Re: [PATCH v2] f2fs: fix potential deadlock in f2fs_balance_fs()

From: Ruipeng Qi

Date: Tue Apr 28 2026 - 23:39:40 EST

On 2026/4/27 16:38, Chao Yu wrote:

On 4/26/26 17:32, ruipengqi wrote:

From: Ruipeng Qi <ruipengqi3@xxxxxxxxx>

When the f2fs filesystem space is nearly exhausted, we encounter deadlock
issues as below:

INFO: task A:1890 blocked for more than 120 seconds.
       Tainted: G           O       6.12.41-g3fe07ddf05ab #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:A    state:D stack:0     pid:1890 tgid:1626 ppid:1153 flags:0x00000204
Call trace:
__switch_to+0xf4/0x158
__schedule+0x27c/0x908
schedule+0x3c/0x118
io_schedule+0x44/0x68
folio_wait_bit_common+0x174/0x370
folio_wait_bit+0x20/0x38
folio_wait_writeback+0x54/0xc8
truncate_inode_partial_folio+0x70/0x1e0
truncate_inode_pages_range+0x1b0/0x450
truncate_pagecache+0x54/0x88
f2fs_file_write_iter+0x3e8/0xb80
do_iter_readv_writev+0xf0/0x1e0
vfs_writev+0x138/0x2c8
do_writev+0x88/0x130
__arm64_sys_writev+0x28/0x40
invoke_syscall+0x50/0x120
el0_svc_common.constprop.0+0xc8/0xf0
do_el0_svc+0x24/0x38
el0_svc+0x30/0xf8
el0t_64_sync_handler+0x120/0x130
el0t_64_sync+0x190/0x198

INFO: task kworker/u8:11:2680853 blocked for more than 120 seconds.
       Tainted: G           O       6.12.41-g3fe07ddf05ab #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u8:11   state:D stack:0     pid:2680853 tgid:2680853 ppid:2      flags:0x00000208
Workqueue: writeback wb_workfn (flush-254:0)
Call trace:
__switch_to+0xf4/0x158
__schedule+0x27c/0x908
schedule+0x3c/0x118
io_schedule+0x44/0x68
folio_wait_bit_common+0x174/0x370
__filemap_get_folio+0x214/0x348
pagecache_get_page+0x20/0x70
f2fs_get_read_data_page+0x150/0x3e8
f2fs_get_lock_data_page+0x2c/0x160
move_data_page+0x50/0x478
do_garbage_collect+0xd38/0x1528
f2fs_gc+0x240/0x7e0
f2fs_balance_fs+0x1a0/0x208
f2fs_write_single_data_page+0x6e4/0x730 //0xfffffe0d6ca08300
f2fs_write_cache_pages+0x378/0x9b0
f2fs_write_data_pages+0x2e4/0x388
do_writepages+0x8c/0x2c8
__writeback_single_inode+0x4c/0x498
writeback_sb_inodes+0x234/0x4a8
__writeback_inodes_wb+0x58/0x118
wb_writeback+0x2f8/0x3c0
wb_workfn+0x2c4/0x508
process_one_work+0x180/0x408
worker_thread+0x258/0x368
kthread+0x118/0x128
ret_from_fork+0x10/0x200

INFO: task kworker/u8:8:2641297 blocked for more than 120 seconds.
       Tainted: G           O       6.12.41-g3fe07ddf05ab #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u8:8    state:D stack:0     pid:2641297 tgid:2641297 ppid:2      flags:0x00000208
Workqueue: writeback wb_workfn (flush-254:0)
Call trace:
__switch_to+0xf4/0x158
__schedule+0x27c/0x908
rt_mutex_schedule+0x30/0x60
__rt_mutex_slowlock_locked.constprop.0+0x460/0x8a8
rwbase_write_lock+0x24c/0x378
down_write+0x1c/0x30
f2fs_balance_fs+0x184/0x208
f2fs_write_inode+0xf4/0x328
__writeback_single_inode+0x370/0x498
writeback_sb_inodes+0x234/0x4a8
__writeback_inodes_wb+0x58/0x118
wb_writeback+0x2f8/0x3c0
wb_workfn+0x2c4/0x508
process_one_work+0x180/0x408
worker_thread+0x258/0x368
kthread+0x118/0x128
ret_from_fork+0x10/0x20

INFO: task B:1902 blocked for more than 120 seconds.
       Tainted: G           O       6.12.41-g3fe07ddf05ab #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:B     state:D stack:0     pid:1902 tgid:1626 ppid:1153 flags:0x0000020c
Call trace:
__switch_to+0xf4/0x158
__schedule+0x27c/0x908
rt_mutex_schedule+0x30/0x60
__rt_mutex_slowlock_locked.constprop.0+0x460/0x8a8
rwbase_write_lock+0x24c/0x378
down_write+0x1c/0x30
f2fs_balance_fs+0x184/0x208
f2fs_map_blocks+0x94c/0x1110
f2fs_file_write_iter+0x228/0xb80
do_iter_readv_writev+0xf0/0x1e0
vfs_writev+0x138/0x2c8
do_writev+0x88/0x130
__arm64_sys_writev+0x28/0x40
invoke_syscall+0x50/0x120
el0_svc_common.constprop.0+0xc8/0xf0
do_el0_svc+0x24/0x38
el0_svc+0x30/0xf8
el0t_64_sync_handler+0x120/0x130
el0t_64_sync+0x190/0x198

