Merge patch series "fs: add i_state helpers"

Christian Brauner <brauner@kernel.org> says:

I've recently looked for some free space in struct inode again because
of some exec kerfuffle we had and while my idea didn't turn into
anything I noticed that we often waste bytes when using wait bit
operations. So I set out to switch that to another mechanism that would
allow us to free up bytes. So this is an attempt to turn i_state from an
unsigned long into an u32 using the individual bytes of i_state as
addresses for the wait var event mechanism (Thanks to Linus for that idea.).

This survives LTP, xfstests on various filesystems, and will-it-scale.

* patches from https://lore.kernel.org/r/20240823-work-i_state-v3-1-5cd5fd207a57@kernel.org:
  inode: make i_state a u32
  inode: port __I_LRU_ISOLATING to var event
  inode: port __I_NEW to var event
  inode: port __I_SYNC to var event
  fs: reorder i_state bits
  fs: add i_state helpers

Link: https://lore.kernel.org/r/20240823-work-i_state-v3-1-5cd5fd207a57@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

6 files changed, 135 insertions, 56 deletions

diff --git a/fs/bcachefs/fs.c b/fs/bcachefs/fs.c
index 94c392abef65..c0900c0c0f8a 100644
--- a/fs/bcachefs/fs.c
+++ b/fs/bcachefs/fs.c
@@ -1644,14 +1644,16 @@ again:
 				break;
 			}
 		} else if (clean_pass && this_pass_clean) {
-			wait_queue_head_t *wq = bit_waitqueue(&inode->v.i_state, __I_NEW);
-			DEFINE_WAIT_BIT(wait, &inode->v.i_state, __I_NEW);
+			struct wait_bit_queue_entry wqe;
+			struct wait_queue_head *wq_head;
 
-			prepare_to_wait(wq, &wait.wq_entry, TASK_UNINTERRUPTIBLE);
+			wq_head = inode_bit_waitqueue(&wqe, &inode->v, __I_NEW);
+			prepare_to_wait_event(wq_head, &wqe.wq_entry,
+					      TASK_UNINTERRUPTIBLE);
 			mutex_unlock(&c->vfs_inodes_lock);
 
 			schedule();
-			finish_wait(wq, &wait.wq_entry);
+			finish_wait(wq_head, &wqe.wq_entry);
 			goto again;
 		}
 	}
diff --git a/fs/dcache.c b/fs/dcache.c
index 1af75fa68638..894e38cdf4d0 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1908,8 +1908,13 @@ void d_instantiate_new(struct dentry *entry, struct inode *inode)
 	__d_instantiate(entry, inode);
 	WARN_ON(!(inode->i_state & I_NEW));
 	inode->i_state &= ~I_NEW & ~I_CREATING;
+	/*
+	 * Pairs with the barrier in prepare_to_wait_event() to make sure
+	 * ___wait_var_event() either sees the bit cleared or
+	 * waitqueue_active() check in wake_up_var() sees the waiter.
+	 */
 	smp_mb();
-	wake_up_bit(&inode->i_state, __I_NEW);
+	inode_wake_up_bit(inode, __I_NEW);
 	spin_unlock(&inode->i_lock);
 }
 EXPORT_SYMBOL(d_instantiate_new);
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 1a5006329f6f..d8bec3c1bb1f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1386,12 +1386,13 @@ static void requeue_io(struct inode *inode, struct bdi_writeback *wb)
 
 static void inode_sync_complete(struct inode *inode)
 {
+	assert_spin_locked(&inode->i_lock);
+
 	inode->i_state &= ~I_SYNC;
 	/* If inode is clean an unused, put it into LRU now... */
 	inode_add_lru(inode);
-	/* Waiters must see I_SYNC cleared before being woken up */
-	smp_mb();
-	wake_up_bit(&inode->i_state, __I_SYNC);
+	/* Called with inode->i_lock which ensures memory ordering. */
+	inode_wake_up_bit(inode, __I_SYNC);
 }
 
 static bool inode_dirtied_after(struct inode *inode, unsigned long t)
@@ -1512,17 +1513,25 @@ static int write_inode(struct inode *inode, struct writeback_control *wbc)
  */
 void inode_wait_for_writeback(struct inode *inode)
 {
-	DEFINE_WAIT_BIT(wq, &inode->i_state, __I_SYNC);
-	wait_queue_head_t *wqh;
+	struct wait_bit_queue_entry wqe;
+	struct wait_queue_head *wq_head;
+
+	assert_spin_locked(&inode->i_lock);
+
+	if (!(inode->i_state & I_SYNC))
+		return;
 
