[PATCH 2/3] fs: inode: Add API to retrieve global next ino using full ino_t width

[Chris Down <chris@xxxxxxxxxxxxxx>]

We can't just wholesale replace get_next_ino to use ino_t and 64-bit
atomics because of a few setbacks:

1. This may break some 32-bit userspace applications on a 64-bit kernel
   which cannot handle a 64-bit ino_t -- see the comment above
   get_next_ino;
2. Some applications inside the kernel already make use of the ino_t
   high bits. For example, overlayfs' xino feature uses these to merge
   inode numbers and fsid indexes to form a new identifier.

As such we need to make using the full width of ino_t an option for
users without being a requirement.

This will later be used to power inode64 in tmpfs, and can be used
elsewhere for other filesystems as desired.

Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Amir Goldstein <amir73il@xxxxxxxxx>
Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: kernel-team@xxxxxx
---
 fs/inode.c         | 41 +++++++++++++++++++++++++++++++++++++----
 include/linux/fs.h |  1 +
 2 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 255a4ae81b65..1d96ad8b71f6 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -878,16 +878,17 @@ static struct inode *find_inode_fast(struct super_block *sb,
  * here to attempt to avoid that.
  */
 #define LAST_INO_BATCH 1024
-static DEFINE_PER_CPU(unsigned int, last_ino);
+static DEFINE_PER_CPU(unsigned int, last_ino_ui);
 
 /*
  * As get_next_ino returns a type with a small width (typically 32 bits),
  * consider reusing inode numbers in your filesystem if you have a private inode
- * cache in order to reduce the risk of wraparound.
+ * cache in order to reduce the risk of wraparound, or consider providing the
+ * option to use get_next_ino_full instead.
  */
 unsigned int get_next_ino(void)
 {
-	unsigned int *p = &get_cpu_var(last_ino);
+	unsigned int *p = &get_cpu_var(last_ino_ui);
 	unsigned int res = *p;
 
 #ifdef CONFIG_SMP
@@ -904,11 +905,43 @@ unsigned int get_next_ino(void)
 	if (unlikely(!res))
 		res++;
 	*p = res;
-	put_cpu_var(last_ino);
+	put_cpu_var(last_ino_ui);
 	return res;
 }
 EXPORT_SYMBOL(get_next_ino);
 
+static DEFINE_PER_CPU(ino_t, last_ino_full);
+
+ino_t get_next_ino_full(void)
+{
+	ino_t *p = &get_cpu_var(last_ino_full);
+	ino_t res = *p;
+
+#ifdef CONFIG_SMP
+	if (unlikely((res & (LAST_INO_BATCH-1)) == 0)) {
+		static atomic64_t shared_last_ino;
+		u64 next = atomic64_add_return(LAST_INO_BATCH,
+					       &shared_last_ino);
+
+		/*
+		 * This might get truncated if ino_t is 32-bit, and so be more
+		 * susceptible to wrap around than on environments where ino_t
+		 * is 64-bit.
+		 */
+		res = next - LAST_INO_BATCH;
+	}
+#endif
+
+	res++;
+	/* get_next_ino_full should not provide a 0 inode number */
+	if (unlikely(!res))
+		res++;
+	*p = res;
+	put_cpu_var(last_ino_full);
+	return res;
+}
+EXPORT_SYMBOL(get_next_ino_full);
+
 /**
  *	new_inode_pseudo 	- obtain an inode
  *	@sb: superblock
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 190c45039359..c73896d993c1 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3053,6 +3053,7 @@ static inline void lockdep_annotate_inode_mutex_key(struct inode *inode) { };
 extern void unlock_new_inode(struct inode *);
 extern void discard_new_inode(struct inode *);
 extern unsigned int get_next_ino(void);
+extern ino_t get_next_ino_full(void);
 extern void evict_inodes(struct super_block *sb);
 
 extern void __iget(struct inode * inode);
-- 
2.24.1