f2fs: fix to avoid updating compression context during writeback

Bai, Shuangpeng <sjb7183@psu.edu> reported a bug as below:

Oops: divide error: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 11441 Comm: syz.0.46 Not tainted 6.17.0 #1 PREEMPT(full)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
RIP: 0010:f2fs_all_cluster_page_ready+0x106/0x550 fs/f2fs/compress.c:857
Call Trace:
 <TASK>
 f2fs_write_cache_pages fs/f2fs/data.c:3078 [inline]
 __f2fs_write_data_pages fs/f2fs/data.c:3290 [inline]
 f2fs_write_data_pages+0x1c19/0x3600 fs/f2fs/data.c:3317
 do_writepages+0x38e/0x640 mm/page-writeback.c:2634
 filemap_fdatawrite_wbc mm/filemap.c:386 [inline]
 __filemap_fdatawrite_range mm/filemap.c:419 [inline]
 file_write_and_wait_range+0x2ba/0x3e0 mm/filemap.c:794
 f2fs_do_sync_file+0x6e6/0x1b00 fs/f2fs/file.c:294
 generic_write_sync include/linux/fs.h:3043 [inline]
 f2fs_file_write_iter+0x76e/0x2700 fs/f2fs/file.c:5259
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x7e9/0xe00 fs/read_write.c:686
 ksys_write+0x19d/0x2d0 fs/read_write.c:738
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xf7/0x470 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The bug was triggered w/ below race condition:

fsync				setattr			ioctl
- f2fs_do_sync_file
 - file_write_and_wait_range
  - f2fs_write_cache_pages
  : inode is non-compressed
  : cc.cluster_size =
    F2FS_I(inode)->i_cluster_size = 0
   - tag_pages_for_writeback
				- f2fs_setattr
				 - truncate_setsize
				 - f2fs_truncate
							- f2fs_fileattr_set
							 - f2fs_setflags_common
							  - set_compress_context
							  : F2FS_I(inode)->i_cluster_size = 4
							  : set_inode_flag(inode, FI_COMPRESSED_FILE)
   - f2fs_compressed_file
   : return true
   - f2fs_all_cluster_page_ready
   : "pgidx % cc->cluster_size" trigger dividing 0 issue

Let's change as below to fix this issue:
- introduce a new atomic type variable .writeback in structure f2fs_inode_info
to track the number of threads which calling f2fs_write_cache_pages().
- use .i_sem lock to protect .writeback update.
- check .writeback before update compression context in f2fs_setflags_common()
to avoid race w/ ->writepages.

Fixes: 4c8ff7095b ("f2fs: support data compression")
Cc: stable@kernel.org
Reported-by: Bai, Shuangpeng <sjb7183@psu.edu>
Tested-by: Bai, Shuangpeng <sjb7183@psu.edu>
Closes: https://lore.kernel.org/lkml/44D8F7B3-68AD-425F-9915-65D27591F93F@psu.edu
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -3222,6 +3222,19 @@ static inline bool __should_serialize_io(struct inode *inode,
 	return false;
 }
 
+static inline void account_writeback(struct inode *inode, bool inc)
+{
+	if (!f2fs_sb_has_compression(F2FS_I_SB(inode)))
+		return;
+
+	f2fs_down_read(&F2FS_I(inode)->i_sem);
+	if (inc)
+		atomic_inc(&F2FS_I(inode)->writeback);
+	else
+		atomic_dec(&F2FS_I(inode)->writeback);
+	f2fs_up_read(&F2FS_I(inode)->i_sem);
+}
+
 static int __f2fs_write_data_pages(struct address_space *mapping,
 						struct writeback_control *wbc,
 						enum iostat_type io_type)
@@ -3267,10 +3280,14 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
 		locked = true;
 	}
 
+	account_writeback(inode, true);
+
 	blk_start_plug(&plug);
 	ret = f2fs_write_cache_pages(mapping, wbc, io_type);
 	blk_finish_plug(&plug);
 
+	account_writeback(inode, false);
+
 	if (locked)
 		mutex_unlock(&sbi->writepages);
 

fs/f2fs/f2fs.h

@@ -947,6 +947,7 @@ struct f2fs_inode_info {
 	unsigned char i_compress_level;		/* compress level (lz4hc,zstd) */
 	unsigned char i_compress_flag;		/* compress flag */
 	unsigned int i_cluster_size;		/* cluster size */
+	atomic_t writeback;			/* count # of writeback thread */
 
 	unsigned int atomic_write_cnt;
 	loff_t original_i_size;		/* original i_size before atomic write */
@@ -4663,7 +4664,7 @@ static inline bool f2fs_disable_compressed_file(struct inode *inode)
 		f2fs_up_write(&fi->i_sem);
 		return true;
 	}
-	if (f2fs_is_mmap_file(inode) ||
+	if (f2fs_is_mmap_file(inode) || atomic_read(&fi->writeback) ||
 		(S_ISREG(inode->i_mode) && F2FS_HAS_BLOCKS(inode))) {
 		f2fs_up_write(&fi->i_sem);
 		return false;

fs/f2fs/file.c

@@ -2128,8 +2128,9 @@ static int f2fs_setflags_common(struct inode *inode, u32 iflags, u32 mask)
 
 			f2fs_down_write(&fi->i_sem);
 			if (!f2fs_may_compress(inode) ||
-					(S_ISREG(inode->i_mode) &&
-					F2FS_HAS_BLOCKS(inode))) {
+				atomic_read(&fi->writeback) ||
+				(S_ISREG(inode->i_mode) &&
+				F2FS_HAS_BLOCKS(inode))) {
 				f2fs_up_write(&fi->i_sem);
 				return -EINVAL;
 			}

fs/f2fs/super.c

@@ -1759,6 +1759,7 @@ static struct inode *f2fs_alloc_inode(struct super_block *sb)
 	atomic_set(&fi->dirty_pages, 0);
 	atomic_set(&fi->i_compr_blocks, 0);
 	atomic_set(&fi->open_count, 0);
+	atomic_set(&fi->writeback, 0);
 	init_f2fs_rwsem(&fi->i_sem);
 	spin_lock_init(&fi->i_size_lock);
 	INIT_LIST_HEAD(&fi->dirty_list);