fs/sysv/itree.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  *  linux/fs/sysv/itree.c
  *
  *  Handling of indirect blocks' trees.
  *  AV, Sep--Dec 2000
  */

 #include <linux/buffer_head.h>
 #include <linux/mount.h>
 #include <linux/mpage.h>
 #include <linux/string.h>
 #include "sysv.h"

 enum {DIRECT = 10, DEPTH = 4};	/* Have triple indirect */

 static inline void dirty_indirect(struct buffer_head *bh, struct inode *inode)
 {
 	mark_buffer_dirty_inode(bh, inode);
 	if (IS_SYNC(inode))
 		sync_dirty_buffer(bh);
 }

 static int block_to_path(struct inode *inode, long block, int offsets[DEPTH])
 {
 	struct super_block *sb = inode->i_sb;
 	struct sysv_sb_info *sbi = SYSV_SB(sb);
 	int ptrs_bits = sbi->s_ind_per_block_bits;
 	unsigned long	indirect_blocks = sbi->s_ind_per_block,
 			double_blocks = sbi->s_ind_per_block_2;
 	int n = 0;

 	if (block < 0) {
 		printk("sysv_block_map: block < 0\n");
 	} else if (block < DIRECT) {
 		offsets[n++] = block;
 	} else if ( (block -= DIRECT) < indirect_blocks) {
 		offsets[n++] = DIRECT;
 		offsets[n++] = block;
 	} else if ((block -= indirect_blocks) < double_blocks) {
 		offsets[n++] = DIRECT+1;
 		offsets[n++] = block >> ptrs_bits;
 		offsets[n++] = block & (indirect_blocks - 1);
 	} else if (((block -= double_blocks) >> (ptrs_bits * 2)) < indirect_blocks) {
 		offsets[n++] = DIRECT+2;
 		offsets[n++] = block >> (ptrs_bits * 2);
 		offsets[n++] = (block >> ptrs_bits) & (indirect_blocks - 1);
 		offsets[n++] = block & (indirect_blocks - 1);
 	} else {
 		/* nothing */;
 	}
 	return n;
 }

 static inline int block_to_cpu(struct sysv_sb_info *sbi, sysv_zone_t nr)
 {
 	return sbi->s_block_base + fs32_to_cpu(sbi, nr);
 }

 typedef struct {
 	sysv_zone_t     *p;
 	sysv_zone_t     key;
 	struct buffer_head *bh;
 } Indirect;

 static DEFINE_RWLOCK(pointers_lock);

 static inline void add_chain(Indirect *p, struct buffer_head *bh, sysv_zone_t *v)
 {
 	p->key = *(p->p = v);
 	p->bh = bh;
 }

 static inline int verify_chain(Indirect *from, Indirect *to)
 {
 	while (from <= to && from->key == *from->p)
 		from++;
 	return (from > to);
 }

 static inline sysv_zone_t *block_end(struct buffer_head *bh)
 {
 	return (sysv_zone_t*)((char*)bh->b_data + bh->b_size);
 }

 static Indirect *get_branch(struct inode *inode,
 			    int depth,
 			    int offsets[],
 			    Indirect chain[],
 			    int *err)
 {
 	struct super_block *sb = inode->i_sb;
 	Indirect *p = chain;
 	struct buffer_head *bh;

 	*err = 0;
 	add_chain(chain, NULL, SYSV_I(inode)->i_data + *offsets);
 	if (!p->key)
 		goto no_block;
 	while (--depth) {
 		int block = block_to_cpu(SYSV_SB(sb), p->key);
 		bh = sb_bread(sb, block);
 		if (!bh)
 			goto failure;
 		read_lock(&pointers_lock);
 		if (!verify_chain(chain, p))
 			goto changed;
 		add_chain(++p, bh, (sysv_zone_t*)bh->b_data + *++offsets);
 		read_unlock(&pointers_lock);
 		if (!p->key)
 			goto no_block;
 	}
 	return NULL;

