1 files changed, 87 insertions, 27 deletions

diff --git a/fs/ubifs/budget.c b/fs/ubifs/budget.c
index 154098157473..73db464cd08b 100644
--- a/fs/ubifs/budget.c
+++ b/fs/ubifs/budget.c
@@ -302,18 +302,6 @@ long long ubifs_calc_available(const struct ubifs_info *c, int min_idx_lebs)
         int subtract_lebs;
         long long available;
 
-        /*
-         * Force the amount available to the total size reported if the used
-         * space is zero.
-         */
-        if (c->lst.total_used <= UBIFS_INO_NODE_SZ &&
-            c->budg_data_growth + c->budg_dd_growth == 0) {
-                /* Do the same calculation as for c->block_cnt */
-                available = c->main_lebs - 2;
-                available *= c->leb_size - c->dark_wm;
-                return available;
-        }
-
         available = c->main_bytes - c->lst.total_used;
 
         /*
@@ -714,34 +702,106 @@ void ubifs_release_dirty_inode_budget(struct ubifs_info *c,
 }
 
 /**
- * ubifs_budg_get_free_space - return amount of free space.
+ * 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 returns amount of free space on the file-system.
+ * 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_budg_get_free_space(struct ubifs_info *c)
+long long ubifs_get_free_space(struct ubifs_info *c)
 {
-        int min_idx_lebs, rsvd_idx_lebs;
+        int min_idx_lebs, rsvd_idx_lebs, lebs;
         long long available, outstanding, free;
 
-        /* Do exactly the same calculations as in 'do_budget_space()' */
         spin_lock(&c->space_lock);
         min_idx_lebs = ubifs_calc_min_idx_lebs(c);
+        outstanding = c->budg_data_growth + c->budg_dd_growth;
 
-        if (min_idx_lebs > c->lst.idx_lebs)
-                rsvd_idx_lebs = min_idx_lebs - c->lst.idx_lebs;
-        else
-                rsvd_idx_lebs = 0;
-
-        if (rsvd_idx_lebs > c->lst.empty_lebs + c->freeable_cnt + c->idx_gc_cnt
-                                - c->lst.taken_empty_lebs) {
+        /*
+         * 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 0;
+                return (long long)c->block_cnt << UBIFS_BLOCK_SHIFT;
         }
 
         available = ubifs_calc_available(c, min_idx_lebs);
-        outstanding = c->budg_data_growth + c->budg_dd_growth;
-        c->min_idx_lebs = 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)