-	lockdep_assert_held(&inode->i_lock);
-	wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
-	while (inode->i_state & I_SYNC) {
+	wq_head = inode_bit_waitqueue(&wqe, inode, __I_SYNC);
+	for (;;) {
+		prepare_to_wait_event(wq_head, &wqe.wq_entry, TASK_UNINTERRUPTIBLE);
+		/* Checking I_SYNC with inode->i_lock guarantees memory ordering. */
+		if (!(inode->i_state & I_SYNC))
+			break;
 		spin_unlock(&inode->i_lock);
-		__wait_on_bit(wqh, &wq, bit_wait,
-			      TASK_UNINTERRUPTIBLE);
+		schedule();
 		spin_lock(&inode->i_lock);
 	}
+	finish_wait(wq_head, &wqe.wq_entry);
 }
 
 /*
@@ -1533,16 +1542,20 @@ void inode_wait_for_writeback(struct inode *inode)
 static void inode_sleep_on_writeback(struct inode *inode)
 	__releases(inode->i_lock)
 {
-	DEFINE_WAIT(wait);
-	wait_queue_head_t *wqh = bit_waitqueue(&inode->i_state, __I_SYNC);
-	int sleep;
+	struct wait_bit_queue_entry wqe;
+	struct wait_queue_head *wq_head;
+	bool sleep;
+
+	assert_spin_locked(&inode->i_lock);
 
-	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
-	sleep = inode->i_state & I_SYNC;
+	wq_head = inode_bit_waitqueue(&wqe, inode, __I_SYNC);
+	prepare_to_wait_event(wq_head, &wqe.wq_entry, TASK_UNINTERRUPTIBLE);
+	/* Checking I_SYNC with inode->i_lock guarantees memory ordering. */
+	sleep = !!(inode->i_state & I_SYNC);
 	spin_unlock(&inode->i_lock);
 	if (sleep)
 		schedule();
-	finish_wait(wqh, &wait);
+	finish_wait(wq_head, &wqe.wq_entry);
 }
 
 /*
diff --git a/fs/inode.c b/fs/inode.c
index ba1645a09603..aacd05749c1f 100644
--- a/fs/inode.c
+++ b/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).
  *
@@ -500,24 +511,35 @@ static void inode_unpin_lru_isolating(struct inode *inode)
 	spin_lock(&inode->i_lock);
 	WARN_ON(!(inode->i_state & I_LRU_ISOLATING));
 	inode->i_state &= ~I_LRU_ISOLATING;
-	smp_mb();
-	wake_up_bit(&inode->i_state, __I_LRU_ISOLATING);
+	/* Called with inode->i_lock which ensures memory ordering. */
+	inode_wake_up_bit(inode, __I_LRU_ISOLATING);
 	spin_unlock(&inode->i_lock);
 }
 
 static void inode_wait_for_lru_isolating(struct inode *inode)
 {
+	struct wait_bit_queue_entry wqe;
+	struct wait_queue_head *wq_head;
+
 	lockdep_assert_held(&inode->i_lock);
-	if (inode->i_state & I_LRU_ISOLATING) {
-		DEFINE_WAIT_BIT(wq, &inode->i_state, __I_LRU_ISOLATING);
-		wait_queue_head_t *wqh;
+	if (!(inode->i_state & I_LRU_ISOLATING))
+		return;
 