 changed:
 	read_unlock(&pointers_lock);
 	brelse(bh);
 	*err = -EAGAIN;
 	goto no_block;
 failure:
 	*err = -EIO;
 no_block:
 	return p;
 }

 static int alloc_branch(struct inode *inode,
 			int num,
 			int *offsets,
 			Indirect *branch)
 {
 	int blocksize = inode->i_sb->s_blocksize;
 	int n = 0;
 	int i;

 	branch[0].key = sysv_new_block(inode->i_sb);
 	if (branch[0].key) for (n = 1; n < num; n++) {
 		struct buffer_head *bh;
 		int parent;
 		/* Allocate the next block */
 		branch[n].key = sysv_new_block(inode->i_sb);
 		if (!branch[n].key)
 			break;
 		/*
 		 * Get buffer_head for parent block, zero it out and set
 		 * the pointer to new one, then send parent to disk.
 		 */
 		parent = block_to_cpu(SYSV_SB(inode->i_sb), branch[n-1].key);
 		bh = sb_getblk(inode->i_sb, parent);
 		if (!bh) {
 			sysv_free_block(inode->i_sb, branch[n].key);
 			break;
 		}
 		lock_buffer(bh);
 		memset(bh->b_data, 0, blocksize);
 		branch[n].bh = bh;
 		branch[n].p = (sysv_zone_t*) bh->b_data + offsets[n];
 		*branch[n].p = branch[n].key;
 		set_buffer_uptodate(bh);
 		unlock_buffer(bh);
 		dirty_indirect(bh, inode);
 	}
 	if (n == num)
 		return 0;

 	/* Allocation failed, free what we already allocated */
 	for (i = 1; i < n; i++)
 		bforget(branch[i].bh);
 	for (i = 0; i < n; i++)
 		sysv_free_block(inode->i_sb, branch[i].key);
 	return -ENOSPC;
 }

 static inline int splice_branch(struct inode *inode,
 				Indirect chain[],
 				Indirect *where,
 				int num)
 {
 	int i;

 	/* Verify that place we are splicing to is still there and vacant */
 	write_lock(&pointers_lock);
 	if (!verify_chain(chain, where-1) || *where->p)
 		goto changed;
 	*where->p = where->key;
 	write_unlock(&pointers_lock);

 	inode_set_ctime_current(inode);

 	/* had we spliced it onto indirect block? */
 	if (where->bh)
 		dirty_indirect(where->bh, inode);

 	if (IS_SYNC(inode))
 		sysv_sync_inode(inode);
 	else
 		mark_inode_dirty(inode);
 	return 0;

 changed:
 	write_unlock(&pointers_lock);
 	for (i = 1; i < num; i++)
 		bforget(where[i].bh);
 	for (i = 0; i < num; i++)
 		sysv_free_block(inode->i_sb, where[i].key);
 	return -EAGAIN;
 }

 static int get_block(struct inode *inode, sector_t iblock, struct buffer_head *bh_result, int create)
 {
 	int err = -EIO;
 	int offsets[DEPTH];
 	Indirect chain[DEPTH];
 	struct super_block *sb = inode->i_sb;
 	Indirect *partial;
 	int left;
 	int depth = block_to_path(inode, iblock, offsets);

 	if (depth == 0)
 		goto out;

 reread:
 	partial = get_branch(inode, depth, offsets, chain, &err);

 	/* Simplest case - block found, no allocation needed */
 	if (!partial) {
 got_it:
 		map_bh(bh_result, sb, block_to_cpu(SYSV_SB(sb),
 					chain[depth-1].key));
 		/* Clean up and exit */
 		partial = chain+depth-1; /* the whole chain */
 		goto cleanup;
 	}

 	/* Next simple case - plain lookup or failed read of indirect block */
 	if (!create || err == -EIO) {
 cleanup:
 		while (partial > chain) {
 			brelse(partial->bh);
 			partial--;
 		}
 out:
 		return err;
 	}

