linux/fs/befs/datastream.c

// SPDX-License-Identifier: GPL-2.0
/*
 * linux/fs/befs/datastream.c
 *
 * Copyright (C) 2001 Will Dyson <[email protected]>
 *
 * Based on portions of file.c by Makoto Kato <[email protected]>
 *
 * Many thanks to Dominic Giampaolo, author of "Practical File System
 * Design with the Be File System", for such a helpful book.
 *
 */

#include <linux/kernel.h>
#include <linux/buffer_head.h>
#include <linux/string.h>

#include "befs.h"
#include "datastream.h"
#include "io.h"

const befs_inode_addr BAD_IADDR = …;

static int befs_find_brun_direct(struct super_block *sb,
				 const befs_data_stream *data,
				 befs_blocknr_t blockno, befs_block_run *run);

static int befs_find_brun_indirect(struct super_block *sb,
				   const befs_data_stream *data,
				   befs_blocknr_t blockno,
				   befs_block_run *run);

static int befs_find_brun_dblindirect(struct super_block *sb,
				      const befs_data_stream *data,
				      befs_blocknr_t blockno,
				      befs_block_run *run);

/**
 * befs_read_datastream - get buffer_head containing data, starting from pos.
 * @sb: Filesystem superblock
 * @ds: datastream to find data with
 * @pos: start of data
 * @off: offset of data in buffer_head->b_data
 *
 * Returns pointer to buffer_head containing data starting with offset @off,
 * if you don't need to know offset just set @off = NULL.
 */
struct buffer_head *
befs_read_datastream(struct super_block *sb, const befs_data_stream *ds,
		     befs_off_t pos, uint *off)
{ … }

/**
 * befs_fblock2brun - give back block run for fblock
 * @sb: the superblock
 * @data: datastream to read from
 * @fblock: the blocknumber with the file position to find
 * @run: The found run is passed back through this pointer
 *
 * Takes a file position and gives back a brun who's starting block
 * is block number fblock of the file.
 *
 * Returns BEFS_OK or BEFS_ERR.
 *
 * Calls specialized functions for each of the three possible
 * datastream regions.
 */
int
befs_fblock2brun(struct super_block *sb, const befs_data_stream *data,
		 befs_blocknr_t fblock, befs_block_run *run)
{ … }

/**
 * befs_read_lsmylink - read long symlink from datastream.
 * @sb: Filesystem superblock
 * @ds: Datastream to read from
 * @buff: Buffer in which to place long symlink data
 * @len: Length of the long symlink in bytes
 *
 * Returns the number of bytes read
 */
size_t
befs_read_lsymlink(struct super_block *sb, const befs_data_stream *ds,
		   void *buff, befs_off_t len)
{ … }

/**
 * befs_count_blocks - blocks used by a file
 * @sb: Filesystem superblock
 * @ds: Datastream of the file
 *
 * Counts the number of fs blocks that the file represented by
 * inode occupies on the filesystem, counting both regular file
 * data and filesystem metadata (and eventually attribute data
 * when we support attributes)
*/

befs_blocknr_t
befs_count_blocks(struct super_block *sb, const befs_data_stream *ds)
{ … }

/**
 * befs_find_brun_direct - find a direct block run in the datastream
 * @sb: the superblock
 * @data: the datastream
 * @blockno: the blocknumber to find
 * @run: The found run is passed back through this pointer
 *
 * Finds the block run that starts at file block number blockno
 * in the file represented by the datastream data, if that
 * blockno is in the direct region of the datastream.
 *
 * Return value is BEFS_OK if the blockrun is found, BEFS_ERR
 * otherwise.
 *
 * Algorithm:
 * Linear search. Checks each element of array[] to see if it
 * contains the blockno-th filesystem block. This is necessary
 * because the block runs map variable amounts of data. Simply
 * keeps a count of the number of blocks searched so far (sum),
 * incrementing this by the length of each block run as we come
 * across it. Adds sum to *count before returning (this is so
 * you can search multiple arrays that are logicaly one array,
 * as in the indirect region code).
 *
 * When/if blockno is found, if blockno is inside of a block
 * run as stored on disk, we offset the start and length members
 * of the block run, so that blockno is the start and len is
 * still valid (the run ends in the same place).
 */
static int
befs_find_brun_direct(struct super_block *sb, const befs_data_stream *data,
		      befs_blocknr_t blockno, befs_block_run *run)
{ … }

/**
 * befs_find_brun_indirect - find a block run in the datastream
 * @sb: the superblock
 * @data: the datastream
 * @blockno: the blocknumber to find
 * @run: The found run is passed back through this pointer
 *
 * Finds the block run that starts at file block number blockno
 * in the file represented by the datastream data, if that
 * blockno is in the indirect region of the datastream.
 *
 * Return value is BEFS_OK if the blockrun is found, BEFS_ERR
 * otherwise.
 *
 * Algorithm:
 * For each block in the indirect run of the datastream, read
 * it in and search through it for search_blk.
 *
 * XXX:
 * Really should check to make sure blockno is inside indirect
 * region.
 */
static int
befs_find_brun_indirect(struct super_block *sb,
			const befs_data_stream *data,
			befs_blocknr_t blockno,
			befs_block_run *run)
{ … }

/**
 * befs_find_brun_dblindirect - find a block run in the datastream
 * @sb: the superblock
 * @data: the datastream
 * @blockno: the blocknumber to find
 * @run: The found run is passed back through this pointer
 *
 * Finds the block run that starts at file block number blockno
 * in the file represented by the datastream data, if that
 * blockno is in the double-indirect region of the datastream.
 *
 * Return value is BEFS_OK if the blockrun is found, BEFS_ERR
 * otherwise.
 *
 * Algorithm:
 * The block runs in the double-indirect region are different.
 * They are always allocated 4 fs blocks at a time, so each
 * block run maps a constant amount of file data. This means
 * that we can directly calculate how many block runs into the
 * double-indirect region we need to go to get to the one that
 * maps a particular filesystem block.
 *
 * We do this in two stages. First we calculate which of the
 * inode addresses in the double-indirect block will point us
 * to the indirect block that contains the mapping for the data,
 * then we calculate which of the inode addresses in that
 * indirect block maps the data block we are after.
 *
 * Oh, and once we've done that, we actually read in the blocks
 * that contain the inode addresses we calculated above. Even
 * though the double-indirect run may be several blocks long,
 * we can calculate which of those blocks will contain the index
 * we are after and only read that one. We then follow it to
 * the indirect block and perform a similar process to find
 * the actual block run that maps the data block we are interested
 * in.
 *
 * Then we offset the run as in befs_find_brun_array() and we are
 * done.
 */
static int
befs_find_brun_dblindirect(struct super_block *sb,
			   const befs_data_stream *data,
			   befs_blocknr_t blockno,
			   befs_block_run *run)
{ … }