-		wqh = bit_waitqueue(&inode->i_state, __I_LRU_ISOLATING);
+	wq_head = inode_bit_waitqueue(&wqe, inode, __I_LRU_ISOLATING);
+	for (;;) {
+		prepare_to_wait_event(wq_head, &wqe.wq_entry, TASK_UNINTERRUPTIBLE);
+		/*
+		 * Checking I_LRU_ISOLATING with inode->i_lock guarantees
+		 * memory ordering.
+		 */
+		if (!(inode->i_state & I_LRU_ISOLATING))
+			break;
 		spin_unlock(&inode->i_lock);
-		__wait_on_bit(wqh, &wq, bit_wait, TASK_UNINTERRUPTIBLE);
+		schedule();
 		spin_lock(&inode->i_lock);
-		WARN_ON(inode->i_state & I_LRU_ISOLATING);
 	}
+	finish_wait(wq_head, &wqe.wq_entry);
+	WARN_ON(inode->i_state & I_LRU_ISOLATING);
 }
 
 /**
@@ -723,7 +745,13 @@ static void evict(struct inode *inode)
 	 * used as an indicator whether blocking on it is safe.
 	 */
 	spin_lock(&inode->i_lock);
-	wake_up_bit(&inode->i_state, __I_NEW);
+	/*
+	 * Pairs with the barrier in prepare_to_wait_event() to make sure
+	 * ___wait_var_event() either sees the bit cleared or
+	 * waitqueue_active() check in wake_up_var() sees the waiter.
+	 */
+	smp_mb();
+	inode_wake_up_bit(inode, __I_NEW);
 	BUG_ON(inode->i_state != (I_FREEING | I_CLEAR));
 	spin_unlock(&inode->i_lock);
 
@@ -1135,8 +1163,13 @@ void unlock_new_inode(struct inode *inode)
 	spin_lock(&inode->i_lock);
 	WARN_ON(!(inode->i_state & I_NEW));
 	inode->i_state &= ~I_NEW & ~I_CREATING;
+	/*
+	 * Pairs with the barrier in prepare_to_wait_event() to make sure
+	 * ___wait_var_event() either sees the bit cleared or
+	 * waitqueue_active() check in wake_up_var() sees the waiter.
+	 */
 	smp_mb();
-	wake_up_bit(&inode->i_state, __I_NEW);
+	inode_wake_up_bit(inode, __I_NEW);
 	spin_unlock(&inode->i_lock);
 }
 EXPORT_SYMBOL(unlock_new_inode);
@@ -1147,8 +1180,13 @@ void discard_new_inode(struct inode *inode)
 	spin_lock(&inode->i_lock);
 	WARN_ON(!(inode->i_state & I_NEW));
 	inode->i_state &= ~I_NEW;
+	/*
+	 * Pairs with the barrier in prepare_to_wait_event() to make sure
+	 * ___wait_var_event() either sees the bit cleared or
+	 * waitqueue_active() check in wake_up_var() sees the waiter.
+	 */
 	smp_mb();
-	wake_up_bit(&inode->i_state, __I_NEW);
+	inode_wake_up_bit(inode, __I_NEW);
 	spin_unlock(&inode->i_lock);
 	iput(inode);
 }
@@ -2337,8 +2375,8 @@ EXPORT_SYMBOL(inode_needs_sync);
  */
 static void __wait_on_freeing_inode(struct inode *inode, bool is_inode_hash_locked)
 {
-	wait_queue_head_t *wq;
-	DEFINE_WAIT_BIT(wait, &inode->i_state, __I_NEW);
+	struct wait_bit_queue_entry wqe;
+	struct wait_queue_head *wq_head;
 
 	/*
 	 * Handle racing against evict(), see that routine for more details.
@@ -2349,14 +2387,14 @@ static void __wait_on_freeing_inode(struct inode *inode, bool is_inode_hash_lock
 		return;
 	}
 
