Currently every ext4 inode gets mapping_metadata_bhs struct although it
is only needed when running without a journal and only for inodes where
any metadata was dirtied. Allocate mapping_metadata_bhs struct on demand
when dirtying the first metadata buffer for the inode.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/ext4/ext4.h      |  2 +-
 fs/ext4/ext4_jbd2.c | 24 +++++++++++++++++++++---
 fs/ext4/fsync.c     | 12 ++++++++----
 fs/ext4/inode.c     |  9 +++++----
 fs/ext4/super.c     |  8 +++++---
 5 files changed, 40 insertions(+), 15 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 94283a991e5c..6bb29a20420f 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1117,7 +1117,7 @@ struct ext4_inode_info {
 	struct rw_semaphore i_data_sem;
 	struct inode vfs_inode;
 	struct jbd2_inode *jinode;
-	struct mapping_metadata_bhs i_metadata_bhs;
+	struct mapping_metadata_bhs *i_metadata_bhs;
 
 	/*
 	 * File creation time. Its function is same as that of
diff --git a/fs/ext4/ext4_jbd2.c b/fs/ext4/ext4_jbd2.c
index 9a8c225f2753..74f05bd0cdde 100644
--- a/fs/ext4/ext4_jbd2.c
+++ b/fs/ext4/ext4_jbd2.c
@@ -350,6 +350,21 @@ int __ext4_journal_get_create_access(const char *where, unsigned int line,
 	return 0;
 }
 
+static void ext4_inode_attach_mmb(struct inode *inode)
+{
+	struct mapping_metadata_bhs *mmb;
+
+	/*
+	 * It's difficult to handle failure when marking buffer dirty without
+	 * leaving filesystem corrupyted
+	 */
+	mmb = kmalloc_obj(*mmb, GFP_KERNEL | __GFP_NOFAIL);
+	mmb_init(mmb, inode->i_mapping);
+	/* Someone swapped another mmb before us? */
+	if (cmpxchg(&EXT4_I(inode)->i_metadata_bhs, NULL, mmb))
+		kfree(mmb);
+}
+
 int __ext4_handle_dirty_metadata(const char *where, unsigned int line,
 				 handle_t *handle, struct inode *inode,
 				 struct buffer_head *bh)
@@ -389,11 +404,14 @@ int __ext4_handle_dirty_metadata(const char *where, unsigned int line,
 					 err);
 		}
 	} else {
-		if (inode)
+		if (inode) {
+			if (!EXT4_I(inode)->i_metadata_bhs)
+				ext4_inode_attach_mmb(inode);
 			mmb_mark_buffer_dirty(bh,
-					      &EXT4_I(inode)->i_metadata_bhs);
-		else
+					      EXT4_I(inode)->i_metadata_bhs);
+		} else {
 			mark_buffer_dirty(bh);
+		}
 		if (inode && inode_needs_sync(inode)) {
 			sync_dirty_buffer(bh);
 			if (buffer_req(bh) && !buffer_uptodate(bh)) {
diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
index 924726dcc85f..e25d365e1179 100644
--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -46,6 +46,7 @@
 static int ext4_sync_parent(struct inode *inode)
 {
 	struct dentry *dentry, *next;
+	struct mapping_metadata_bhs *mmb;
 	int ret = 0;
 
 	if (!ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY))
@@ -68,9 +69,12 @@ static int ext4_sync_parent(struct inode *inode)
 		 * through ext4_evict_inode()) and so we are safe to flush
 		 * metadata blocks and the inode.
 		 */
-		ret = mmb_sync(&EXT4_I(inode)->i_metadata_bhs);
-		if (ret)
-			break;
+		mmb = READ_ONCE(EXT4_I(inode)->i_metadata_bhs);
+		if (mmb) {
+			ret = mmb_sync(mmb);
+			if (ret)
+				break;
+		}
 		ret = sync_inode_metadata(inode, 1);
 		if (ret)
 			break;
@@ -89,7 +93,7 @@ static int ext4_fsync_nojournal(struct file *file, loff_t start, loff_t end,
 	};
 	int ret;
 
-	ret = mmb_fsync_noflush(file, &EXT4_I(inode)->i_metadata_bhs,
+	ret = mmb_fsync_noflush(file, EXT4_I(inode)->i_metadata_bhs,
 				start, end, datasync);
 	if (ret)
 		return ret;
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index c2c2d6ac7f3d..3e66e9510909 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -195,9 +195,8 @@ void ext4_evict_inode(struct inode *inode)
 			ext4_warning_inode(inode, "data will be lost");
 
 		truncate_inode_pages_final(&inode->i_data);
-		/* Avoid mballoc special inode which has no proper iops */
-		if (!EXT4_SB(inode->i_sb)->s_journal)
-			mmb_sync(&EXT4_I(inode)->i_metadata_bhs);
+		if (EXT4_I(inode)->i_metadata_bhs)
+			mmb_sync(EXT4_I(inode)->i_metadata_bhs);
 		goto no_delete;
 	}
 
@@ -3451,6 +3450,7 @@ static bool ext4_release_folio(struct folio *folio, gfp_t wait)
 static bool ext4_inode_datasync_dirty(struct inode *inode)
 {
 	journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
+	struct mapping_metadata_bhs *mmb;
 
 	if (journal) {
 		if (jbd2_transaction_committed(journal,
@@ -3461,8 +3461,9 @@ static bool ext4_inode_datasync_dirty(struct inode *inode)
 		return true;
 	}
 
+	mmb = READ_ONCE(EXT4_I(inode)->i_metadata_bhs);
 	/* Any metadata buffers to write? */
-	if (mmb_has_buffers(&EXT4_I(inode)->i_metadata_bhs))
+	if (mmb && mmb_has_buffers(mmb))
 		return true;
 	return inode_state_read_once(inode) & I_DIRTY_DATASYNC;
 }
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 6a77db4d3124..92134ea4620c 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1430,7 +1430,7 @@ static struct inode *ext4_alloc_inode(struct super_block *sb)
 	INIT_WORK(&ei->i_rsv_conversion_work, ext4_end_io_rsv_work);
 	ext4_fc_init_inode(&ei->vfs_inode);
 	spin_lock_init(&ei->i_fc_lock);
-	mmb_init(&ei->i_metadata_bhs, &ei->vfs_inode.i_data);
+	ei->i_metadata_bhs = NULL;
 	return &ei->vfs_inode;
 }
 
@@ -1527,8 +1527,10 @@ static void destroy_inodecache(void)
 void ext4_clear_inode(struct inode *inode)
 {
 	ext4_fc_del(inode);
-	if (!EXT4_SB(inode->i_sb)->s_journal)
-		mmb_invalidate(&EXT4_I(inode)->i_metadata_bhs);
+	if (EXT4_I(inode)->i_metadata_bhs) {
+		mmb_invalidate(EXT4_I(inode)->i_metadata_bhs);
+		kfree(EXT4_I(inode)->i_metadata_bhs);
+	}
 	clear_inode(inode);
 	ext4_discard_preallocations(inode);
 	/*
-- 
2.51.0