tmpfs: fix off-by-one in max_blocks checks

If you fill up a tmpfs, df was showing

  tmpfs                   460800         -         -   -  /tmp

because of an off-by-one in the max_blocks checks.  Fix it so df shows

  tmpfs                   460800    460800         0 100% /tmp
Signed-off-by: NHugh Dickins <hughd@google.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -421,7 +421,8 @@ static swp_entry_t *shmem_swp_alloc(struct shmem_inode_info *info, unsigned long
 		 * a waste to allocate index if we cannot allocate data.
 		 */
 		if (sbinfo->max_blocks) {
-			if (percpu_counter_compare(&sbinfo->used_blocks, (sbinfo->max_blocks - 1)) > 0)
+			if (percpu_counter_compare(&sbinfo->used_blocks,
+						sbinfo->max_blocks - 1) >= 0)
 				return ERR_PTR(-ENOSPC);
 			percpu_counter_inc(&sbinfo->used_blocks);
 			spin_lock(&inode->i_lock);
@@ -1397,7 +1398,8 @@ static int shmem_getpage(struct inode *inode, unsigned long idx,
 		shmem_swp_unmap(entry);
 		sbinfo = SHMEM_SB(inode->i_sb);
 		if (sbinfo->max_blocks) {
-			if ((percpu_counter_compare(&sbinfo->used_blocks, sbinfo->max_blocks) > 0) ||
+			if (percpu_counter_compare(&sbinfo->used_blocks,
+						sbinfo->max_blocks) >= 0 ||
 			    shmem_acct_block(info->flags)) {
 				spin_unlock(&info->lock);
 				error = -ENOSPC;