ext4: fix a potential assertion failure due to improperly dirtied buffer

On an old kernel version(4.19, ext3, data=journal, pagesize=64k),
an assertion failure will occasionally be triggered by the line below:
-----------
jbd2_journal_commit_transaction
{
...
J_ASSERT_BH(bh, !buffer_dirty(bh));
/*
* The buffer on BJ_Forget list and not jbddirty means
...
}
-----------

The same condition may also be applied to the lattest kernel version.

When blocksize < pagesize and we truncate a file, there can be buffers in
the mapping tail page beyond i_size. These buffers will be filed to
transaction's BJ_Forget list by ext4_journalled_invalidatepage() during
truncation. When the transaction doing truncate starts committing, we can
grow the file again. This calls __block_write_begin() which allocates new
blocks under these buffers in the tail page we go through the branch:

                        if (buffer_new(bh)) {
                                clean_bdev_bh_alias(bh);
                                if (folio_test_uptodate(folio)) {
                                        clear_buffer_new(bh);
                                        set_buffer_uptodate(bh);
                                        mark_buffer_dirty(bh);
                                        continue;
                                }
                                ...
                        }

Hence buffers on BJ_Forget list of the committing transaction get marked
dirty and this triggers the jbd2 assertion.

Teach ext4_block_write_begin() to properly handle files with data
journalling by avoiding dirtying them directly. Instead of
folio_zero_new_buffers() we use ext4_journalled_zero_new_buffers() which
takes care of handling journalling. We also don't need to mark new uptodate
buffers as dirty in ext4_block_write_begin(). That will be either done
either by block_commit_write() in case of success or by
folio_zero_new_buffers() in case of failure.

Reported-by: Baolin Liu <liubaolin@kylinos.cn>
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Shida Zhang <zhangshida@kylinos.cn>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240830053739.3588573-4-zhangshida@kylinos.cn
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: Shida Zhang

fs/ext4/ext4.h

@@ -3853,7 +3853,8 @@ static inline int ext4_buffer_uptodate(struct buffer_head *bh)
 	return buffer_uptodate(bh);
 }
 
-extern int ext4_block_write_begin(struct folio *folio, loff_t pos, unsigned len,
+extern int ext4_block_write_begin(handle_t *handle, struct folio *folio,
+				  loff_t pos, unsigned len,
 				  get_block_t *get_block);
 #endif	/* __KERNEL__ */
 

fs/ext4/inline.c

@@ -601,10 +601,11 @@ static int ext4_convert_inline_data_to_extent(struct address_space *mapping,
 		goto out;
 
 	if (ext4_should_dioread_nolock(inode)) {
-		ret = ext4_block_write_begin(folio, from, to,
+		ret = ext4_block_write_begin(handle, folio, from, to,
 					     ext4_get_block_unwritten);
 	} else
-		ret = ext4_block_write_begin(folio, from, to, ext4_get_block);
+		ret = ext4_block_write_begin(handle, folio, from, to,
+					     ext4_get_block);
 
 	if (!ret && ext4_should_journal_data(inode)) {
 		ret = ext4_walk_page_buffers(handle, inode,
@@ -856,7 +857,7 @@ static int ext4_da_convert_inline_data_to_extent(struct address_space *mapping,
 			goto out;
 	}
 
-	ret = ext4_block_write_begin(folio, 0, inline_size,
+	ret = ext4_block_write_begin(NULL, folio, 0, inline_size,
 				     ext4_da_get_block_prep);
 	if (ret) {
 		up_read(&EXT4_I(inode)->xattr_sem);

fs/ext4/inode.c

@@ -49,6 +49,11 @@
 
 #include <trace/events/ext4.h>
 
+static void ext4_journalled_zero_new_buffers(handle_t *handle,
+					    struct inode *inode,
+					    struct folio *folio,
+					    unsigned from, unsigned to);
+
 static __u32 ext4_inode_csum(struct inode *inode, struct ext4_inode *raw,
 			      struct ext4_inode_info *ei)
 {
@@ -1023,7 +1028,8 @@ int do_journal_get_write_access(handle_t *handle, struct inode *inode,
 	return ret;
 }
 
