[v8,1/5] fs: add infrastructure for multigrain timestamps

diff --git a/fs/inode.c b/fs/inode.c
index 84bc3c76e5cc..f3d68e4b8df7 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -168,6 +168,8 @@  int inode_init_always(struct super_block *sb, struct inode *inode)
 	inode->i_fop = &no_open_fops;
 	inode->i_ino = 0;
 	inode->__i_nlink = 1;
+	inode->__i_ctime.tv_sec = 0;
+	inode->__i_ctime.tv_nsec = 0;
 	inode->i_opflags = 0;
 	if (sb->s_xattr)
 		inode->i_opflags |= IOP_XATTR;
@@ -2102,10 +2104,52 @@  int file_remove_privs(struct file *file)
 }
 EXPORT_SYMBOL(file_remove_privs);
 
+/**
+ * current_mgtime - Return FS time (possibly fine-grained)
+ * @inode: inode.
+ *
+ * Return the current time truncated to the time granularity supported by
+ * the fs, as suitable for a ctime/mtime change. If the ctime is flagged
+ * as having been QUERIED, get a fine-grained timestamp.
+ */
+struct timespec64 current_mgtime(struct inode *inode)
+{
+	struct timespec64 now, ctime;
+	atomic_long_t *pnsec = (atomic_long_t *)&inode->__i_ctime.tv_nsec;
+	long nsec = atomic_long_read(pnsec);
+
+	if (nsec & I_CTIME_QUERIED) {
+		ktime_get_real_ts64(&now);
+		return timestamp_truncate(now, inode);
+	}
+
+	ktime_get_coarse_real_ts64(&now);
+	now = timestamp_truncate(now, inode);
+
+	/*
+	 * If we've recently fetched a fine-grained timestamp
+	 * then the coarse-grained one may still be earlier than the
+	 * existing ctime. Just keep the existing value if so.
+	 */
+	ctime = inode_get_ctime(inode);
+	if (timespec64_compare(&ctime, &now) > 0)
+		now = ctime;
+
+	return now;
+}
+EXPORT_SYMBOL(current_mgtime);
+
+static struct timespec64 current_ctime(struct inode *inode)
+{
+	if (is_mgtime(inode))
+		return current_mgtime(inode);
+	return current_time(inode);
+}
+
 static int inode_needs_update_time(struct inode *inode)
 {
 	int sync_it = 0;
-	struct timespec64 now = current_time(inode);
+	struct timespec64 now = current_ctime(inode);
 	struct timespec64 ctime;
 
 	/* First try to exhaust all avenues to not sync */
@@ -2527,6 +2571,13 @@  struct timespec64 current_time(struct inode *inode)
 }
 EXPORT_SYMBOL(current_time);
 
+/*
+ * Coarse timer ticks happen (roughly) every jiffy. If we see a coarse time
+ * more than 2 jiffies earlier than the current ctime, then we need to
+ * update it. This is the max delta allowed (in ns).
+ */
+#define COARSE_TIME_MAX_DELTA (2 / HZ * NSEC_PER_SEC)
+
 /**
  * inode_set_ctime_current - set the ctime to current_time
  * @inode: inode
@@ -2536,9 +2587,54 @@  EXPORT_SYMBOL(current_time);
  */
 struct timespec64 inode_set_ctime_current(struct inode *inode)
 {
-	struct timespec64 now = current_time(inode);
+	struct timespec64 now;
+	struct timespec64 ctime;
+
+	ctime.tv_nsec = READ_ONCE(inode->__i_ctime.tv_nsec);
+	if (!(ctime.tv_nsec & I_CTIME_QUERIED)) {
+		now = current_time(inode);
+
+		/* Just copy it into place if it's not multigrain */
+		if (!is_mgtime(inode)) {
+			inode_set_ctime_to_ts(inode, now);
+			return now;
+		}
 