 	/*
 	 * Indirect block might be removed by truncate while we were
 	 * reading it. Handling of that case (forget what we've got and
 	 * reread) is taken out of the main path.
 	 */
 	if (err == -EAGAIN)
 		goto changed;

 	left = (chain + depth) - partial;
 	err = alloc_branch(inode, left, offsets+(partial-chain), partial);
 	if (err)
 		goto cleanup;

 	if (splice_branch(inode, chain, partial, left) < 0)
 		goto changed;

 	set_buffer_new(bh_result);
 	goto got_it;

 changed:
 	while (partial > chain) {
 		brelse(partial->bh);
 		partial--;
 	}
 	goto reread;
 }

 static inline int all_zeroes(sysv_zone_t *p, sysv_zone_t *q)
 {
 	while (p < q)
 		if (*p++)
 			return 0;
 	return 1;
 }

 static Indirect *find_shared(struct inode *inode,
 				int depth,
 				int offsets[],
 				Indirect chain[],
 				sysv_zone_t *top)
 {
 	Indirect *partial, *p;
 	int k, err;

 	*top = 0;
 	for (k = depth; k > 1 && !offsets[k-1]; k--)
 		;
 	partial = get_branch(inode, k, offsets, chain, &err);

 	write_lock(&pointers_lock);
 	if (!partial)
 		partial = chain + k-1;
 	/*
 	 * If the branch acquired continuation since we've looked at it -
 	 * fine, it should all survive and (new) top doesn't belong to us.
 	 */
 	if (!partial->key && *partial->p) {
 		write_unlock(&pointers_lock);
 		goto no_top;
 	}
 	for (p=partial; p>chain && all_zeroes((sysv_zone_t*)p->bh->b_data,p->p); p--)
 		;
 	/*
 	 * OK, we've found the last block that must survive. The rest of our
 	 * branch should be detached before unlocking. However, if that rest
 	 * of branch is all ours and does not grow immediately from the inode
 	 * it's easier to cheat and just decrement partial->p.
 	 */
 	if (p == chain + k - 1 && p > chain) {
 		p->p--;
 	} else {
 		*top = *p->p;
 		*p->p = 0;
 	}
 	write_unlock(&pointers_lock);

 	while (partial > p) {
 		brelse(partial->bh);
 		partial--;
 	}
 no_top:
 	return partial;
 }

 static inline void free_data(struct inode *inode, sysv_zone_t *p, sysv_zone_t *q)
 {
 	for ( ; p < q ; p++) {
 		sysv_zone_t nr = *p;
 		if (nr) {
 			*p = 0;
 			sysv_free_block(inode->i_sb, nr);
 			mark_inode_dirty(inode);
 		}
 	}
 }

 static void free_branches(struct inode *inode, sysv_zone_t *p, sysv_zone_t *q, int depth)
 {
 	struct buffer_head * bh;
 	struct super_block *sb = inode->i_sb;

 	if (depth--) {
 		for ( ; p < q ; p++) {
 			int block;
 			sysv_zone_t nr = *p;
 			if (!nr)
 				continue;
 			*p = 0;
 			block = block_to_cpu(SYSV_SB(sb), nr);
 			bh = sb_bread(sb, block);
 			if (!bh)
 				continue;
 			free_branches(inode, (sysv_zone_t*)bh->b_data,
 					block_end(bh), depth);
 			bforget(bh);
 			sysv_free_block(sb, nr);
 			mark_inode_dirty(inode);
 		}
 	} else
 		free_data(inode, p, q);
 }

 void sysv_truncate (struct inode * inode)
 {
 	sysv_zone_t *i_data = SYSV_I(inode)->i_data;
 	int offsets[DEPTH];
 	Indirect chain[DEPTH];
 	Indirect *partial;
 	sysv_zone_t nr = 0;
 	int n;
 	long iblock;
 	unsigned blocksize;

 	if (!(S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode) ||
 	    S_ISLNK(inode->i_mode)))
 		return;

 	blocksize = inode->i_sb->s_blocksize;
 	iblock = (inode->i_size + blocksize-1)
 					>> inode->i_sb->s_blocksize_bits;

