ubifs-utils: Import UBIFS libs from linux kernel

Import UBIFS libs from linux kernel. Next patches will replace ubifs
related source code with implementation of linux kernel, which makes
userspace implementation be same with linux kernel, then fsck.ubifs
can resuse the code.

Notice: lpt.c is modified with [1] applied. ubifs.h and orphan.c are
modified with [2] applied, journal.c is modified with [3] reverted(
because fsck runs in a single thread, so waitqueue is not needed to
be implemented in userspace.).
[1] https://lore.kernel.org/linux-mtd/20231228014112.2836317-13-chengzhihao1@huawei.com/
[2] https://lore.kernel.org/linux-mtd/20240410073751.2522830-1-chengzhihao1@huawei.com/
[3] https://lore.kernel.org/linux-mtd/20240122063103.359501-1-chengzhihao1@huawei.com/

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

1 files changed, 714 insertions, 0 deletions

diff --git a/ubifs-utils/libubifs/budget.c b/ubifs-utils/libubifs/budget.c
new file mode 100644
index 0000000..d76eb7b
--- /dev/null
+++ b/ubifs-utils/libubifs/budget.c
@@ -0,0 +1,714 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * This file is part of UBIFS.
+ *
+ * Copyright (C) 2006-2008 Nokia Corporation.
+ *
+ * Authors: Adrian Hunter
+ *          Artem Bityutskiy (Битюцкий Артём)
+ */
+
+/*
+ * This file implements the budgeting sub-system which is responsible for UBIFS
+ * space management.
+ *
+ * Factors such as compression, wasted space at the ends of LEBs, space in other
+ * journal heads, the effect of updates on the index, and so on, make it
+ * impossible to accurately predict the amount of space needed. Consequently
+ * approximations are used.
+ */
+
+#include "ubifs.h"
+#include <linux/writeback.h>
+#include <linux/math64.h>
+
+/*
+ * When pessimistic budget calculations say that there is no enough space,
+ * UBIFS starts writing back dirty inodes and pages, doing garbage collection,
+ * or committing. The below constant defines maximum number of times UBIFS
+ * repeats the operations.
+ */
+#define MAX_MKSPC_RETRIES 3
+
+/*
+ * The below constant defines amount of dirty pages which should be written
+ * back at when trying to shrink the liability.
+ */
+#define NR_TO_WRITE 16
+
+/**
+ * shrink_liability - write-back some dirty pages/inodes.
+ * @c: UBIFS file-system description object
+ * @nr_to_write: how many dirty pages to write-back
+ *
+ * This function shrinks UBIFS liability by means of writing back some amount
+ * of dirty inodes and their pages.
+ *
+ * Note, this function synchronizes even VFS inodes which are locked
+ * (@i_mutex) by the caller of the budgeting function, because write-back does
+ * not touch @i_mutex.
+ */
+static void shrink_liability(struct ubifs_info *c, int nr_to_write)
+{
+	down_read(&c->vfs_sb->s_umount);
+	writeback_inodes_sb_nr(c->vfs_sb, nr_to_write, WB_REASON_FS_FREE_SPACE);
+	up_read(&c->vfs_sb->s_umount);
+}
+
+/**
+ * run_gc - run garbage collector.
+ * @c: UBIFS file-system description object
+ *
+ * This function runs garbage collector to make some more free space. Returns
+ * zero if a free LEB has been produced, %-EAGAIN if commit is required, and a
+ * negative error code in case of failure.
+ */
+static int run_gc(struct ubifs_info *c)
+{
+	int lnum;
+
+	/* Make some free space by garbage-collecting dirty space */
+	down_read(&c->commit_sem);
+	lnum = ubifs_garbage_collect(c, 1);
+	up_read(&c->commit_sem);
+	if (lnum < 0)
+		return lnum;
+
+	/* GC freed one LEB, return it to lprops */
+	dbg_budg("GC freed LEB %d", lnum);
+	return ubifs_return_leb(c, lnum);
+}
+
+/**
+ * get_liability - calculate current liability.
+ * @c: UBIFS file-system description object
+ *
+ * This function calculates and returns current UBIFS liability, i.e. the
+ * amount of bytes UBIFS has "promised" to write to the media.
