Linux 2.6.28.1

[linux/fpc-iii.git] / fs / ubifs / budget.c

/*
 * This file is part of UBIFS.
 *
 * Copyright (C) 2006-2008 Nokia Corporation.
 *
 * This program is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 as published by
 * the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
 * more details.
 *
 * You should have received a copy of the GNU General Public License along with
 * this program; if not, write to the Free Software Foundation, Inc., 51
 * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 *
 * 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 <asm/div64.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 constants define maximum number of times UBIFS
 * repeats the operations.
 */
#define MAX_SHRINK_RETRIES 8
#define MAX_GC_RETRIES     4
#define MAX_CMT_RETRIES    2
#define MAX_NOSPC_RETRIES  1

/*
 * 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

/**
 * struct retries_info - information about re-tries while making free space.
 * @prev_liability: previous liability
 * @shrink_cnt: how many times the liability was shrinked
 * @shrink_retries: count of liability shrink re-tries (increased when
 *                  liability does not shrink)
 * @try_gc: GC should be tried first
 * @gc_retries: how many times GC was run
 * @cmt_retries: how many times commit has been done
 * @nospc_retries: how many times GC returned %-ENOSPC
 *
 * Since we consider budgeting to be the fast-path, and this structure has to
 * be allocated on stack and zeroed out, we make it smaller using bit-fields.
 */
struct retries_info {
        long long prev_liability;
        unsigned int shrink_cnt;
        unsigned int shrink_retries:5;
        unsigned int try_gc:1;
        unsigned int gc_retries:4;
        unsigned int cmt_retries:3;
        unsigned int nospc_retries:1;
};

/**
 * 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. Returns the amount of pages which were
 * written back. The returned value does not include dirty inodes which were
 * synchronized.
 *
 * 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 int shrink_liability(struct ubifs_info *c, int nr_to_write)
{
        int nr_written;
        struct writeback_control wbc = {
                .sync_mode   = WB_SYNC_NONE,
                .range_end   = LLONG_MAX,
                .nr_to_write = nr_to_write,
        };

        generic_sync_sb_inodes(c->vfs_sb, &wbc);
        nr_written = nr_to_write - wbc.nr_to_write;

        if (!nr_written) {
                /*
                 * Re-try again but wait on pages/inodes which are being
                 * written-back concurrently (e.g., by pdflush).
                 */
                memset(&wbc, 0, sizeof(struct writeback_control));
                wbc.sync_mode   = WB_SYNC_ALL;
                wbc.range_end   = LLONG_MAX;
                wbc.nr_to_write = nr_to_write;
                generic_sync_sb_inodes(c->vfs_sb, &wbc);
                nr_written = nr_to_write - wbc.nr_to_write;
        }

        dbg_budg("%d pages were written back", nr_written);
        return nr_written;
}


/**
 * 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 err, 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);
        err = ubifs_return_leb(c, lnum);
        if (err)
                return err;
        return 0;
}

/**
 * make_free_space - make more free space on the file-system.
 * @c: UBIFS file-system description object
 * @ri: information about previous invocations of this function
 *
 * 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 then 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, struct retries_info *ri)
{
        int err;

        /*
         * If we have some dirty pages and inodes (liability), try to write
         * them back unless this was tried too many times without effect
         * already.
         */
        if (ri->shrink_retries < MAX_SHRINK_RETRIES && !ri->try_gc) {
                long long liability;

                spin_lock(&c->space_lock);
                liability = c->budg_idx_growth + c->budg_data_growth +
                            c->budg_dd_growth;
                spin_unlock(&c->space_lock);

                if (ri->prev_liability >= liability) {
                        /* Liability does not shrink, next time try GC then */
                        ri->shrink_retries += 1;
                        if (ri->gc_retries < MAX_GC_RETRIES)
                                ri->try_gc = 1;
                        dbg_budg("liability did not shrink: retries %d of %d",
                                 ri->shrink_retries, MAX_SHRINK_RETRIES);
                }

                dbg_budg("force write-back (count %d)", ri->shrink_cnt);
                shrink_liability(c, NR_TO_WRITE + ri->shrink_cnt);

                ri->prev_liability = liability;
                ri->shrink_cnt += 1;
                return -EAGAIN;
        }