 	block_truncate_page(inode->i_mapping, inode->i_size, get_block);

 	n = block_to_path(inode, iblock, offsets);
 	if (n == 0)
 		return;

 	if (n == 1) {
 		free_data(inode, i_data+offsets[0], i_data + DIRECT);
 		goto do_indirects;
 	}

 	partial = find_shared(inode, n, offsets, chain, &nr);
 	/* Kill the top of shared branch (already detached) */
 	if (nr) {
 		if (partial == chain)
 			mark_inode_dirty(inode);
 		else
 			dirty_indirect(partial->bh, inode);
 		free_branches(inode, &nr, &nr+1, (chain+n-1) - partial);
 	}
 	/* Clear the ends of indirect blocks on the shared branch */
 	while (partial > chain) {
 		free_branches(inode, partial->p + 1, block_end(partial->bh),
 				(chain+n-1) - partial);
 		dirty_indirect(partial->bh, inode);
 		brelse (partial->bh);
 		partial--;
 	}
 do_indirects:
 	/* Kill the remaining (whole) subtrees (== subtrees deeper than...) */
 	while (n < DEPTH) {
 		nr = i_data[DIRECT + n - 1];
 		if (nr) {
 			i_data[DIRECT + n - 1] = 0;
 			mark_inode_dirty(inode);
 			free_branches(inode, &nr, &nr+1, n);
 		}
 		n++;
 	}
 	inode_set_mtime_to_ts(inode, inode_set_ctime_current(inode));
 	if (IS_SYNC(inode))
 		sysv_sync_inode (inode);
 	else
 		mark_inode_dirty(inode);
 }

 static unsigned sysv_nblocks(struct super_block *s, loff_t size)
 {
 	struct sysv_sb_info *sbi = SYSV_SB(s);
 	int ptrs_bits = sbi->s_ind_per_block_bits;
 	unsigned blocks, res, direct = DIRECT, i = DEPTH;
 	blocks = (size + s->s_blocksize - 1) >> s->s_blocksize_bits;
 	res = blocks;
 	while (--i && blocks > direct) {
 		blocks = ((blocks - direct - 1) >> ptrs_bits) + 1;
 		res += blocks;
 		direct = 1;
 	}
 	return res;
 }

 int sysv_getattr(struct mnt_idmap *idmap, const struct path *path,
 		 struct kstat *stat, u32 request_mask, unsigned int flags)
 {
 	struct super_block *s = path->dentry->d_sb;
 	generic_fillattr(&nop_mnt_idmap, request_mask, d_inode(path->dentry),
 			 stat);
 	stat->blocks = (s->s_blocksize / 512) * sysv_nblocks(s, stat->size);
 	stat->blksize = s->s_blocksize;
 	return 0;
 }

 static int sysv_writepages(struct address_space *mapping,
 		struct writeback_control *wbc)
 {
 	return mpage_writepages(mapping, wbc, get_block);
 }

 static int sysv_read_folio(struct file *file, struct folio *folio)
 {
 	return block_read_full_folio(folio, get_block);
 }

 int sysv_prepare_chunk(struct page *page, loff_t pos, unsigned len)
 {
 	return __block_write_begin(page, pos, len, get_block);
 }

 static void sysv_write_failed(struct address_space *mapping, loff_t to)
 {
 	struct inode *inode = mapping->host;

 	if (to > inode->i_size) {
 		truncate_pagecache(inode, inode->i_size);
 		sysv_truncate(inode);
 	}
 }

 static int sysv_write_begin(struct file *file, struct address_space *mapping,
 			loff_t pos, unsigned len,
 			struct page **pagep, void **fsdata)
 {
 	int ret;

 	ret = block_write_begin(mapping, pos, len, pagep, get_block);
 	if (unlikely(ret))
 		sysv_write_failed(mapping, pos + len);

 	return ret;
 }

 static sector_t sysv_bmap(struct address_space *mapping, sector_t block)
 {
 	return generic_block_bmap(mapping,block,get_block);
 }