+ */
+static long long get_liability(struct ubifs_info *c)
+{
+	long long liab;
+
+	spin_lock(&c->space_lock);
+	liab = c->bi.idx_growth + c->bi.data_growth + c->bi.dd_growth;
+	spin_unlock(&c->space_lock);
+	return liab;
+}
+
+/**
+ * make_free_space - make more free space on the file-system.
+ * @c: UBIFS file-system description object
+ *
+ * This function is called when an operation cannot be budgeted because there
+ * is supposedly no free space. But in most cases there is some free space:
+ *   o budgeting is pessimistic, so it always budgets more than it is actually
+ *     needed, so shrinking the liability is one way to make free space - the
+ *     cached data will take less space then it was budgeted for;
+ *   o GC may turn some dark space into free space (budgeting treats dark space
+ *     as not available);
+ *   o commit may free some LEB, i.e., turn freeable LEBs into free LEBs.
+ *
+ * So this function tries to do the above. Returns %-EAGAIN if some free space
+ * was presumably made and the caller has to re-try budgeting the operation.
+ * Returns %-ENOSPC if it couldn't do more free space, and other negative error
+ * codes on failures.
+ */
+static int make_free_space(struct ubifs_info *c)
+{
+	int err, retries = 0;
+	long long liab1, liab2;
+
+	do {
+		liab1 = get_liability(c);
+		/*
+		 * We probably have some dirty pages or inodes (liability), try
+		 * to write them back.
+		 */
+		dbg_budg("liability %lld, run write-back", liab1);
+		shrink_liability(c, NR_TO_WRITE);
+
+		liab2 = get_liability(c);
+		if (liab2 < liab1)
+			return -EAGAIN;
+
+		dbg_budg("new liability %lld (not shrunk)", liab2);
+
+		/* Liability did not shrink again, try GC */
+		dbg_budg("Run GC");
+		err = run_gc(c);
+		if (!err)
+			return -EAGAIN;
+
+		if (err != -EAGAIN && err != -ENOSPC)
+			/* Some real error happened */
+			return err;
+
+		dbg_budg("Run commit (retries %d)", retries);
+		err = ubifs_run_commit(c);
+		if (err)
+			return err;
+	} while (retries++ < MAX_MKSPC_RETRIES);
+
+	return -ENOSPC;
+}
+
+/**
+ * ubifs_calc_min_idx_lebs - calculate amount of LEBs for the index.
+ * @c: UBIFS file-system description object
+ *
+ * This function calculates and returns the number of LEBs which should be kept
+ * for index usage.
+ */
+int ubifs_calc_min_idx_lebs(struct ubifs_info *c)
+{
+	int idx_lebs;
+	long long idx_size;
+
+	idx_size = c->bi.old_idx_sz + c->bi.idx_growth + c->bi.uncommitted_idx;
+	/* And make sure we have thrice the index size of space reserved */
+	idx_size += idx_size << 1;
+	/*
+	 * We do not maintain 'old_idx_size' as 'old_idx_lebs'/'old_idx_bytes'
+	 * pair, nor similarly the two variables for the new index size, so we
+	 * have to do this costly 64-bit division on fast-path.
+	 */
+	idx_lebs = div_u64(idx_size + c->idx_leb_size - 1, c->idx_leb_size);
+	/*
+	 * The index head is not available for the in-the-gaps method, so add an
+	 * extra LEB to compensate.
+	 */
+	idx_lebs += 1;
+	if (idx_lebs < MIN_INDEX_LEBS)
+		idx_lebs = MIN_INDEX_LEBS;
+	return idx_lebs;
+}
+
+/**
+ * ubifs_calc_available - calculate available FS space.
+ * @c: UBIFS file-system description object
+ * @min_idx_lebs: minimum number of LEBs reserved for the index
+ *
+ * This function calculates and returns amount of FS space available for use.
+ */
+long long ubifs_calc_available(const struct ubifs_info *c, int min_idx_lebs)
+{
+	int subtract_lebs;
+	long long available;
+
+	available = c->main_bytes - c->lst.total_used;
+
+	/*
+	 * Now 'available' contains theoretically available flash space
+	 * assuming there is no index, so we have to subtract the space which
+	 * is reserved for the index.
+	 */
+	subtract_lebs = min_idx_lebs;
+
+	/* Take into account that GC reserves one LEB for its own needs */
+	subtract_lebs += 1;
+
+	/*
+	 * Since different write types go to different heads, we should
+	 * reserve one leb for each head.
