Merge pull request #61 from jason50123/external-journal

Implement mounting/unmounting for external journal device

Makefile

@@ -10,6 +10,9 @@ all: $(MKFS)
 
 IMAGE ?= test.img
 IMAGESIZE ?= 200
+JOURNAL ?= journal.img
+JOURNALSIZE ?= 8
+
 # To test max files(40920) in directory, the image size should be at least 159.85 MiB
 # 40920 * 4096(block size) ~= 159.85 MiB
 
@@ -20,12 +23,18 @@ $(IMAGE): $(MKFS)
 	dd if=/dev/zero of=${IMAGE} bs=1M count=${IMAGESIZE}
 	./$< $(IMAGE)
 
+journal: $(JOURNAL)
+
+$(JOURNAL):
+	dd if=/dev/zero of=$(JOURNAL) bs=1M count=$(JOURNALSIZE)
+	mke2fs -b 4096 -O journal_dev $(JOURNAL)
+
 check: all
 	script/test.sh $(IMAGE) $(IMAGESIZE) $(MKFS)
 
 clean:
 	make -C $(KDIR) M=$(PWD) clean
 	rm -f *~ $(PWD)/*.ur-safe
-	rm -f $(MKFS) $(IMAGE)
+	rm -f $(MKFS) $(IMAGE) $(JOURNAL)
 
-.PHONY: all clean
+.PHONY: all clean journal

README.md

@@ -172,9 +172,85 @@ struct simplefs_extent
                                  +---------+
 ```
 
-## TODO
+### journalling support
 
-- journalling support
+Simplefs now includes support for an external journal device, leveraging the journaling block device (jbd2) subsystem in the Linux kernel. This enhancement improves the file system's resilience by maintaining a log of changes, which helps prevent corruption and facilitates recovery in the event of a crash or power failure.
+
+
+The journaling support in simplefs is implemented using the jbd2 subsystem, which is a widely-used journaling layer in Linux. Currently, simplefs primarily stores the journal-related information in an external journal device.
+
+For a detailed introduction to journaling, please refer to these two websites:
+[Journal(jbd2) document](https://www.kernel.org/doc/html/latest/filesystems/ext4/journal.html)
+[Journal(jbd2) api](https://docs.kernel.org/filesystems/journalling.html) 
+
+External journal device disk layout:
+
++--------------------+------------------+---------------------------+--------------+
+| Journal Superblock | Descriptor Block | Metadata/Data ( modified ) | Commit Block |
++--------------------+------------------+---------------------------+--------------+
+
+Hint:
+Each transaction starts with a descriptor block, followed by several metadata blocks or data blocks, and ends with a commit block. Every modified metadata (such as inode, bitmap, etc.) occupies its own block. Currently, simplefs primarily records "extent" metadata.
+
+
+How to Enable Journaling in simplefs: 
+
+Step 1: Create the Journal Disk Image
+To create an 8MB disk image for the journal, use the following make command:
+
+Note: 
+Assuming an 8 MB size for the external journal device, which is an arbitrary choice for now, I will set the journal block length to a fixed 2048, calculated by dividing the device size by the block size (4096 bytes).
+
+```shell
+$ make journal
+```
+
+Step 2:  Make sure you've loaded the SimpleFS Kernel Module
+
+```shell
+$ insmod simplefs/simplefs.ko
+```
+
+Step 3: Setup the Loop Device for the Journal
+Find an available loop device and associate it with the journal image:
+
+``` shell
+$ loop_device=$(losetup -f)
+$ losetup $loop_device /simplefs/journal.img
+```
+
+You shall get the following kernel messages:
+```
+loop0: detected capacity change from 0 to 16384
+```
+
+Step 4: Mount the SimpleFS File System with the External Journal
+Mount the SimpleFS file system along with the external journal device using the following command:
+
+```shell
+mount -o loop,rw,owner,group,users,journal_path="$loop_device" -t simplefs /simplefs/test.img /test
+```
+
+Corresponding kernel message:
+```
+loop1: detected capacity change from 0 to 409600
+simplefs: simplefs_parse_options: parsing options 'owner,group,journal_path=/dev/loop0'
+simplefs: '/dev/loop1' mount success
+```
+
+Current Limitations and Known Issues
+
+1. External Journal Device Size:
+
+- The exact size of the external journal device cannot be determined. As a temporary solution, the size is set by dividing the device size by the block size, with the external journal device size fixed at 8 MB.
+
+2. Metadata Recording:
+
+- At present, only "extent" metadata is recorded. In the future, additional metadata such as "super block" and inode metadata can be included.
+
+3. Implementation of External Journal Device:
+
+- Only the external journal device is implemented. Future improvements can include the use of an internal journal (inode journal). However, this will require the addition of a bmap function and appropriate adjustments to the disk partition during mkfs.
 
 ## License
 

simplefs.h

@@ -39,6 +39,9 @@
  * |      blocks   |  rest of the blocks
  * +---------------+
  */
+#ifdef __KERNEL__
+#include <linux/jbd2.h>
+#endif
 
 struct simplefs_inode {
     uint32_t i_mode;   /* File mode */
@@ -71,6 +74,10 @@ struct simplefs_sb_info {
     uint32_t nr_free_blocks; /* Number of free blocks */
 
 #ifdef __KERNEL__
+    journal_t *journal;
+    struct block_device *s_journal_bdev; /* v5.10+ external journal device */
+    struct bdev_handle
+        *s_journal_bdev_handle;  /* v6.7+ external journal device */
     unsigned long *ifree_bitmap; /* In-memory free inodes bitmap */
     unsigned long *bfree_bitmap; /* In-memory free blocks bitmap */
 #endif