-int ext4_block_write_begin(struct folio *folio, loff_t pos, unsigned len,
+int ext4_block_write_begin(handle_t *handle, struct folio *folio,
+			   loff_t pos, unsigned len,
 			   get_block_t *get_block)
 {
 	unsigned from = pos & (PAGE_SIZE - 1);
@@ -1037,6 +1043,7 @@ int ext4_block_write_begin(struct folio *folio, loff_t pos, unsigned len,
 	struct buffer_head *bh, *head, *wait[2];
 	int nr_wait = 0;
 	int i;
+	bool should_journal_data = ext4_should_journal_data(inode);
 
 	BUG_ON(!folio_test_locked(folio));
 	BUG_ON(from > PAGE_SIZE);
@@ -1066,10 +1073,22 @@ int ext4_block_write_begin(struct folio *folio, loff_t pos, unsigned len,
 			if (err)
 				break;
 			if (buffer_new(bh)) {
+				/*
+				 * We may be zeroing partial buffers or all new
+				 * buffers in case of failure. Prepare JBD2 for
+				 * that.
+				 */
+				if (should_journal_data)
+					do_journal_get_write_access(handle,
+								    inode, bh);
 				if (folio_test_uptodate(folio)) {
-					clear_buffer_new(bh);
+					/*
+					 * Unlike __block_write_begin() we leave
+					 * dirtying of new uptodate buffers to
+					 * ->write_end() time or
+					 * folio_zero_new_buffers().
+					 */
 					set_buffer_uptodate(bh);
-					mark_buffer_dirty(bh);
 					continue;
 				}
 				if (block_end > to || block_start < from)
@@ -1099,7 +1118,11 @@ int ext4_block_write_begin(struct folio *folio, loff_t pos, unsigned len,
 			err = -EIO;
 	}
 	if (unlikely(err)) {
-		folio_zero_new_buffers(folio, from, to);
+		if (should_journal_data)
+			ext4_journalled_zero_new_buffers(handle, inode, folio,
+							 from, to);
+		else
+			folio_zero_new_buffers(folio, from, to);
 	} else if (fscrypt_inode_uses_fs_layer_crypto(inode)) {
 		for (i = 0; i < nr_wait; i++) {
 			int err2;
@@ -1197,10 +1220,11 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
 	folio_wait_stable(folio);
 
 	if (ext4_should_dioread_nolock(inode))
-		ret = ext4_block_write_begin(folio, pos, len,
+		ret = ext4_block_write_begin(handle, folio, pos, len,
 					     ext4_get_block_unwritten);
 	else
-		ret = ext4_block_write_begin(folio, pos, len, ext4_get_block);
+		ret = ext4_block_write_begin(handle, folio, pos, len,
+					     ext4_get_block);
 	if (!ret && ext4_should_journal_data(inode)) {
 		ret = ext4_walk_page_buffers(handle, inode,
 					     folio_buffers(folio), from, to,
@@ -2926,7 +2950,8 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
 
-	ret = ext4_block_write_begin(folio, pos, len, ext4_da_get_block_prep);
+	ret = ext4_block_write_begin(NULL, folio, pos, len,
+				     ext4_da_get_block_prep);
 	if (ret < 0) {
 		folio_unlock(folio);
 		folio_put(folio);
@@ -6183,7 +6208,8 @@ vm_fault_t ext4_page_mkwrite(struct vm_fault *vmf)
 		if (folio_pos(folio) + len > size)
 			len = size - folio_pos(folio);
 
-		err = __block_write_begin(&folio->page, 0, len, ext4_get_block);
+		err = ext4_block_write_begin(handle, folio, 0, len,
+					     ext4_get_block);
 		if (!err) {
 			ret = VM_FAULT_SIGBUS;
 			if (ext4_journal_folio_buffers(handle, folio, len))