+	 */
+	subtract_lebs += c->jhead_cnt;
+
+	/* We also reserve one LEB for deletions, which bypass budgeting */
+	subtract_lebs += 1;
+
+	available -= (long long)subtract_lebs * c->leb_size;
+
+	/* Subtract the dead space which is not available for use */
+	available -= c->lst.total_dead;
+
+	/*
+	 * Subtract dark space, which might or might not be usable - it depends
+	 * on the data which we have on the media and which will be written. If
+	 * this is a lot of uncompressed or not-compressible data, the dark
+	 * space cannot be used.
+	 */
+	available -= c->lst.total_dark;
+
+	/*
+	 * However, there is more dark space. The index may be bigger than
+	 * @min_idx_lebs. Those extra LEBs are assumed to be available, but
+	 * their dark space is not included in total_dark, so it is subtracted
+	 * here.
+	 */
+	if (c->lst.idx_lebs > min_idx_lebs) {
+		subtract_lebs = c->lst.idx_lebs - min_idx_lebs;
+		available -= subtract_lebs * c->dark_wm;
+	}
+
+	/* The calculations are rough and may end up with a negative number */
+	return available > 0 ? available : 0;
+}
+
+/**
+ * can_use_rp - check whether the user is allowed to use reserved pool.
+ * @c: UBIFS file-system description object
+ *
+ * UBIFS has so-called "reserved pool" which is flash space reserved
+ * for the superuser and for uses whose UID/GID is recorded in UBIFS superblock.
+ * This function checks whether current user is allowed to use reserved pool.
+ * Returns %1  current user is allowed to use reserved pool and %0 otherwise.
+ */
+static int can_use_rp(struct ubifs_info *c)
+{
+	if (uid_eq(current_fsuid(), c->rp_uid) || capable(CAP_SYS_RESOURCE) ||
+	    (!gid_eq(c->rp_gid, GLOBAL_ROOT_GID) && in_group_p(c->rp_gid)))
+		return 1;
+	return 0;
+}
+
+/**
+ * do_budget_space - reserve flash space for index and data growth.
+ * @c: UBIFS file-system description object
+ *
+ * This function makes sure UBIFS has enough free LEBs for index growth and
+ * data.
+ *
+ * When budgeting index space, UBIFS reserves thrice as many LEBs as the index
+ * would take if it was consolidated and written to the flash. This guarantees
+ * that the "in-the-gaps" commit method always succeeds and UBIFS will always
+ * be able to commit dirty index. So this function basically adds amount of
+ * budgeted index space to the size of the current index, multiplies this by 3,
+ * and makes sure this does not exceed the amount of free LEBs.
+ *
+ * Notes about @c->bi.min_idx_lebs and @c->lst.idx_lebs variables:
+ * o @c->lst.idx_lebs is the number of LEBs the index currently uses. It might
+ *    be large, because UBIFS does not do any index consolidation as long as
+ *    there is free space. IOW, the index may take a lot of LEBs, but the LEBs
+ *    will contain a lot of dirt.
+ * o @c->bi.min_idx_lebs is the number of LEBS the index presumably takes. IOW,
+ *    the index may be consolidated to take up to @c->bi.min_idx_lebs LEBs.
+ *
+ * This function returns zero in case of success, and %-ENOSPC in case of
+ * failure.
+ */
+static int do_budget_space(struct ubifs_info *c)
+{
+	long long outstanding, available;
+	int lebs, rsvd_idx_lebs, min_idx_lebs;
+
+	/* First budget index space */
+	min_idx_lebs = ubifs_calc_min_idx_lebs(c);
+
+	/* Now 'min_idx_lebs' contains number of LEBs to reserve */
+	if (min_idx_lebs > c->lst.idx_lebs)
+		rsvd_idx_lebs = min_idx_lebs - c->lst.idx_lebs;
+	else
+		rsvd_idx_lebs = 0;
+
+	/*
+	 * The number of LEBs that are available to be used by the index is:
+	 *
+	 *    @c->lst.empty_lebs + @c->freeable_cnt + @c->idx_gc_cnt -
+	 *    @c->lst.taken_empty_lebs
+	 *
+	 * @c->lst.empty_lebs are available because they are empty.
