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

["Darrick J. Wong" <djwong@kernel.org>]

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
> 
>