Merge branch 'rk/inode_lock' into dev

These are ilock patches which helps improve the current inode lock scalabiliy
problem in ext4 DIO mixed read/write workload case. The problem was first
reported by Joseph [1]. This should help improve mixed read/write workload
cases for databases which use directIO.

These patches are based upon upstream discussion with Jan Kara & Joseph [2].

The problem really is that in case of DIO overwrites, we start with
a exclusive lock and then downgrade it later to shared lock. This causes a
scalability problem in case of mixed DIO read/write workload case.
i.e. if we have any ongoing DIO reads and then comes a DIO writes,
(since writes starts with excl. inode lock) then it has to wait until the
shared lock is released (which only happens when DIO read is completed).
Same is true for vice versa as well.
The same can be easily observed with perf-tools trace analysis [3].

For more details, including performance numbers, please see [4].

[1] https://lore.kernel.org/linux-ext4/1566871552-60946-4-git-send-email-joseph.qi@linux.alibaba.com/
[2] https://lore.kernel.org/linux-ext4/20190910215720.GA7561@quack2.suse.cz/
[3] https://raw.githubusercontent.com/riteshharjani/LinuxStudy/master/ext4/perf.report
[4] https://lore.kernel.org/r/20191212055557.11151-1-riteshh@linux.ibm.com

fs/ext4/file.c

@@ -88,9 +88,10 @@ static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	struct inode *inode = file_inode(iocb->ki_filp);
 	ssize_t ret;
 
-	if (!inode_trylock_shared(inode)) {
-		if (iocb->ki_flags & IOCB_NOWAIT)
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		if (!inode_trylock_shared(inode))
 			return -EAGAIN;
+	} else {
 		inode_lock_shared(inode);
 	}
 	/*
@@ -165,19 +166,25 @@ static int ext4_release_file(struct inode *inode, struct file *filp)
  * threads are at work on the same unwritten block, they must be synchronized
  * or one thread will zero the other's data, causing corruption.
  */
-static int
-ext4_unaligned_aio(struct inode *inode, struct iov_iter *from, loff_t pos)
+static bool
+ext4_unaligned_io(struct inode *inode, struct iov_iter *from, loff_t pos)
 {
 	struct super_block *sb = inode->i_sb;
-	int blockmask = sb->s_blocksize - 1;
-
-	if (pos >= ALIGN(i_size_read(inode), sb->s_blocksize))
-		return 0;
+	unsigned long blockmask = sb->s_blocksize - 1;
 
 	if ((pos | iov_iter_alignment(from)) & blockmask)
-		return 1;
+		return true;
 
-	return 0;
+	return false;
+}
+
+static bool
+ext4_extending_io(struct inode *inode, loff_t offset, size_t len)
+{
+	if (offset + len > i_size_read(inode) ||
+	    offset + len > EXT4_I(inode)->i_disksize)
+		return true;
+	return false;
 }
 
 /* Is IO overwriting allocated and initialized blocks? */
@@ -203,7 +210,8 @@ static bool ext4_overwrite_io(struct inode *inode, loff_t pos, loff_t len)
 	return err == blklen && (map.m_flags & EXT4_MAP_MAPPED);
 }
 
-static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
+static ssize_t ext4_generic_write_checks(struct kiocb *iocb,
+					 struct iov_iter *from)
 {
 	struct inode *inode = file_inode(iocb->ki_filp);
 	ssize_t ret;
@@ -227,11 +235,21 @@ static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
 		iov_iter_truncate(from, sbi->s_bitmap_maxbytes - iocb->ki_pos);
 	}
 
+	return iov_iter_count(from);
+}
+
+static ssize_t ext4_write_checks(struct kiocb *iocb, struct iov_iter *from)
+{
+	ssize_t ret, count;
+
+	count = ext4_generic_write_checks(iocb, from);
+	if (count <= 0)
+		return count;
+
 	ret = file_modified(iocb->ki_filp);
 	if (ret)
 		return ret;
-
-	return iov_iter_count(from);
+	return count;
 }
 
 static ssize_t ext4_buffered_write_iter(struct kiocb *iocb,
@@ -363,62 +381,136 @@ static const struct iomap_dio_ops ext4_dio_write_ops = {
 	.end_io = ext4_dio_write_end_io,
 };
 