+	 * @c->freeable_cnt are available because they contain only free and
+	 * dirty space, @c->idx_gc_cnt are available because they are index
+	 * LEBs that have been garbage collected and are awaiting the commit
+	 * before they can be used. And the in-the-gaps method will grab these
+	 * if it needs them. @c->lst.taken_empty_lebs are empty LEBs that have
+	 * already been allocated for some purpose.
+	 *
+	 * Note, @c->idx_gc_cnt is included to both @c->lst.empty_lebs (because
+	 * these LEBs are empty) and to @c->lst.taken_empty_lebs (because they
+	 * are taken until after the commit).
+	 *
+	 * Note, @c->lst.taken_empty_lebs may temporarily be higher by one
+	 * because of the way we serialize LEB allocations and budgeting. See a
+	 * comment in 'ubifs_find_free_space()'.
+	 */
+	lebs = c->lst.empty_lebs + c->freeable_cnt + c->idx_gc_cnt -
+	       c->lst.taken_empty_lebs;
+	if (unlikely(rsvd_idx_lebs > lebs)) {
+		dbg_budg("out of indexing space: min_idx_lebs %d (old %d), rsvd_idx_lebs %d",
+			 min_idx_lebs, c->bi.min_idx_lebs, rsvd_idx_lebs);
+		return -ENOSPC;
+	}
+
+	available = ubifs_calc_available(c, min_idx_lebs);
+	outstanding = c->bi.data_growth + c->bi.dd_growth;
+
+	if (unlikely(available < outstanding)) {
+		dbg_budg("out of data space: available %lld, outstanding %lld",
+			 available, outstanding);
+		return -ENOSPC;
+	}
+
+	if (available - outstanding <= c->rp_size && !can_use_rp(c))
+		return -ENOSPC;
+
+	c->bi.min_idx_lebs = min_idx_lebs;
+	return 0;
+}
+
+/**
+ * calc_idx_growth - calculate approximate index growth from budgeting request.
+ * @c: UBIFS file-system description object
+ * @req: budgeting request
+ *
+ * For now we assume each new node adds one znode. But this is rather poor
+ * approximation, though.
+ */
+static int calc_idx_growth(const struct ubifs_info *c,
+			   const struct ubifs_budget_req *req)
+{
+	int znodes;
+
+	znodes = req->new_ino + (req->new_page << UBIFS_BLOCKS_PER_PAGE_SHIFT) +
+		 req->new_dent;
+	return znodes * c->max_idx_node_sz;
+}
+
+/**
+ * calc_data_growth - calculate approximate amount of new data from budgeting
+ * request.
+ * @c: UBIFS file-system description object
+ * @req: budgeting request
+ */
+static int calc_data_growth(const struct ubifs_info *c,
+			    const struct ubifs_budget_req *req)
+{
+	int data_growth;
+
+	data_growth = req->new_ino  ? c->bi.inode_budget : 0;
+	if (req->new_page)
+		data_growth += c->bi.page_budget;
+	if (req->new_dent)
+		data_growth += c->bi.dent_budget;
+	data_growth += req->new_ino_d;
+	return data_growth;
+}
+
+/**
+ * calc_dd_growth - calculate approximate amount of data which makes other data
+ * dirty from budgeting request.
+ * @c: UBIFS file-system description object
+ * @req: budgeting request
+ */
+static int calc_dd_growth(const struct ubifs_info *c,
+			  const struct ubifs_budget_req *req)
+{
+	int dd_growth;
+
+	dd_growth = req->dirtied_page ? c->bi.page_budget : 0;
+
+	if (req->dirtied_ino)
+		dd_growth += c->bi.inode_budget * req->dirtied_ino;
+	if (req->mod_dent)
+		dd_growth += c->bi.dent_budget;
+	dd_growth += req->dirtied_ino_d;
+	return dd_growth;
+}
+
+/**
+ * ubifs_budget_space - ensure there is enough space to complete an operation.
+ * @c: UBIFS file-system description object
+ * @req: budget request
+ *
+ * This function allocates budget for an operation. It uses pessimistic
+ * approximation of how much flash space the operation needs. The goal of this
+ * function is to make sure UBIFS always has flash space to flush all dirty
+ * pages, dirty inodes, and dirty znodes (liability). This function may force
+ * commit, garbage-collection or write-back. Returns zero in case of success,
+ * %-ENOSPC if there is no free space and other negative error codes in case of
+ * failures.
