrfs_free_block(struct super_block *sb, u64 block);
> +u64 ftrfs_alloc_inode_num(struct super_block *sb);
> +
> +/* dir.c */
> +struct dentry *ftrfs_lookup(struct inode *dir, struct dentry *dentry,
> + unsigned int flags);
> +
> +/* namei.c */
> +int ftrfs_write_inode(struct inode *inode, struct writeback_control *wbc);
> --
> 2.52.0
>
>