        /*
         * Try to run garbage collector unless it was already tried too many
         * times.
         */
        if (ri->gc_retries < MAX_GC_RETRIES) {
                ri->gc_retries += 1;
                dbg_budg("run GC, retries %d of %d",
                         ri->gc_retries, MAX_GC_RETRIES);

                ri->try_gc = 0;
                err = run_gc(c);
                if (!err)
                        return -EAGAIN;

                if (err == -EAGAIN) {
                        dbg_budg("GC asked to commit");
                        err = ubifs_run_commit(c);
                        if (err)
                                return err;
                        return -EAGAIN;
                }

                if (err != -ENOSPC)
                        return err;

                /*
                 * GC could not make any progress. If this is the first time,
                 * then it makes sense to try to commit, because it might make
                 * some dirty space.
                 */
                dbg_budg("GC returned -ENOSPC, retries %d",
                         ri->nospc_retries);
                if (ri->nospc_retries >= MAX_NOSPC_RETRIES)
                        return err;
                ri->nospc_retries += 1;
        }

        /* Neither GC nor write-back helped, try to commit */
        if (ri->cmt_retries < MAX_CMT_RETRIES) {
                ri->cmt_retries += 1;
                dbg_budg("run commit, retries %d of %d",
                         ri->cmt_retries, MAX_CMT_RETRIES);
                err = ubifs_run_commit(c);
                if (err)
                        return err;
                return -EAGAIN;
        }
        return -ENOSPC;
}

/**
 * ubifs_calc_min_idx_lebs - calculate amount of eraseblocks for the index.
 * @c: UBIFS file-system description object
 *
 * This function calculates and returns the number of eraseblocks which should
 * be kept for index usage.
 */
int ubifs_calc_min_idx_lebs(struct ubifs_info *c)
{
        int ret;
        uint64_t idx_size;

        idx_size = c->old_idx_sz + c->budg_idx_growth + c->budg_uncommitted_idx;

        /* And make sure we have thrice the index size of space reserved */
        idx_size = 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.
         */
        if (do_div(idx_size, c->leb_size - c->max_idx_node_sz))
                ret = idx_size + 1;
        else
                ret = idx_size;
        /*
         * The index head is not available for the in-the-gaps method, so add an
         * extra LEB to compensate.
         */
        ret += 1;
        /*
         * At present the index needs at least 2 LEBs: one for the index head
         * and one for in-the-gaps method (which currently does not cater for
         * the index head and so excludes it from consideration).
         */
        if (ret < 2)
                ret = 2;
        return ret;
}

/**
 * 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;

        /*
         * The GC journal head LEB is not really accessible. And since
         * different write types go to different heads, we may count only on
         * one head's space.
         */
        subtract_lebs += c->jhead_cnt - 1;

        /* 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 (current->fsuid == c->rp_uid || capable(CAP_SYS_RESOURCE) ||
            (c->rp_gid != 0 && 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 eraseblocks 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 eraseblocks.
 *
 * Notes about @c->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->min_idx_lebs is the the index presumably takes. IOW, the index may be
 *   consolidated to take up to @c->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->min_idx_lebs,
                         rsvd_idx_lebs);
                return -ENOSPC;
        }

        available = ubifs_calc_available(c, min_idx_lebs);
        outstanding = c->budg_data_growth + c->budg_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->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->inode_budget : 0;
        if (req->new_page)
                data_growth += c->page_budget;
        if (req->new_dent)
                data_growth += c->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->page_budget : 0;

        if (req->dirtied_ino)
                dd_growth += c->inode_budget << (req->dirtied_ino - 1);
        if (req->mod_dent)
                dd_growth += c->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 uninitialized_var(cmt_retries), uninitialized_var(wb_retries);
        int err, idx_growth, data_growth, dd_growth;
        struct retries_info ri;

        ubifs_assert(req->new_page <= 1);
        ubifs_assert(req->dirtied_page <= 1);
        ubifs_assert(req->new_dent <= 1);
        ubifs_assert(req->mod_dent <= 1);
        ubifs_assert(req->new_ino <= 1);
        ubifs_assert(req->new_ino_d <= UBIFS_MAX_INO_DATA);
        ubifs_assert(req->dirtied_ino <= 4);
        ubifs_assert(req->dirtied_ino_d <= UBIFS_MAX_INO_DATA * 4);
        ubifs_assert(!(req->new_ino_d & 7));
        ubifs_assert(!(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);
        memset(&ri, 0, sizeof(struct retries_info));

again:
        spin_lock(&c->space_lock);
        ubifs_assert(c->budg_idx_growth >= 0);
        ubifs_assert(c->budg_data_growth >= 0);
        ubifs_assert(c->budg_dd_growth >= 0);