-	wq = bit_waitqueue(&inode->i_state, __I_NEW);
-	prepare_to_wait(wq, &wait.wq_entry, TASK_UNINTERRUPTIBLE);
+	wq_head = inode_bit_waitqueue(&wqe, inode, __I_NEW);
+	prepare_to_wait_event(wq_head, &wqe.wq_entry, TASK_UNINTERRUPTIBLE);
 	spin_unlock(&inode->i_lock);
 	rcu_read_unlock();
 	if (is_inode_hash_locked)
 		spin_unlock(&inode_hash_lock);
 	schedule();
-	finish_wait(wq, &wait.wq_entry);
+	finish_wait(wq_head, &wqe.wq_entry);
 	if (is_inode_hash_locked)
 		spin_lock(&inode_hash_lock);
 	rcu_read_lock();
diff --git a/include/linux/fs.h b/include/linux/fs.h
index cda1f2545ea0..a1ef8c65d828 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -681,7 +681,8 @@ struct inode {
 #endif
 
 	/* Misc */
-	unsigned long		i_state;
+	u32			i_state;
+	/* 32-bit hole */
 	struct rw_semaphore	i_rwsem;
 
 	unsigned long		dirtied_when;	/* jiffies of first dirtying */
@@ -744,6 +745,21 @@ 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)
@@ -2395,28 +2411,32 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
  *			i_count.
  *
  * Q: What is the difference between I_WILL_FREE and I_FREEING?
+ *
+ * __I_{SYNC,NEW,LRU_ISOLATING} are used to derive unique addresses to wait
+ * upon. There's one free address left.
  */
-#define I_DIRTY_SYNC		(1 << 0)
-#define I_DIRTY_DATASYNC	(1 << 1)
-#define I_DIRTY_PAGES		(1 << 2)
-#define __I_NEW			3
+#define __I_NEW			0
 #define I_NEW			(1 << __I_NEW)
-#define I_WILL_FREE		(1 << 4)
-#define I_FREEING		(1 << 5)
-#define I_CLEAR			(1 << 6)
-#define __I_SYNC		7
+#define __I_SYNC		1
 #define I_SYNC			(1 << __I_SYNC)
-#define I_REFERENCED		(1 << 8)
+#define __I_LRU_ISOLATING	2
+#define I_LRU_ISOLATING		(1 << __I_LRU_ISOLATING)
+
+#define I_DIRTY_SYNC		(1 << 3)
+#define I_DIRTY_DATASYNC	(1 << 4)
+#define I_DIRTY_PAGES		(1 << 5)
+#define I_WILL_FREE		(1 << 6)
+#define I_FREEING		(1 << 7)
+#define I_CLEAR			(1 << 8)
+#define I_REFERENCED		(1 << 9)
 #define I_LINKABLE		(1 << 10)
 #define I_DIRTY_TIME		(1 << 11)
-#define I_WB_SWITCH		(1 << 13)
-#define I_OVL_INUSE		(1 << 14)
-#define I_CREATING		(1 << 15)
-#define I_DONTCACHE		(1 << 16)
-#define I_SYNC_QUEUED		(1 << 17)
-#define I_PINNING_NETFS_WB	(1 << 18)
-#define __I_LRU_ISOLATING	19
-#define I_LRU_ISOLATING		(1 << __I_LRU_ISOLATING)
+#define I_WB_SWITCH		(1 << 12)
+#define I_OVL_INUSE		(1 << 13)
+#define I_CREATING		(1 << 14)
+#define I_DONTCACHE		(1 << 15)
+#define I_SYNC_QUEUED		(1 << 16)
+#define I_PINNING_NETFS_WB	(1 << 17)
 
 #define I_DIRTY_INODE (I_DIRTY_SYNC | I_DIRTY_DATASYNC)
 #define I_DIRTY (I_DIRTY_INODE | I_DIRTY_PAGES)
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 56b85841ae4c..8f651bb0a1a5 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -200,7 +200,8 @@ void inode_io_list_del(struct inode *inode);
 /* writeback.h requires fs.h; it, too, is not included from here. */
 static inline void wait_on_inode(struct inode *inode)
 {
-	wait_on_bit(&inode->i_state, __I_NEW, TASK_UNINTERRUPTIBLE);
+	wait_var_event(inode_state_wait_address(inode, __I_NEW),
+		       !(READ_ONCE(inode->i_state) & I_NEW));
 }
 
 #ifdef CONFIG_CGROUP_WRITEBACK