[PATCH] shmem: no sbinfo for tmpfs mount?

Some might want a tmpfs mount with the improved scalability afforded by
omitting shmem superblock accounting; or some might just want to test it in an
externally-visible tmpfs mount instance.

Adopt the convention that mount option -o nr_blocks=0,nr_inodes=0 means
without resource limits, and hence no shmem_sb_info.  Not recommended for
general use, but no worse than ramfs.

Disallow remounting from unlimited to limited (no accounting has been done so
far, so no idea whether it's permissible), and from limited to unlimited
(because we'd need then to free the sbinfo, and visit each inode to reset its
i_blocks to 0: why bother?).
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/filesystems/tmpfs.txt

@@ -71,6 +71,12 @@ can be changed on remount.  The size parameter also accepts a suffix %
 to limit this tmpfs instance to that percentage of your physical RAM:
 the default, when neither size nor nr_blocks is specified, is size=50%
 
+If both nr_blocks (or size) and nr_inodes are set to 0, neither blocks
+nor inodes will be limited in that instance.  It is generally unwise to
+mount with such options, since it allows any user with write access to
+use up all the memory on the machine; but enhances the scalability of
+that instance in a system with many cpus making intensive use of it.
+
 
 To specify the initial root directory you can use the following mount
 options:

mm/shmem.c

@@ -1528,13 +1528,16 @@ static int shmem_statfs(struct super_block *sb, struct kstatfs *buf)
 
 	buf->f_type = TMPFS_MAGIC;
 	buf->f_bsize = PAGE_CACHE_SIZE;
-	spin_lock(&sbinfo->stat_lock);
-	buf->f_blocks = sbinfo->max_blocks;
-	buf->f_bavail = buf->f_bfree = sbinfo->free_blocks;
-	buf->f_files = sbinfo->max_inodes;
-	buf->f_ffree = sbinfo->free_inodes;
-	spin_unlock(&sbinfo->stat_lock);
 	buf->f_namelen = NAME_MAX;
+	if (sbinfo) {
+		spin_lock(&sbinfo->stat_lock);
+		buf->f_blocks = sbinfo->max_blocks;
+		buf->f_bavail = buf->f_bfree = sbinfo->free_blocks;
+		buf->f_files = sbinfo->max_inodes;
+		buf->f_ffree = sbinfo->free_inodes;
+		spin_unlock(&sbinfo->stat_lock);
+	}
+	/* else leave those fields 0 like simple_statfs */
 	return 0;
 }
 
@@ -1591,13 +1594,15 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
 	 * but each new link needs a new dentry, pinning lowmem, and
 	 * tmpfs dentries cannot be pruned until they are unlinked.
 	 */
-	spin_lock(&sbinfo->stat_lock);
-	if (!sbinfo->free_inodes) {
+	if (sbinfo) {
+		spin_lock(&sbinfo->stat_lock);
+		if (!sbinfo->free_inodes) {
+			spin_unlock(&sbinfo->stat_lock);
+			return -ENOSPC;
+		}
+		sbinfo->free_inodes--;
 		spin_unlock(&sbinfo->stat_lock);
-		return -ENOSPC;
 	}
-	sbinfo->free_inodes--;
-	spin_unlock(&sbinfo->stat_lock);
 
 	dir->i_size += BOGO_DIRENT_SIZE;
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = CURRENT_TIME;
@@ -1614,9 +1619,11 @@ static int shmem_unlink(struct inode *dir, struct dentry *dentry)
 
 	if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode)) {
 		struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
-		spin_lock(&sbinfo->stat_lock);
-		sbinfo->free_inodes++;
-		spin_unlock(&sbinfo->stat_lock);
+		if (sbinfo) {
+			spin_lock(&sbinfo->stat_lock);
+			sbinfo->free_inodes++;
+			spin_unlock(&sbinfo->stat_lock);
+		}
 	}
 
 	dir->i_size -= BOGO_DIRENT_SIZE;
@@ -1827,11 +1834,21 @@ static int shmem_parse_options(char *options, int *mode, uid_t *uid, gid_t *gid,
 static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
 {
 	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
-	unsigned long max_blocks = sbinfo->max_blocks;
-	unsigned long max_inodes = sbinfo->max_inodes;
+	unsigned long max_blocks = 0;
+	unsigned long max_inodes = 0;
 
+	if (sbinfo) {
+		max_blocks = sbinfo->max_blocks;
+		max_inodes = sbinfo->max_inodes;
+	}
 	if (shmem_parse_options(data, NULL, NULL, NULL, &max_blocks, &max_inodes))
 		return -EINVAL;
+	/* Keep it simple: disallow limited <-> unlimited remount */
+	if ((max_blocks || max_inodes) == !sbinfo)
+		return -EINVAL;
+	/* But allow the pointless unlimited -> unlimited remount */
+	if (!sbinfo)
+		return 0;
 	return shmem_set_size(sbinfo, max_blocks, max_inodes);
 }
 #endif
@@ -1853,22 +1870,27 @@ static int shmem_fill_super(struct super_block *sb,
 	int err = -ENOMEM;
 
 #ifdef CONFIG_TMPFS
+	unsigned long blocks = 0;
+	unsigned long inodes = 0;
+
 	/*
 	 * Per default we only allow half of the physical ram per
 	 * tmpfs instance, limiting inodes to one per page of lowmem;
 	 * but the internal instance is left unlimited.
 	 */
 	if (!(sb->s_flags & MS_NOUSER)) {
-		struct shmem_sb_info *sbinfo;
-		unsigned long blocks = totalram_pages / 2;
-		unsigned long inodes = totalram_pages - totalhigh_pages;
+		blocks = totalram_pages / 2;
+		inodes = totalram_pages - totalhigh_pages;
 		if (inodes > blocks)
 			inodes = blocks;
 
 		if (shmem_parse_options(data, &mode,
 					&uid, &gid, &blocks, &inodes))
 			return -EINVAL;
+	}
 
+	if (blocks || inodes) {
+		struct shmem_sb_info *sbinfo;
 		sbinfo = kmalloc(sizeof(struct shmem_sb_info), GFP_KERNEL);
 		if (!sbinfo)
 			return -ENOMEM;