+/*
+ * The intention here is to start with shared lock acquired then see if any
+ * condition requires an exclusive inode lock. If yes, then we restart the
+ * whole operation by releasing the shared lock and acquiring exclusive lock.
+ *
+ * - For unaligned_io we never take shared lock as it may cause data corruption
+ *   when two unaligned IO tries to modify the same block e.g. while zeroing.
+ *
+ * - For extending writes case we don't take the shared lock, since it requires
+ *   updating inode i_disksize and/or orphan handling with exclusive lock.
+ *
+ * - shared locking will only be true mostly with overwrites. Otherwise we will
+ *   switch to exclusive i_rwsem lock.
+ */
+static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct iov_iter *from,
+				     bool *ilock_shared, bool *extend)
+{
+	struct file *file = iocb->ki_filp;
+	struct inode *inode = file_inode(file);
+	loff_t offset;
+	size_t count;
+	ssize_t ret;
+
+restart:
+	ret = ext4_generic_write_checks(iocb, from);
+	if (ret <= 0)
+		goto out;
+
+	offset = iocb->ki_pos;
+	count = ret;
+	if (ext4_extending_io(inode, offset, count))
+		*extend = true;
+	/*
+	 * Determine whether the IO operation will overwrite allocated
+	 * and initialized blocks.
+	 * We need exclusive i_rwsem for changing security info
+	 * in file_modified().
+	 */
+	if (*ilock_shared && (!IS_NOSEC(inode) || *extend ||
+	     !ext4_overwrite_io(inode, offset, count))) {
+		inode_unlock_shared(inode);
+		*ilock_shared = false;
+		inode_lock(inode);
+		goto restart;
+	}
+
+	ret = file_modified(file);
+	if (ret < 0)
+		goto out;
+
+	return count;
+out:
+	if (*ilock_shared)
+		inode_unlock_shared(inode);
+	else
+		inode_unlock(inode);
+	return ret;
+}
+
 static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 {
 	ssize_t ret;
-	size_t count;
-	loff_t offset;
 	handle_t *handle;
 	struct inode *inode = file_inode(iocb->ki_filp);
-	bool extend = false, overwrite = false, unaligned_aio = false;
+	loff_t offset = iocb->ki_pos;
+	size_t count = iov_iter_count(from);
+	bool extend = false, unaligned_io = false;
+	bool ilock_shared = true;
+
+	/*
+	 * We initially start with shared inode lock unless it is
+	 * unaligned IO which needs exclusive lock anyways.
+	 */
+	if (ext4_unaligned_io(inode, from, offset)) {
+		unaligned_io = true;
+		ilock_shared = false;
+	}
+	/*
+	 * Quick check here without any i_rwsem lock to see if it is extending
+	 * IO. A more reliable check is done in ext4_dio_write_checks() with
+	 * proper locking in place.
+	 */
+	if (offset + count > i_size_read(inode))
+		ilock_shared = false;
 
 	if (iocb->ki_flags & IOCB_NOWAIT) {
-		if (!inode_trylock(inode))
-			return -EAGAIN;
+		if (ilock_shared) {
+			if (!inode_trylock_shared(inode))
+				return -EAGAIN;
+		} else {
+			if (!inode_trylock(inode))
+				return -EAGAIN;
+		}
 	} else {
-		inode_lock(inode);
+		if (ilock_shared)
+			inode_lock_shared(inode);
+		else
+			inode_lock(inode);
 	}
 
+	/* Fallback to buffered I/O if the inode does not support direct I/O. */
 	if (!ext4_dio_supported(inode)) {
-		inode_unlock(inode);
-		/*
-		 * Fallback to buffered I/O if the inode does not support
-		 * direct I/O.
-		 */
+		if (ilock_shared)
+			inode_unlock_shared(inode);
+		else
+			inode_unlock(inode);
 		return ext4_buffered_write_iter(iocb, from);
 	}
 
-	ret = ext4_write_checks(iocb, from);
-	if (ret <= 0) {
-		inode_unlock(inode);
+	ret = ext4_dio_write_checks(iocb, from, &ilock_shared, &extend);
+	if (ret <= 0)
 		return ret;
-	}
 
-	/*
-	 * Unaligned asynchronous direct I/O must be serialized among each
-	 * other as the zeroing of partial blocks of two competing unaligned
-	 * asynchronous direct I/O writes can result in data corruption.
-	 */
 	offset = iocb->ki_pos;
-	count = iov_iter_count(from);
-	if (ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS) &&
-	    !is_sync_kiocb(iocb) && ext4_unaligned_aio(inode, from, offset)) {
-		unaligned_aio = true;
-		inode_dio_wait(inode);
-	}
+	count = ret;
 
 	/*
-	 * Determine whether the I/O will overwrite allocated and initialized
-	 * blocks. If so, check to see whether it is possible to take the
-	 * dioread_nolock path.
+	 * Unaligned direct IO must be serialized among each other as zeroing
+	 * of partial blocks of two competing unaligned IOs can result in data
+	 * corruption.
+	 *
+	 * So we make sure we don't allow any unaligned IO in flight.
+	 * For IOs where we need not wait (like unaligned non-AIO DIO),
+	 * below inode_dio_wait() may anyway become a no-op, since we start
+	 * with exclusive lock.
 	 */
-	if (!unaligned_aio && ext4_overwrite_io(inode, offset, count) &&
-	    ext4_should_dioread_nolock(inode)) {
-		overwrite = true;
-		downgrade_write(&inode->i_rwsem);
-	}
+	if (unaligned_io)
+		inode_dio_wait(inode);
 
-	if (offset + count > EXT4_I(inode)->i_disksize) {
+	if (extend) {
 		handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
 		if (IS_ERR(handle)) {
 			ret = PTR_ERR(handle);
@@ -431,18 +523,17 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
 			goto out;
 		}
 
-		extend = true;
 		ext4_journal_stop(handle);
 	}
 
 	ret = iomap_dio_rw(iocb, from, &ext4_iomap_ops, &ext4_dio_write_ops,
-			   is_sync_kiocb(iocb) || unaligned_aio || extend);
+			   is_sync_kiocb(iocb) || unaligned_io || extend);
 
 	if (extend)
 		ret = ext4_handle_inode_extension(inode, offset, ret, count);
 
 out:
-	if (overwrite)
+	if (ilock_shared)
 		inode_unlock_shared(inode);
 	else
 		inode_unlock(inode);
@@ -487,9 +578,10 @@ ext4_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	bool extend = false;
 	struct inode *inode = file_inode(iocb->ki_filp);
 
-	if (!inode_trylock(inode)) {
-		if (iocb->ki_flags & IOCB_NOWAIT)
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		if (!inode_trylock(inode))
 			return -EAGAIN;
+	} else {
 		inode_lock(inode);
 	}