+ */
+int ubifs_budget_space(struct ubifs_info *c, struct ubifs_budget_req *req)
+{
+	int err, idx_growth, data_growth, dd_growth, retried = 0;
+
+	ubifs_assert(c, req->new_page <= 1);
+	ubifs_assert(c, req->dirtied_page <= 1);
+	ubifs_assert(c, req->new_dent <= 1);
+	ubifs_assert(c, req->mod_dent <= 1);
+	ubifs_assert(c, req->new_ino <= 1);
+	ubifs_assert(c, req->new_ino_d <= UBIFS_MAX_INO_DATA);
+	ubifs_assert(c, req->dirtied_ino <= 4);
+	ubifs_assert(c, req->dirtied_ino_d <= UBIFS_MAX_INO_DATA * 4);
+	ubifs_assert(c, !(req->new_ino_d & 7));
+	ubifs_assert(c, !(req->dirtied_ino_d & 7));
+
+	data_growth = calc_data_growth(c, req);
+	dd_growth = calc_dd_growth(c, req);
+	if (!data_growth && !dd_growth)
+		return 0;
+	idx_growth = calc_idx_growth(c, req);
+
+again:
+	spin_lock(&c->space_lock);
+	ubifs_assert(c, c->bi.idx_growth >= 0);
+	ubifs_assert(c, c->bi.data_growth >= 0);
+	ubifs_assert(c, c->bi.dd_growth >= 0);
+
+	if (unlikely(c->bi.nospace) && (c->bi.nospace_rp || !can_use_rp(c))) {
+		dbg_budg("no space");
+		spin_unlock(&c->space_lock);
+		return -ENOSPC;
+	}
+
+	c->bi.idx_growth += idx_growth;
+	c->bi.data_growth += data_growth;
+	c->bi.dd_growth += dd_growth;
+
+	err = do_budget_space(c);
+	if (likely(!err)) {
+		req->idx_growth = idx_growth;
+		req->data_growth = data_growth;
+		req->dd_growth = dd_growth;
+		spin_unlock(&c->space_lock);
+		return 0;
+	}
+
+	/* Restore the old values */
+	c->bi.idx_growth -= idx_growth;
+	c->bi.data_growth -= data_growth;
+	c->bi.dd_growth -= dd_growth;
+	spin_unlock(&c->space_lock);
+
+	if (req->fast) {
+		dbg_budg("no space for fast budgeting");
+		return err;
+	}
+
+	err = make_free_space(c);
+	cond_resched();
+	if (err == -EAGAIN) {
+		dbg_budg("try again");
+		goto again;
+	} else if (err == -ENOSPC) {
+		if (!retried) {
+			retried = 1;
+			dbg_budg("-ENOSPC, but anyway try once again");
+			goto again;
+		}
+		dbg_budg("FS is full, -ENOSPC");
+		c->bi.nospace = 1;
+		if (can_use_rp(c) || c->rp_size == 0)
+			c->bi.nospace_rp = 1;
+		smp_wmb();
+	} else
+		ubifs_err(c, "cannot budget space, error %d", err);
+	return err;
+}
+
+/**
+ * ubifs_release_budget - release budgeted free space.
+ * @c: UBIFS file-system description object
+ * @req: budget request
+ *
+ * This function releases the space budgeted by 'ubifs_budget_space()'. Note,
+ * since the index changes (which were budgeted for in @req->idx_growth) will
+ * only be written to the media on commit, this function moves the index budget
+ * from @c->bi.idx_growth to @c->bi.uncommitted_idx. The latter will be zeroed
+ * by the commit operation.