 const struct address_space_operations sysv_aops = {
 	.dirty_folio = block_dirty_folio,
 	.invalidate_folio = block_invalidate_folio,
 	.read_folio = sysv_read_folio,
 	.writepages = sysv_writepages,
 	.write_begin = sysv_write_begin,
 	.write_end = generic_write_end,
 	.migrate_folio = buffer_migrate_folio,
 	.bmap = sysv_bmap
 };
	// SPDX-License-Identifier: GPL-2.0
	/*
	* linux/fs/sysv/itree.c
	*
	* Handling of indirect blocks' trees.
	* AV, Sep--Dec 2000
	*/

	#include <linux/buffer_head.h>
	#include <linux/mount.h>
	#include <linux/mpage.h>
	#include <linux/string.h>
	#include "sysv.h"

	enum {DIRECT = 10, DEPTH = 4}; /* Have triple indirect */

	static inline void dirty_indirect(struct buffer_head bh, struct inode inode)
	{
	mark_buffer_dirty_inode(bh, inode);
	if (IS_SYNC(inode))
	sync_dirty_buffer(bh);
	}

	static int block_to_path(struct inode *inode, long block, int offsets[DEPTH])
	{
	struct super_block *sb = inode->i_sb;
	struct sysv_sb_info *sbi = SYSV_SB(sb);
	int ptrs_bits = sbi->s_ind_per_block_bits;
	unsigned long indirect_blocks = sbi->s_ind_per_block,
	double_blocks = sbi->s_ind_per_block_2;
	int n = 0;

	if (block < 0) {
	printk("sysv_block_map: block < 0\n");
	} else if (block < DIRECT) {
	offsets[n++] = block;
	} else if ( (block -= DIRECT) < indirect_blocks) {
	offsets[n++] = DIRECT;
	offsets[n++] = block;
	} else if ((block -= indirect_blocks) < double_blocks) {
	offsets[n++] = DIRECT+1;
	offsets[n++] = block >> ptrs_bits;
	offsets[n++] = block & (indirect_blocks - 1);
	} else if (((block -= double_blocks) >> (ptrs_bits * 2)) < indirect_blocks) {
	offsets[n++] = DIRECT+2;
	offsets[n++] = block >> (ptrs_bits * 2);
	offsets[n++] = (block >> ptrs_bits) & (indirect_blocks - 1);
	offsets[n++] = block & (indirect_blocks - 1);
	} else {
	/* nothing */;
	}
	return n;
	}

	static inline int block_to_cpu(struct sysv_sb_info *sbi, sysv_zone_t nr)
	{
	return sbi->s_block_base + fs32_to_cpu(sbi, nr);
	}

	typedef struct {
	sysv_zone_t *p;
	sysv_zone_t key;
	struct buffer_head *bh;
	} Indirect;

	static DEFINE_RWLOCK(pointers_lock);

	static inline void add_chain(Indirect p, struct buffer_head bh, sysv_zone_t *v)
	{
	p->key = *(p->p = v);
	p->bh = bh;
	}

	static inline int verify_chain(Indirect from, Indirect to)
	{
	while (from <= to && from->key == *from->p)
	from++;
	return (from > to);
	}

	static inline sysv_zone_t block_end(struct buffer_head bh)
	{
	return (sysv_zone_t)((char)bh->b_data + bh->b_size);
	}

	static Indirect get_branch(struct inode inode,
	int depth,
	int offsets[],
	Indirect chain[],
	int *err)
	{
	struct super_block *sb = inode->i_sb;
	Indirect *p = chain;
	struct buffer_head *bh;