        if (unlikely(c->nospace) && (c->nospace_rp || !can_use_rp(c))) {
                dbg_budg("no space");
                spin_unlock(&c->space_lock);
                return -ENOSPC;
        }

        c->budg_idx_growth += idx_growth;
        c->budg_data_growth += data_growth;
        c->budg_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->budg_idx_growth -= idx_growth;
        c->budg_data_growth -= data_growth;
        c->budg_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, &ri);
        if (err == -EAGAIN) {
                dbg_budg("try again");
                cond_resched();
                goto again;
        } else if (err == -ENOSPC) {
                dbg_budg("FS is full, -ENOSPC");
                c->nospace = 1;
                if (can_use_rp(c) || c->rp_size == 0)
                        c->nospace_rp = 1;
                smp_wmb();
        } else
                ubifs_err("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->budg_idx_growth to @c->budg_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(req->new_page <= 1);
        ubifs_assert(req->dirtied_page <= 1);
        ubifs_assert(req->new_dent <= 1);
        ubifs_assert(req->mod_dent <= 1);
        ubifs_assert(req->new_ino <= 1);
        ubifs_assert(req->new_ino_d <= UBIFS_MAX_INO_DATA);
        ubifs_assert(req->dirtied_ino <= 4);
        ubifs_assert(req->dirtied_ino_d <= UBIFS_MAX_INO_DATA * 4);
        ubifs_assert(!(req->new_ino_d & 7));
        ubifs_assert(!(req->dirtied_ino_d & 7));
        if (!req->recalculate) {
                ubifs_assert(req->idx_growth >= 0);
                ubifs_assert(req->data_growth >= 0);
                ubifs_assert(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->nospace = c->nospace_rp = 0;
        smp_wmb();

        spin_lock(&c->space_lock);
        c->budg_idx_growth -= req->idx_growth;
        c->budg_uncommitted_idx += req->idx_growth;
        c->budg_data_growth -= req->data_growth;
        c->budg_dd_growth -= req->dd_growth;
        c->min_idx_lebs = ubifs_calc_min_idx_lebs(c);

        ubifs_assert(c->budg_idx_growth >= 0);
        ubifs_assert(c->budg_data_growth >= 0);
        ubifs_assert(c->budg_dd_growth >= 0);
        ubifs_assert(c->min_idx_lebs < c->main_lebs);
        ubifs_assert(!(c->budg_idx_growth & 7));
        ubifs_assert(!(c->budg_data_growth & 7));
        ubifs_assert(!(c->budg_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 then
 * 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->budg_idx_growth -= c->max_idx_node_sz << UBIFS_BLOCKS_PER_PAGE_SHIFT;
        /* Release the data growth reservation */
        c->budg_data_growth -= c->page_budget;
        /* Increase the dirty data growth reservation instead */
        c->budg_dd_growth += c->page_budget;
        /* And re-calculate the indexing space reservation */
        c->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.
 */
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));
        req.dd_growth = c->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 indexind nodes as well. This introduces additional
 * overhead, and UBIFS it has to report sligtly less free space to meet the
 * above expectetion.
 *
 * 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, uint64_t 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 reseves 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;
        do_div(free, divisor);
        return free;
}

/**
 * ubifs_get_free_space - 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,
 * alighment, wastage at the end of eraseblocks, etc), it cannot report real
 * amount of free flash space it has (well, because not all dirty space is
 * reclamable, UBIFS does not actually know the real amount). If UBIFS did so,
 * it would bread user expectetion 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(struct ubifs_info *c)
{
        int min_idx_lebs, rsvd_idx_lebs, lebs;
        long long available, outstanding, free;

        spin_lock(&c->space_lock);
        min_idx_lebs = ubifs_calc_min_idx_lebs(c);
        outstanding = c->budg_data_growth + c->budg_dd_growth;

        /*
         * Force the amount available to the total size reported if the used
         * space is zero.
         */
        if (c->lst.total_used <= UBIFS_INO_NODE_SZ && !outstanding) {
                spin_unlock(&c->space_lock);
                return (long long)c->block_cnt << UBIFS_BLOCK_SHIFT;
        }

        available = ubifs_calc_available(c, 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 (min_idx_lebs > c->lst.idx_lebs)
                rsvd_idx_lebs = 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);
        spin_unlock(&c->space_lock);

        if (available > outstanding)
                free = ubifs_reported_space(c, available - outstanding);
        else
                free = 0;
        return free;
}