JFFS2: Add parameter to reserve disk space for root

diff --git a/fs/jffs2/jffs2_fs_sb.h b/fs/jffs2/jffs2_fs_sb.h
index 55a0c1d..0d00bf2 100644
--- a/fs/jffs2/jffs2_fs_sb.h
+++ b/fs/jffs2/jffs2_fs_sb.h
@@ -32,6 +32,13 @@  struct jffs2_inodirty;
 struct jffs2_mount_opts {
 	bool override_compr;
 	unsigned int compr;
+
+	/* The size of the reserved pool. The reserved pool is the JFFS2 flash
+	 * space which may only be used by root cannot be used by the other
+	 * users. This is implemented simply by means of not allowing the
+	 * latter users to write to the file system if the amount if the
+	 * available space is less then 'rp_size'. */
+	unsigned int rp_size;
 };
 
 /* A struct for the overall file system control.  Pointers to
diff --git a/fs/jffs2/nodemgmt.c b/fs/jffs2/nodemgmt.c
index 6784d1e..7d4c869 100644
--- a/fs/jffs2/nodemgmt.c
+++ b/fs/jffs2/nodemgmt.c
@@ -18,6 +18,37 @@ 
 #include "nodelist.h"
 #include "debug.h"
 
+/*
+ * Check whether the user is allowed to write.
+ */
+static int jffs2_rp_can_write(struct jffs2_sb_info *c)
+{
+	uint32_t avail;
+	struct jffs2_mount_opts *opts = &c->mount_opts;
+
+	avail = c->dirty_size + c->free_size + c->unchecked_size +
+		c->erasing_size - c->resv_blocks_write * c->sector_size
+		- c->nospc_dirty_size;
+
+	if (avail < 2 * opts->rp_size)
+		D1(printk(KERN_DEBUG "rpsize %u, dirty_size %u, free_size %u, "
+			"erasing_size %u, unchecked_size %u, "
+			"nr_erasing_blocks %u, avail %u, resrv %u\n",
+			opts->rp_size, c->dirty_size, c->free_size,
+			c->erasing_size, c->unchecked_size,
+			c->nr_erasing_blocks, avail, c->nospc_dirty_size));
+
+	if (avail > opts->rp_size)
+		return 1;
+
+	/* Always allow root */
+	if (current_fsuid() == 0 || current_fsgid() == 0)
+		return 1;
+
+	D1(printk(KERN_DEBUG "forbid writing\n"));
+	return 0;
+}
+
 /**
  *	jffs2_reserve_space - request physical space to write nodes to flash
  *	@c: superblock info
@@ -55,6 +86,15 @@  int jffs2_reserve_space(struct jffs2_sb_info *c, uint32_t minsize,
 
 	spin_lock(&c->erase_completion_lock);
 
+	/*
+	 * Check if the free space is greater then size of the reserved pool.
+	 * If not, only allow root to proceed with writing.
+	 */
+	if (prio != ALLOC_DELETION && !jffs2_rp_can_write(c)) {
+		ret = -ENOSPC;
+		goto out;
+	}
+
 	/* this needs a little more thought (true <tglx> :)) */
 	while(ret == -EAGAIN) {
 		while(c->nr_free_blocks + c->nr_erasing_blocks < blocksneeded) {
@@ -158,6 +198,8 @@  int jffs2_reserve_space(struct jffs2_sb_info *c, uint32_t minsize,
 			jffs2_dbg(1, "%s(): ret is %d\n", __func__, ret);
 		}
 	}
+
+out:
 	spin_unlock(&c->erase_completion_lock);
 	if (!ret)
 		ret = jffs2_prealloc_raw_node_refs(c, c->nextblock, 1);
diff --git a/fs/jffs2/super.c b/fs/jffs2/super.c
index f9916f3..3dba438 100644
--- a/fs/jffs2/super.c
+++ b/fs/jffs2/super.c
@@ -105,6 +105,8 @@  static int jffs2_show_options(struct seq_file *s, struct dentry *root)
 
 	if (opts->override_compr)
 		seq_printf(s, ",compr=%s", jffs2_compr_name(opts->compr));
+	if (opts->rp_size)
+		seq_printf(s, ",rp_size=%u", opts->rp_size / 1024);
 
 	return 0;
 }
@@ -171,15 +173,18 @@  static const struct export_operations jffs2_export_ops = {
  * JFFS2 mount options.
  *
  * Opt_override_compr: override default compressor
+ * Opt_rp_size: size of reserved pool in KiB
  * Opt_err: just end of array marker
  */
 enum {
 	Opt_override_compr,
+	Opt_rp_size,
 	Opt_err,
 };
 
 static const match_table_t tokens = {
 	{Opt_override_compr, "compr=%s"},
+	{Opt_rp_size, "rp_size=%u"},
 	{Opt_err, NULL},
 };
 
@@ -187,6 +192,7 @@  static int jffs2_parse_options(struct jffs2_sb_info *c, char *data)
 {
 	substring_t args[MAX_OPT_ARGS];
 	char *p, *name;
+	unsigned int opt;
 
 	if (!data)
 		return 0;
@@ -224,6 +230,17 @@  static int jffs2_parse_options(struct jffs2_sb_info *c, char *data)
 			kfree(name);
 			c->mount_opts.override_compr = true;
 			break;
+		case Opt_rp_size:
+			if (match_int(&args[0], &opt))
+				return -EINVAL;
+			opt *= 1024;
+			if (opt > c->mtd->size) {
+				pr_warn("Too large reserve pool specified, "
+					"max is %llu KB\n", c->mtd->size / 1024);
+				return -EINVAL;
+			}
+			c->mount_opts.rp_size = opt;
+			break;
 		default:
 			pr_err("Error: unrecognized mount option '%s' or missing value\n",
 			       p);