[PATCH] f2fs: compress: fix to handle race between truncate and writeback

[Chao Yu <chao@kernel.org>]

From: Chao Yu <chao@kernel.org>

fsstress reports a kernel BUG in f2fs_truncate_partial_cluster():

kernel BUG at fs/f2fs/compress.c:1238!
RIP: 0010:f2fs_truncate_partial_cluster+0x292/0x2a0
Call Trace:
 <TASK>
 f2fs_truncate+0xf6/0x210
 f2fs_setattr+0x6b7/0x770
 notify_change+0x33b/0x520
 do_truncate+0xc2/0xf0
 vfs_truncate+0x153/0x1d0
 ksys_truncate+0x78/0xd0
 __x64_sys_truncate+0x16/0x20
 do_syscall_64+0xbe/0x540

The root cause is that Thread A (truncate) and Thread B (background
writeback or fsync) can race as follows:

Thread A                             Thread B
- f2fs_setattr
 - f2fs_truncate
  - f2fs_truncate_blocks
   - f2fs_truncate_partial_cluster
    - f2fs_is_compressed_cluster
      return 1
                                     - f2fs_write_cache_pages
                                      - f2fs_write_multi_pages
                                       - f2fs_write_raw_pages
                                        - f2fs_write_single_data_page
                                          dn.data_blkaddr != COMPRESS_ADDR
                                          (cluster converted to normal)
    - f2fs_prepare_compress_overwrite
     - f2fs_is_compressed_cluster
       return 0
     - return 0
    - f2fs_bug_on(sbi, err == 0): BUG!

Writeback path does not acquire i_gc_rwsem or filemap_invalidate_lock.
When a compressed cluster fails compression during writeback, it is
overwritten with raw data blocks. If Thread A checked
f2fs_is_compressed_cluster() before the conversion, but calls
f2fs_prepare_compress_overwrite() after the conversion,
f2fs_prepare_compress_overwrite() returns 0 because the cluster is no
longer a compressed cluster.

To fix this, remove the f2fs_bug_on() and retry checking the cluster status
when f2fs_prepare_compress_overwrite() returns 0, so that it can fall back
to f2fs_do_truncate_blocks() to handle it as a normal cluster.

Cc: stable@kernel.org
Fixes: 3265d3db1f16 ("f2fs: support partial truncation on compressed inode")
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/compress.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/f2fs/compress.c b/fs/f2fs/compress.c
index ce88092d9ce2..e1cc7185428e 100644
--- a/fs/f2fs/compress.c
+++ b/fs/f2fs/compress.c
@@ -1222,6 +1222,7 @@ int f2fs_truncate_partial_cluster(struct inode *inode, u64 from, bool lock)
 	int i;
 	int err;
 
+repeat:
 	err = f2fs_is_compressed_cluster(inode, start_idx);
 	if (err < 0)
 		return err;
@@ -1233,11 +1234,10 @@ int f2fs_truncate_partial_cluster(struct inode *inode, u64 from, bool lock)
 	/* truncate compressed cluster */
 	err = f2fs_prepare_compress_overwrite(inode, &pagep,
 						start_idx, &fsdata);
+	if (err == 0)
+		goto repeat;
 
-	/* should not be a normal cluster */
-	f2fs_bug_on(F2FS_I_SB(inode), err == 0);
-
-	if (err <= 0)
+	if (err < 0)
 		return err;
 
 	rpages = fsdata;
-- 
2.49.0