+ */
+void ubifs_release_budget(struct ubifs_info *c, struct ubifs_budget_req *req)
+{
+	ubifs_assert(c, req->new_page <= 1);
+	ubifs_assert(c, req->dirtied_page <= 1);
+	ubifs_assert(c, req->new_dent <= 1);
+	ubifs_assert(c, req->mod_dent <= 1);
+	ubifs_assert(c, req->new_ino <= 1);
+	ubifs_assert(c, req->new_ino_d <= UBIFS_MAX_INO_DATA);
+	ubifs_assert(c, req->dirtied_ino <= 4);
+	ubifs_assert(c, req->dirtied_ino_d <= UBIFS_MAX_INO_DATA * 4);
+	ubifs_assert(c, !(req->new_ino_d & 7));
+	ubifs_assert(c, !(req->dirtied_ino_d & 7));
+	if (!req->recalculate) {
+		ubifs_assert(c, req->idx_growth >= 0);
+		ubifs_assert(c, req->data_growth >= 0);
+		ubifs_assert(c, req->dd_growth >= 0);
+	}
+
+	if (req->recalculate) {
+		req->data_growth = calc_data_growth(c, req);
+		req->dd_growth = calc_dd_growth(c, req);
+		req->idx_growth = calc_idx_growth(c, req);
+	}
+
+	if (!req->data_growth && !req->dd_growth)
+		return;
+
+	c->bi.nospace = c->bi.nospace_rp = 0;
+	smp_wmb();
+
+	spin_lock(&c->space_lock);
+	c->bi.idx_growth -= req->idx_growth;
+	c->bi.uncommitted_idx += req->idx_growth;
+	c->bi.data_growth -= req->data_growth;
+	c->bi.dd_growth -= req->dd_growth;
+	c->bi.min_idx_lebs = ubifs_calc_min_idx_lebs(c);
+
+	ubifs_assert(c, c->bi.idx_growth >= 0);
+	ubifs_assert(c, c->bi.data_growth >= 0);
+	ubifs_assert(c, c->bi.dd_growth >= 0);
+	ubifs_assert(c, c->bi.min_idx_lebs < c->main_lebs);
+	ubifs_assert(c, !(c->bi.idx_growth & 7));
+	ubifs_assert(c, !(c->bi.data_growth & 7));
+	ubifs_assert(c, !(c->bi.dd_growth & 7));
+	spin_unlock(&c->space_lock);
+}
+
+/**
+ * ubifs_convert_page_budget - convert budget of a new page.
+ * @c: UBIFS file-system description object
+ *
+ * This function converts budget which was allocated for a new page of data to
+ * the budget of changing an existing page of data. The latter is smaller than
+ * the former, so this function only does simple re-calculation and does not
+ * involve any write-back.
+ */
+void ubifs_convert_page_budget(struct ubifs_info *c)
+{
+	spin_lock(&c->space_lock);
+	/* Release the index growth reservation */
+	c->bi.idx_growth -= c->max_idx_node_sz << UBIFS_BLOCKS_PER_PAGE_SHIFT;
+	/* Release the data growth reservation */
+	c->bi.data_growth -= c->bi.page_budget;
+	/* Increase the dirty data growth reservation instead */
+	c->bi.dd_growth += c->bi.page_budget;
+	/* And re-calculate the indexing space reservation */
+	c->bi.min_idx_lebs = ubifs_calc_min_idx_lebs(c);
+	spin_unlock(&c->space_lock);
+}
+
+/**
+ * ubifs_release_dirty_inode_budget - release dirty inode budget.
+ * @c: UBIFS file-system description object
+ * @ui: UBIFS inode to release the budget for
+ *
+ * This function releases budget corresponding to a dirty inode. It is usually
+ * called when after the inode has been written to the media and marked as
+ * clean. It also causes the "no space" flags to be cleared.
+ */
+void ubifs_release_dirty_inode_budget(struct ubifs_info *c,
+				      struct ubifs_inode *ui)
+{
+	struct ubifs_budget_req req;
+
+	memset(&req, 0, sizeof(struct ubifs_budget_req));
+	/* The "no space" flags will be cleared because dd_growth is > 0 */
+	req.dd_growth = c->bi.inode_budget + ALIGN(ui->data_len, 8);
+	ubifs_release_budget(c, &req);
+}
+
+/**
+ * ubifs_reported_space - calculate reported free space.
+ * @c: the UBIFS file-system description object
+ * @free: amount of free space
+ *
+ * This function calculates amount of free space which will be reported to
+ * user-space. User-space application tend to expect that if the file-system
+ * (e.g., via the 'statfs()' call) reports that it has N bytes available, they
+ * are able to write a file of size N. UBIFS attaches node headers to each data
+ * node and it has to write indexing nodes as well. This introduces additional
+ * overhead, and UBIFS has to report slightly less free space to meet the above
+ * expectations.
+ *
+ * This function assumes free space is made up of uncompressed data nodes and
+ * full index nodes (one per data node, tripled because we always allow enough
+ * space to write the index thrice).
