[RFC PATCH v2 3/5] ext4: data=journal: write-protect pages on submit inode data buffers callback

[Mauricio Faria de Oliveira <mfo@canonical.com>]

This implements the journal's j_submit_inode_data_buffers() callback
to write-protect the inode's pages with write_cache_pages(), and use
a writepage callback to redirty pages with buffers that are not part
of the committing transaction or the next transaction.

And set a no-op function as j_finish_inode_data_buffers() callback
(nothing needed other than the write-protect above.)

Currently, the inode is added to the transaction's inode list in the
__ext4_journalled_writepage() function.
---
 fs/ext4/inode.c |  4 +++
 fs/ext4/super.c | 65 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 10dd470876b3..978ccde8454f 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1911,6 +1911,10 @@ static int __ext4_journalled_writepage(struct page *page,
 		err = ext4_walk_page_buffers(handle, page_bufs, 0, len, NULL,
 					     write_end_fn);
 	}
+	if (ret == 0)
+		ret = err;
+	// XXX: is this correct for inline data inodes?
+	err = ext4_jbd2_inode_add_write(handle, inode, 0, len);
 	if (ret == 0)
 		ret = err;
 	EXT4_I(inode)->i_datasync_tid = handle->h_transaction->t_tid;
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 330957ed1f05..38aaac6572ea 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -472,6 +472,66 @@ static void ext4_journal_commit_callback(journal_t *journal, transaction_t *txn)
 	spin_unlock(&sbi->s_md_lock);
 }
 
+/*
+ * This writepage callback for write_cache_pages()
+ * takes care of a few cases after page cleaning.
+ *
+ * write_cache_pages() already checks for dirty pages
+ * and calls clear_page_dirty_for_io(), which we want,
+ * to write protect the pages.
+ *
+ * However, we have to redirty a page in two cases:
+ * 1) some buffer is not part of the committing transaction
+ * 2) some buffer already has b_next_transaction set
+ */
+
+static int ext4_journalled_writepage_callback(struct page *page,
+					      struct writeback_control *wbc,
+					      void *data)
+{
+	transaction_t *transaction = (transaction_t *) data;
+	struct buffer_head *bh, *head;
+	struct journal_head *jh;
+
+	// XXX: any chance of !bh here?
+	bh = head = page_buffers(page);
+	do {
+		jh = bh2jh(bh);
+		if (!jh || jh->b_transaction != transaction ||
+		    jh->b_next_transaction) {
+			redirty_page_for_writepage(wbc, page);
+			goto out;
+		}
+	} while ((bh = bh->b_this_page) != head);
+
+out:
+	return AOP_WRITEPAGE_ACTIVATE;
+}
+
+static int ext4_journalled_submit_inode_data_buffers(struct jbd2_inode *jinode)
+{
+	struct address_space *mapping = jinode->i_vfs_inode->i_mapping;
+	transaction_t *transaction = jinode->i_transaction;
+	loff_t dirty_start = jinode->i_dirty_start;
+	loff_t dirty_end = jinode->i_dirty_end;
+
+	struct writeback_control wbc = {
+		.sync_mode =  WB_SYNC_ALL,
+		.nr_to_write = mapping->nrpages * 2,
+		.range_start = dirty_start,
+		.range_end = dirty_end,
+        };
+
+	return write_cache_pages(mapping, &wbc,
+				 ext4_journalled_writepage_callback,
+				 transaction);
+}
+
+static int ext4_journalled_finish_inode_data_buffers(struct jbd2_inode *jinode)
+{
+	return 0;
+}
+
 static bool system_going_down(void)
 {
 	return system_state == SYSTEM_HALT || system_state == SYSTEM_POWER_OFF
@@ -4599,6 +4659,11 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		ext4_msg(sb, KERN_ERR, "can't mount with "
 			"journal_async_commit in data=ordered mode");
 		goto failed_mount_wq;
+	} else if (test_opt(sb, DATA_FLAGS) == EXT4_MOUNT_JOURNAL_DATA) {
+		sbi->s_journal->j_submit_inode_data_buffers =
+			ext4_journalled_submit_inode_data_buffers;
+		sbi->s_journal->j_finish_inode_data_buffers =
+			ext4_journalled_finish_inode_data_buffers;
 	}
 
 	set_task_ioprio(sbi->s_journal->j_task, journal_ioprio);
-- 
2.17.1