fs: add i_state helpers

The i_state member is an unsigned long so that it can be used with the
wait bit infrastructure which expects unsigned long. This wastes 4 bytes
which we're unlikely to ever use. Switch to using the var event wait
mechanism using the address of the bit. Thanks to Linus for the address
idea.

Link: https://lore.kernel.org/r/20240823-work-i_state-v3-1-5cd5fd207a57@kernel.orgReviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/inode.c

@@ -472,6 +472,17 @@ static void __inode_add_lru(struct inode *inode, bool rotate)
 		inode->i_state |= I_REFERENCED;
 }
 
+struct wait_queue_head *inode_bit_waitqueue(struct wait_bit_queue_entry *wqe,
+					    struct inode *inode, u32 bit)
+{
+        void *bit_address;
+
+        bit_address = inode_state_wait_address(inode, bit);
+        init_wait_var_entry(wqe, bit_address, 0);
+        return __var_waitqueue(bit_address);
+}
+EXPORT_SYMBOL(inode_bit_waitqueue);
+
 /*
  * Add inode to LRU if needed (inode is unused and clean).
  *

include/linux/fs.h

@@ -744,6 +744,21 @@ struct inode {
 	void			*i_private; /* fs or device private pointer */
 } __randomize_layout;
 
+/*
+ * Get bit address from inode->i_state to use with wait_var_event()
+ * infrastructre.
+ */
+#define inode_state_wait_address(inode, bit) ((char *)&(inode)->i_state + (bit))
+
+struct wait_queue_head *inode_bit_waitqueue(struct wait_bit_queue_entry *wqe,
+					    struct inode *inode, u32 bit);
+
+static inline void inode_wake_up_bit(struct inode *inode, u32 bit)
+{
+	/* Caller is responsible for correct memory barriers. */
+	wake_up_var(inode_state_wait_address(inode, bit));
+}
+
 struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode);
 
 static inline unsigned int i_blocksize(const struct inode *node)