+ *
+ * Note, the calculation is pessimistic, which means that most of the time
+ * UBIFS reports less space than it actually has.
+ */
+long long ubifs_reported_space(const struct ubifs_info *c, long long free)
+{
+	int divisor, factor, f;
+
+	/*
+	 * Reported space size is @free * X, where X is UBIFS block size
+	 * divided by UBIFS block size + all overhead one data block
+	 * introduces. The overhead is the node header + indexing overhead.
+	 *
+	 * Indexing overhead calculations are based on the following formula:
+	 * I = N/(f - 1) + 1, where I - number of indexing nodes, N - number
+	 * of data nodes, f - fanout. Because effective UBIFS fanout is twice
+	 * as less than maximum fanout, we assume that each data node
+	 * introduces 3 * @c->max_idx_node_sz / (@c->fanout/2 - 1) bytes.
+	 * Note, the multiplier 3 is because UBIFS reserves thrice as more space
+	 * for the index.
+	 */
+	f = c->fanout > 3 ? c->fanout >> 1 : 2;
+	factor = UBIFS_BLOCK_SIZE;
+	divisor = UBIFS_MAX_DATA_NODE_SZ;
+	divisor += (c->max_idx_node_sz * 3) / (f - 1);
+	free *= factor;
+	return div_u64(free, divisor);
+}
+
+/**
+ * ubifs_get_free_space_nolock - return amount of free space.
+ * @c: UBIFS file-system description object
+ *
+ * This function calculates amount of free space to report to user-space.
+ *
+ * Because UBIFS may introduce substantial overhead (the index, node headers,
+ * alignment, wastage at the end of LEBs, etc), it cannot report real amount of
+ * free flash space it has (well, because not all dirty space is reclaimable,
+ * UBIFS does not actually know the real amount). If UBIFS did so, it would
+ * bread user expectations about what free space is. Users seem to accustomed
+ * to assume that if the file-system reports N bytes of free space, they would
+ * be able to fit a file of N bytes to the FS. This almost works for
+ * traditional file-systems, because they have way less overhead than UBIFS.
+ * So, to keep users happy, UBIFS tries to take the overhead into account.
+ */
+long long ubifs_get_free_space_nolock(struct ubifs_info *c)
+{
+	int rsvd_idx_lebs, lebs;
+	long long available, outstanding, free;
+
+	ubifs_assert(c, c->bi.min_idx_lebs == ubifs_calc_min_idx_lebs(c));
+	outstanding = c->bi.data_growth + c->bi.dd_growth;
+	available = ubifs_calc_available(c, c->bi.min_idx_lebs);
+
+	/*
+	 * When reporting free space to user-space, UBIFS guarantees that it is
+	 * possible to write a file of free space size. This means that for
+	 * empty LEBs we may use more precise calculations than
+	 * 'ubifs_calc_available()' is using. Namely, we know that in empty
+	 * LEBs we would waste only @c->leb_overhead bytes, not @c->dark_wm.
+	 * Thus, amend the available space.
+	 *
+	 * Note, the calculations below are similar to what we have in
+	 * 'do_budget_space()', so refer there for comments.
+	 */
+	if (c->bi.min_idx_lebs > c->lst.idx_lebs)
+		rsvd_idx_lebs = c->bi.min_idx_lebs - c->lst.idx_lebs;
+	else
+		rsvd_idx_lebs = 0;
+	lebs = c->lst.empty_lebs + c->freeable_cnt + c->idx_gc_cnt -
+	       c->lst.taken_empty_lebs;
+	lebs -= rsvd_idx_lebs;
+	available += lebs * (c->dark_wm - c->leb_overhead);
+
+	if (available > outstanding)
+		free = ubifs_reported_space(c, available - outstanding);
+	else
+		free = 0;
+	return free;
+}
+
+/**
+ * ubifs_get_free_space - return amount of free space.
+ * @c: UBIFS file-system description object
+ *
+ * This function calculates and returns amount of free space to report to
+ * user-space.
+ */
+long long ubifs_get_free_space(struct ubifs_info *c)
+{
+	long long free;
+
+	spin_lock(&c->space_lock);
+	free = ubifs_get_free_space_nolock(c);
+	spin_unlock(&c->space_lock);
+
+	return free;
+}