[RFC PATCH] <linux/taggedptr.h>: Introduce tagged pointer

[Gao Xiang]

Currently kernel has scattered tagged pointer usages hacked
by hand in plain code, without a unique and portable functionset
to highlight the tagged pointer itself and wrap these hacked code
in order to clean up all over meaningless magic masks.

Therefore, this patch introduces simple generic methods to fold
tags into a pointer integer. It currently reuses the last 2 bits
of the pointer for tags, which are safely for all modern platforms.

In addition, it will also be used for the upcoming EROFS filesystem,
which heavily uses tagged pointer approach for high performance
and reducing extra memory allocation.

Refer to:
https://en.wikipedia.org/wiki/Tagged_pointer

To: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Kate Stewart <kstewart@xxxxxxxxxxxxxxxxxxx>
Cc: Philippe Ombredanne <pombredanne@xxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Chao Yu <yuchao0@xxxxxxxxxx>
Cc: Miao Xie <miaoxie@xxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-erofs@xxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Gao Xiang <gaoxiang25@xxxxxxxxxx>
---

Tagged pointers are good for performance and memory saving,
hoping for a generic taggedptr approach to clean up redundant dirty code.

Any comments, suggestions or alternative approaches are welcomed. :)

Thanks,
Gao Xiang

 fs/file.c                 | 19 ++++++------
 include/linux/file.h      |  7 +++--
 include/linux/taggedptr.h | 76 +++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 90 insertions(+), 12 deletions(-)
 create mode 100644 include/linux/taggedptr.h

diff --git a/fs/file.c b/fs/file.c
index 7ffd6e9..6d6f185 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -727,7 +727,7 @@ struct file *fget_raw(unsigned int fd)
  * The fput_needed flag returned by fget_light should be passed to the
  * corresponding fput_light.
  */
-static unsigned long __fget_light(unsigned int fd, fmode_t mask)
+static taggedptr_t __fget_light(unsigned int fd, fmode_t mask)
 {
 	struct files_struct *files = current->files;
 	struct file *file;
@@ -736,33 +736,34 @@ static unsigned long __fget_light(unsigned int fd, fmode_t mask)
 		file = __fcheck_files(files, fd);
 		if (!file || unlikely(file->f_mode & mask))
 			return 0;
-		return (unsigned long)file;
+		return taggedptr_fold(file, 0);
 	} else {
 		file = __fget(fd, mask);
 		if (!file)
 			return 0;
-		return FDPUT_FPUT | (unsigned long)file;
+		return taggedptr_fold(file, FDPUT_FPUT);
 	}
 }
-unsigned long __fdget(unsigned int fd)
+
+taggedptr_t __fdget(unsigned int fd)
 {
 	return __fget_light(fd, FMODE_PATH);
 }
 EXPORT_SYMBOL(__fdget);
 
-unsigned long __fdget_raw(unsigned int fd)
+taggedptr_t __fdget_raw(unsigned int fd)
 {
 	return __fget_light(fd, 0);
 }
 
-unsigned long __fdget_pos(unsigned int fd)
+taggedptr_t __fdget_pos(unsigned int fd)
 {
-	unsigned long v = __fdget(fd);
-	struct file *file = (struct file *)(v & ~3);
+	taggedptr_t v = __fdget(fd);
+	struct file *file = taggedptr_unfold_ptr(v);
 
 	if (file && (file->f_mode & FMODE_ATOMIC_POS)) {
 		if (file_count(file) > 1) {
-			v |= FDPUT_POS_UNLOCK;
+			taggedptr_set_tags(v, FDPUT_POS_UNLOCK);
 			mutex_lock(&file->f_pos_lock);
 		}
 	}
diff --git a/include/linux/file.h b/include/linux/file.h
index 279720d..af81f68 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -9,6 +9,7 @@
 #include <linux/compiler.h>
 #include <linux/types.h>
 #include <linux/posix_types.h>
+#include <linux/taggedptr.h>
 
 struct file;
 
@@ -42,9 +43,9 @@ static inline void fdput(struct fd fd)
 
 extern struct file *fget(unsigned int fd);
 extern struct file *fget_raw(unsigned int fd);
-extern unsigned long __fdget(unsigned int fd);
-extern unsigned long __fdget_raw(unsigned int fd);
-extern unsigned long __fdget_pos(unsigned int fd);
+extern taggedptr_t __fdget(unsigned int fd);
+extern taggedptr_t __fdget_raw(unsigned int fd);
+extern taggedptr_t __fdget_pos(unsigned int fd);
 extern void __f_unlock_pos(struct file *);
 
 static inline struct fd __to_fd(unsigned long v)
diff --git a/include/linux/taggedptr.h b/include/linux/taggedptr.h
new file mode 100644
index 0000000..1805394
--- /dev/null
+++ b/include/linux/taggedptr.h
@@ -0,0 +1,76 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Tagged pointer implementation
+ *
+ * Copyright (C) 2018 Gao Xiang <gaoxiang25@xxxxxxxxxx>
+ */
+#ifndef _LINUX_TAGGEDPTR_H
+#define _LINUX_TAGGEDPTR_H
+
+#include <linux/types.h>
+#include <linux/build_bug.h>
+
+/*
+ * mark these special integers as another type
+ * in order to highlight the tagged pointer usage.
+ */
+typedef uintptr_t	taggedptr_t;
+
+/*
+ * generally for all architectures, the last 2 bits of
+ * pointer can be used safely
+ */
+#ifndef TAGGEDPTR_TAGS_BITS
+#define TAGGEDPTR_TAGS_BITS	2
+#endif
+
+#define TAGGEDPTR_TAGS_MASK	((1 << TAGGEDPTR_TAGS_BITS) - 1)
+
+extern void __compiletime_error("bad taggedptr tags")
+	__bad_taggedptr_tags(void);
+
+/* encode the tagged pointer */
+static inline taggedptr_t taggedptr_fold(void *ptr, unsigned int tags)
+{
+	if (__builtin_constant_p(tags) && (tags & ~TAGGEDPTR_TAGS_MASK))
+		__bad_taggedptr_tags();
+
+	return (taggedptr_t)ptr | tags;
+}
+
+static inline void *taggedptr_unfold_ptr(taggedptr_t tptr)
+{
+	return (void *)(tptr & ~TAGGEDPTR_TAGS_MASK);
+}
+
+static inline unsigned int taggedptr_unfold_tags(taggedptr_t tptr)
+{
+	return tptr & TAGGEDPTR_TAGS_MASK;
+}
+
+static inline taggedptr_t taggedptr_replace_tags(taggedptr_t tptr,
+						 unsigned int tags)
+{
+	return taggedptr_fold(taggedptr_unfold_ptr(tptr), tags);
+}
+
+static inline taggedptr_t taggedptr_set_tags(taggedptr_t tptr,
+					     unsigned int tags)
+{
+	if (__builtin_constant_p(tags) && (tags & ~TAGGEDPTR_TAGS_MASK))
+		__bad_taggedptr_tags();
+
+	return tptr |= tags;
+}
+
+static inline taggedptr_t taggedptr_clear_tags(taggedptr_t tptr,
+					       unsigned int tags)
+{
+	if (__builtin_constant_p(tags) && (tags & ~TAGGEDPTR_TAGS_MASK))
+		__bad_taggedptr_tags();
+
+	return tptr &= ~tags;
+}
+
+#endif
+
-- 
1.9.1