-	inode_set_ctime(inode, now.tv_sec, now.tv_nsec);
+		/*
+		 * If we've recently updated with a fine-grained timestamp,
+		 * then the coarse-grained one may still be earlier than the
+		 * existing ctime. Just keep the existing value if so.
+		 */
+		ctime.tv_sec = inode->__i_ctime.tv_sec;
+		if (timespec64_compare(&ctime, &now) > 0) {
+			struct timespec64	limit = now;
+
+			/*
+			 * If the current coarse-grained clock is earlier than
+			 * it should be, then that's an indication that there
+			 * may have been a backward clock jump, and that the
+			 * update should not be skipped.
+			 */
+			timespec64_add_ns(&limit, COARSE_TIME_MAX_DELTA);
+			if (timespec64_compare(&ctime, &limit) < 0)
+				return ctime;
+		}
+
+		/*
+		 * Ctime updates are usually protected by the inode_lock, but
+		 * we can still race with someone setting the QUERIED flag.
+		 * Try to swap the new nsec value into place. If it's changed
+		 * in the interim, then just go with a fine-grained timestamp.
+		 */
+		if (cmpxchg(&inode->__i_ctime.tv_nsec, ctime.tv_nsec,
+			    now.tv_nsec) != ctime.tv_nsec)
+			goto fine_grained;
+		inode->__i_ctime.tv_sec = now.tv_sec;
+		return now;
+	}
+fine_grained:
+	ktime_get_real_ts64(&now);
+	inode_set_ctime_to_ts(inode, timestamp_truncate(now, inode));
 	return now;
 }
 EXPORT_SYMBOL(inode_set_ctime_current);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b528f063e8ff..91239a4c1a65 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1508,18 +1508,65 @@  static inline bool fsuidgid_has_mapping(struct super_block *sb,
 	       kgid_has_mapping(fs_userns, kgid);
 }
 
+struct timespec64 current_mgtime(struct inode *inode);
 struct timespec64 current_time(struct inode *inode);
 struct timespec64 inode_set_ctime_current(struct inode *inode);
 
+/*
+ * Multigrain timestamps
+ *
+ * Conditionally use fine-grained ctime and mtime timestamps when there
+ * are users actively observing them via getattr. The primary use-case
+ * for this is NFS clients that use the ctime to distinguish between
+ * different states of the file, and that are often fooled by multiple
+ * operations that occur in the same coarse-grained timer tick.
+ *
+ * The kernel always keeps normalized struct timespec64 values in the ctime,
+ * which means that only the first 30 bits of the value are used. Use the
+ * 31st bit of the ctime's tv_nsec field as a flag to indicate that the value
+ * has been queried since it was last updated.
+ */
+#define I_CTIME_QUERIED		(1L<<30)
+
 /**
  * inode_get_ctime - fetch the current ctime from the inode
  * @inode: inode from which to fetch ctime
  *
- * Grab the current ctime from the inode and return it.
+ * Grab the current ctime from the inode, mask off the I_CTIME_QUERIED
+ * flag and return it. This is mostly intended for use by internal consumers
+ * of the ctime that aren't concerned with ensuring a fine-grained update on
+ * the next change (e.g. when preparing to store the value in the backing store
+ * for later retrieval).
+ *
+ * This is safe to call regardless of whether the underlying filesystem
+ * is using multigrain timestamps.
  */
 static inline struct timespec64 inode_get_ctime(const struct inode *inode)
 {
-	return inode->__i_ctime;
+	struct timespec64 ctime;
+
+	ctime.tv_sec = inode->__i_ctime.tv_sec;
+	ctime.tv_nsec = inode->__i_ctime.tv_nsec & ~I_CTIME_QUERIED;
+
+	return ctime;
+}
+
+/**
+ * inode_query_ctime - fetch the current ctime from inode and flag it
+ * @inode: inode from which to fetch and flag
+ *
+ * Grab the current ctime from the inode, mask off the I_CTIME_QUERIED
+ * flag and return it. This version also marks the inode as needing a fine
+ * grained timestamp update in the future.
+ */
+static inline struct timespec64 inode_query_ctime(const struct inode *inode)
+{
+	struct timespec64 ctime;
+	atomic_long_t *pnsec = (atomic_long_t *)&inode->__i_ctime.tv_nsec;
+
+	ctime.tv_sec = inode->__i_ctime.tv_sec;
+	ctime.tv_nsec = atomic_long_fetch_or(I_CTIME_QUERIED, pnsec) & ~I_CTIME_QUERIED;
+	return ctime;
 }
 
 /**
@@ -2305,6 +2352,7 @@  struct file_system_type {
 #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
 #define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
 #define FS_ALLOW_IDMAP         32      /* FS has been updated to handle vfs idmappings. */
+#define FS_MGTIME		64	/* FS uses multigrain timestamps */
 #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
 	int (*init_fs_context)(struct fs_context *);
 	const struct fs_parameter_spec *parameters;
@@ -2328,6 +2376,17 @@  struct file_system_type {
 
 #define MODULE_ALIAS_FS(NAME) MODULE_ALIAS("fs-" NAME)
 
+/**
+ * is_mgtime: is this inode using multigrain timestamps
+ * @inode: inode to test for multigrain timestamps
+ *
+ * Return true if the inode uses multigrain timestamps, false otherwise.
+ */
+static inline bool is_mgtime(const struct inode *inode)
+{
+	return inode->i_sb->s_type->fs_flags & FS_MGTIME;
+}
+
 extern struct dentry *mount_bdev(struct file_system_type *fs_type,
 	int flags, const char *dev_name, void *data,
 	int (*fill_super)(struct super_block *, void *, int));