	*err = 0;
	add_chain(chain, NULL, SYSV_I(inode)->i_data + *offsets);
	if (!p->key)
	goto no_block;
	while (--depth) {
	int block = block_to_cpu(SYSV_SB(sb), p->key);
	bh = sb_bread(sb, block);
	if (!bh)
	goto failure;
	read_lock(&pointers_lock);
	if (!verify_chain(chain, p))
	goto changed;
	add_chain(++p, bh, (sysv_zone_t)bh->b_data + ++offsets);
	read_unlock(&pointers_lock);
	if (!p->key)
	goto no_block;
	}
	return NULL;

	changed:
	read_unlock(&pointers_lock);
	brelse(bh);
	*err = -EAGAIN;
	goto no_block;
	failure:
	*err = -EIO;
	no_block:
	return p;
	}

	static int alloc_branch(struct inode *inode,
	int num,
	int *offsets,
	Indirect *branch)
	{
	int blocksize = inode->i_sb->s_blocksize;
	int n = 0;
	int i;

	branch[0].key = sysv_new_block(inode->i_sb);
	if (branch[0].key) for (n = 1; n < num; n++) {
	struct buffer_head *bh;
	int parent;
	/* Allocate the next block */
	branch[n].key = sysv_new_block(inode->i_sb);
	if (!branch[n].key)
	break;
	/*
	* Get buffer_head for parent block, zero it out and set
	* the pointer to new one, then send parent to disk.
	*/
	parent = block_to_cpu(SYSV_SB(inode->i_sb), branch[n-1].key);
	bh = sb_getblk(inode->i_sb, parent);
	if (!bh) {
	sysv_free_block(inode->i_sb, branch[n].key);
	break;
	}
	lock_buffer(bh);
	memset(bh->b_data, 0, blocksize);
	branch[n].bh = bh;
	branch[n].p = (sysv_zone_t*) bh->b_data + offsets[n];
	*branch[n].p = branch[n].key;
	set_buffer_uptodate(bh);
	unlock_buffer(bh);
	dirty_indirect(bh, inode);
	}
	if (n == num)
	return 0;

	/* Allocation failed, free what we already allocated */
	for (i = 1; i < n; i++)
	bforget(branch[i].bh);
	for (i = 0; i < n; i++)
	sysv_free_block(inode->i_sb, branch[i].key);
	return -ENOSPC;
	}

	static inline int splice_branch(struct inode *inode,
	Indirect chain[],
	Indirect *where,
	int num)
	{
	int i;

	/* Verify that place we are splicing to is still there and vacant */
	write_lock(&pointers_lock);
	if (!verify_chain(chain, where-1) \|\| *where->p)
	goto changed;
	*where->p = where->key;
	write_unlock(&pointers_lock);

	inode_set_ctime_current(inode);

	/* had we spliced it onto indirect block? */
	if (where->bh)
	dirty_indirect(where->bh, inode);

	if (IS_SYNC(inode))
	sysv_sync_inode(inode);
	else
	mark_inode_dirty(inode);
	return 0;

	changed:
	write_unlock(&pointers_lock);
	for (i = 1; i < num; i++)
	bforget(where[i].bh);
	for (i = 0; i < num; i++)
	sysv_free_block(inode->i_sb, where[i].key);
	return -EAGAIN;
	}

	static int get_block(struct inode inode, sector_t iblock, struct buffer_head bh_result, int create)
	{
	int err = -EIO;
	int offsets[DEPTH];
	Indirect chain[DEPTH];
	struct super_block *sb = inode->i_sb;
	Indirect *partial;
	int left;
	int depth = block_to_path(inode, iblock, offsets);

	if (depth == 0)
	goto out;

	reread:
	partial = get_branch(inode, depth, offsets, chain, &err);

	/* Simplest case - block found, no allocation needed */
	if (!partial) {
	got_it:
	map_bh(bh_result, sb, block_to_cpu(SYSV_SB(sb),
	chain[depth-1].key));
	/* Clean up and exit */
	partial = chain+depth-1; /* the whole chain */
	goto cleanup;
	}

	/* Next simple case - plain lookup or failed read of indirect block */
	if (!create \|\| err == -EIO) {
	cleanup:
	while (partial > chain) {
	brelse(partial->bh);
	partial--;
	}
	out:
	return err;
	}

