[PATCH 6.12 497/567] f2fs: fix to avoid updating compression context during writeback

Greg Kroah-Hartman
2026-01-06T17:43:34Z

From: Chao Yu <chao@kernel.org>

[ Upstream commit 10b591e7fb7cdc8c1e53e9c000dc0ef7069aaa76 ]

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: 4c8ff7095bef ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/f2fs/data.c | 17 +++++++++++++++++
fs/f2fs/f2fs.h | 3 ++-
fs/f2fs/file.c | 5 +++--
fs/f2fs/super.c | 1 +
4 files changed, 23 insertions(+), 3 deletions(-)

--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -3272,6 +3272,19 @@ static inline bool __should_serialize_io
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)
@@ -3321,10 +3334,14 @@ static int __f2fs_write_data_pages(struc
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);

--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -887,6 +887,7 @@ 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 */
@@ -4540,7 +4541,7 @@ static inline bool f2fs_disable_compress
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;
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2071,8 +2071,9 @@ static int f2fs_setflags_common(struct i

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;
}
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1426,6 +1426,7 @@ static struct inode *f2fs_alloc_inode(st
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);