	/*
	* Indirect block might be removed by truncate while we were
	* reading it. Handling of that case (forget what we've got and
	* reread) is taken out of the main path.
	*/
	if (err == -EAGAIN)
	goto changed;

	left = (chain + depth) - partial;
	err = alloc_branch(inode, left, offsets+(partial-chain), partial);
	if (err)
	goto cleanup;

	if (splice_branch(inode, chain, partial, left) < 0)
	goto changed;

	set_buffer_new(bh_result);
	goto got_it;

	changed:
	while (partial > chain) {
	brelse(partial->bh);
	partial--;
	}
	goto reread;
	}

	static inline int all_zeroes(sysv_zone_t p, sysv_zone_t q)
	{
	while (p < q)
	if (*p++)
	return 0;
	return 1;
	}

	static Indirect find_shared(struct inode inode,
	int depth,
	int offsets[],
	Indirect chain[],
	sysv_zone_t *top)
	{
	Indirect partial, p;
	int k, err;

	*top = 0;
	for (k = depth; k > 1 && !offsets[k-1]; k--)
	;
	partial = get_branch(inode, k, offsets, chain, &err);

	write_lock(&pointers_lock);
	if (!partial)
	partial = chain + k-1;
	/*
	* If the branch acquired continuation since we've looked at it -
	* fine, it should all survive and (new) top doesn't belong to us.
	*/
	if (!partial->key && *partial->p) {
	write_unlock(&pointers_lock);
	goto no_top;
	}
	for (p=partial; p>chain && all_zeroes((sysv_zone_t*)p->bh->b_data,p->p); p--)
	;
	/*
	* OK, we've found the last block that must survive. The rest of our
	* branch should be detached before unlocking. However, if that rest
	* of branch is all ours and does not grow immediately from the inode
	* it's easier to cheat and just decrement partial->p.
	*/
	if (p == chain + k - 1 && p > chain) {
	p->p--;
	} else {
	top = p->p;
	*p->p = 0;
	}
	write_unlock(&pointers_lock);

	while (partial > p) {
	brelse(partial->bh);
	partial--;
	}
	no_top:
	return partial;
	}

	static inline void free_data(struct inode inode, sysv_zone_t p, sysv_zone_t *q)
	{
	for ( ; p < q ; p++) {
	sysv_zone_t nr = *p;
	if (nr) {
	*p = 0;
	sysv_free_block(inode->i_sb, nr);
	mark_inode_dirty(inode);
	}
	}
	}

	static void free_branches(struct inode inode, sysv_zone_t p, sysv_zone_t *q, int depth)
	{
	struct buffer_head * bh;
	struct super_block *sb = inode->i_sb;

	if (depth--) {
	for ( ; p < q ; p++) {
	int block;
	sysv_zone_t nr = *p;
	if (!nr)
	continue;
	*p = 0;
	block = block_to_cpu(SYSV_SB(sb), nr);
	bh = sb_bread(sb, block);
	if (!bh)
	continue;
	free_branches(inode, (sysv_zone_t*)bh->b_data,
	block_end(bh), depth);
	bforget(bh);
	sysv_free_block(sb, nr);
	mark_inode_dirty(inode);
	}
	} else
	free_data(inode, p, q);
	}

	void sysv_truncate (struct inode * inode)
	{
	sysv_zone_t *i_data = SYSV_I(inode)->i_data;
	int offsets[DEPTH];
	Indirect chain[DEPTH];
	Indirect *partial;
	sysv_zone_t nr = 0;
	int n;
	long iblock;
	unsigned blocksize;

	if (!(S_ISREG(inode->i_mode) \|\| S_ISDIR(inode->i_mode) \|\|
	S_ISLNK(inode->i_mode)))
	return;

	blocksize = inode->i_sb->s_blocksize;
	iblock = (inode->i_size + blocksize-1)
	>> inode->i_sb->s_blocksize_bits;

	block_truncate_page(inode->i_mapping, inode->i_size, get_block);

	n = block_to_path(inode, iblock, offsets);
	if (n == 0)
	return;

	if (n == 1) {
	free_data(inode, i_data+offsets[0], i_data + DIRECT);
	goto do_indirects;
	}

	partial = find_shared(inode, n, offsets, chain, &nr);
	/* Kill the top of shared branch (already detached) */
	if (nr) {
	if (partial == chain)
	mark_inode_dirty(inode);
	else
	dirty_indirect(partial->bh, inode);
	free_branches(inode, &nr, &nr+1, (chain+n-1) - partial);
	}
	/* Clear the ends of indirect blocks on the shared branch */
	while (partial > chain) {
	free_branches(inode, partial->p + 1, block_end(partial->bh),
	(chain+n-1) - partial);
	dirty_indirect(partial->bh, inode);
	brelse (partial->bh);
	partial--;
	}
	do_indirects:
	/* Kill the remaining (whole) subtrees (== subtrees deeper than...) */
	while (n < DEPTH) {
	nr = i_data[DIRECT + n - 1];
	if (nr) {
	i_data[DIRECT + n - 1] = 0;
	mark_inode_dirty(inode);
	free_branches(inode, &nr, &nr+1, n);
	}
	n++;
	}
	inode_set_mtime_to_ts(inode, inode_set_ctime_current(inode));
	if (IS_SYNC(inode))
	sysv_sync_inode (inode);
	else
	mark_inode_dirty(inode);
	}

	static unsigned sysv_nblocks(struct super_block *s, loff_t size)
	{
	struct sysv_sb_info *sbi = SYSV_SB(s);
	int ptrs_bits = sbi->s_ind_per_block_bits;
	unsigned blocks, res, direct = DIRECT, i = DEPTH;
	blocks = (size + s->s_blocksize - 1) >> s->s_blocksize_bits;
	res = blocks;
	while (--i && blocks > direct) {
	blocks = ((blocks - direct - 1) >> ptrs_bits) + 1;
	res += blocks;
	direct = 1;
	}
	return res;
	}

	int sysv_getattr(struct mnt_idmap idmap, const struct path path,
	struct kstat *stat, u32 request_mask, unsigned int flags)
	{
	struct super_block *s = path->dentry->d_sb;
	generic_fillattr(&nop_mnt_idmap, request_mask, d_inode(path->dentry),
	stat);
	stat->blocks = (s->s_blocksize / 512) * sysv_nblocks(s, stat->size);
	stat->blksize = s->s_blocksize;
	return 0;
	}

	static int sysv_writepages(struct address_space *mapping,
	struct writeback_control *wbc)
	{
	return mpage_writepages(mapping, wbc, get_block);
	}

	static int sysv_read_folio(struct file file, struct folio folio)
	{
	return block_read_full_folio(folio, get_block);
	}

	int sysv_prepare_chunk(struct page *page, loff_t pos, unsigned len)
	{
	return __block_write_begin(page, pos, len, get_block);
	}

	static void sysv_write_failed(struct address_space *mapping, loff_t to)
	{
	struct inode *inode = mapping->host;

	if (to > inode->i_size) {
	truncate_pagecache(inode, inode->i_size);
	sysv_truncate(inode);
	}
	}

	static int sysv_write_begin(struct file file, struct address_space mapping,
	loff_t pos, unsigned len,
	struct page pagep, void fsdata)
	{
	int ret;

	ret = block_write_begin(mapping, pos, len, pagep, get_block);
	if (unlikely(ret))
	sysv_write_failed(mapping, pos + len);

	return ret;
	}

	static sector_t sysv_bmap(struct address_space *mapping, sector_t block)
	{
	return generic_block_bmap(mapping,block,get_block);
	}

	const struct address_space_operations sysv_aops = {
	.dirty_folio = block_dirty_folio,
	.invalidate_folio = block_invalidate_folio,
	.read_folio = sysv_read_folio,
	.writepages = sysv_writepages,
	.write_begin = sysv_write_begin,
	.write_end = generic_write_end,
	.migrate_folio = buffer_migrate_folio,
	.bmap = sysv_bmap
	};