INFO: task sync:2769849 blocked for more than 120 seconds.
       Tainted: G           O       6.12.41-g3fe07ddf05ab #1
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:sync            state:D stack:0     pid:2769849 tgid:2769849 ppid:736    flags:0x0000020c
Call trace:
__switch_to+0xf4/0x158
__schedule+0x27c/0x908
schedule+0x3c/0x118
wb_wait_for_completion+0xb0/0xe8
sync_inodes_sb+0xc8/0x2b0
sync_inodes_one_sb+0x24/0x38
iterate_supers+0xa8/0x138
ksys_sync+0x54/0xc8
__arm64_sys_sync+0x18/0x30
invoke_syscall+0x50/0x120
el0_svc_common.constprop.0+0xc8/0xf0
do_el0_svc+0x24/0x38
el0_svc+0x30/0xf8
el0t_64_sync_handler+0x120/0x130
el0t_64_sync+0x190/0x198

The root cause is a potential deadlock between the following tasks:

kworker/u8:11                Thread A
- f2fs_write_single_data_page
- f2fs_do_write_data_page
   - folio_start_writeback(X)
   - f2fs_outplace_write_data
    - bio_add_folio(X)
- folio_unlock(X)
                    - truncate_inode_pages_range
                     - __filemap_get_folio(X, FGP_LOCK)
                     - truncate_inode_partial_folio(X)
                      - folio_wait_writeback(X)
- f2fs_balance_fs
   - f2fs_gc
    - do_garbage_collect
     - move_data_page
      - f2fs_get_lock_data_page
       - __filemap_get_folio(X, FGP_LOCK)

Both threads try to access folio X. Thread A holds the lock but waits
for writeback, while kworker waits for the lock. This causes a deadlock.

Other threads also enter D state, waiting for locks such as gc_lock and
writepages.

OPU/IPU DATA folio are all affected by this issue. To avoid such
potential deadlocks, always commit these cached folios before
triggering f2fs_gc() in f2fs_balance_fs().

v2:
- Commit cached OPU/IPU folios, not just OPU folios as in v1.

Suggested-by: Chao <chao@xxxxxxxxxx>
Signed-off-by: Ruipeng Qi <ruipengqi3@xxxxxxxxx>
---
fs/f2fs/data.c    | 26 ++++++++++++++++++++++++++
fs/f2fs/f2fs.h    | 1 +
fs/f2fs/segment.c | 9 +++++++++
3 files changed, 36 insertions(+)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 338df7a2aea6..fd03366b3228 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -939,6 +939,32 @@ void f2fs_submit_merged_ipu_write(struct f2fs_sb_info *sbi,
      }
}
+void f2fs_submit_all_merged_ipu_writes(struct f2fs_sb_info *sbi)
+{
+    struct bio_entry *be, *tmp;
+    struct f2fs_bio_info *io;
+    enum temp_type temp;
+    LIST_HEAD(list);
+
+    for (temp = HOT; temp < NR_TEMP_TYPE; temp++) {
+        io = sbi->write_io[DATA] + temp;
+
+        if (list_empty(&io->bio_list))
+            continue;

Needs to be covered w/ bio_list_lock to avoid race condition.

Hi,Chao

The lockless list_empty() here is intentional and acceptable.

If list_empty() returns true but the list becomes non-empty
afterwards (due to race), the newly added bio will be submitted
by the subsequent write path, so no bio will be lost.

Similar patterns exist in the kernel, e.g.:
net/rfkill/core.c: rfkill_fop_read()
    /* since we re-check and it just compares pointers,
     * using !list_empty() without locking isn't a problem
     */
fs/f2fs/data.c: f2fs_submit_merged_ipu_write()
    list_empty() is also used without holding bio_list_lock
    as a lockless pre-check

If you'd prefer, we can add a comment to make the intent clear:

    /* list_empty() without lock is safe here - READ_ONCE()
     * ensures pointer read atomicity. A false negative is
     * acceptable since any bio added concurrently will be
     * submitted by the next write path.
     */
    if (list_empty(&io->bio_list))
        continue;

+
+        f2fs_down_write(&io->bio_list_lock);
+        list_splice_init(&io->bio_list, &list);
+        f2fs_up_write(&io->bio_list_lock);
+
+        list_for_each_entry_safe(be, tmp, &list, list) {
+            f2fs_submit_write_bio(sbi, be->bio, DATA);
+            del_bio_entry(be);
+        }
+

Unnecessary blank line.

Thanks,

Thanks for your correction. Will fix in v3.
    v3:
    - Fixed minor grammatical issues
    - Add comment on lockless list_empty() to explain why it is safe
without holding bio_list_lock

Thanks,

+    }
+
+}
+
int f2fs_merge_page_bio(struct f2fs_io_info *fio)
{
      struct bio *bio = *fio->bio;
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index bb34e864d0ef..e9038ab1b2bd 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -4148,6 +4148,7 @@ void f2fs_submit_merged_write_folio(struct f2fs_sb_info *sbi,
                  struct folio *folio, enum page_type type);
void f2fs_submit_merged_ipu_write(struct f2fs_sb_info *sbi,
                      struct bio **bio, struct folio *folio);
+void f2fs_submit_all_merged_ipu_writes(struct f2fs_sb_info *sbi);
void f2fs_flush_merged_writes(struct f2fs_sb_info *sbi);
int f2fs_submit_page_bio(struct f2fs_io_info *fio);
int f2fs_merge_page_bio(struct f2fs_io_info *fio);
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 6a97fe76712b..856ffe91b94f 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -454,6 +454,15 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need)
          io_schedule();
          finish_wait(&sbi->gc_thread->fggc_wq, &wait);
      } else {
+
+        /*
+         * Submit all cached OPU/IPU DATA bios before triggering
+         * foreground GC to avoid potential deadlocks.
+         */
+
+        f2fs_submit_merged_write(sbi, DATA);
+        f2fs_submit_all_merged_ipu_writes(sbi);
+
          struct f2fs_gc_control gc_control = {
              .victim_segno = NULL_SEGNO,
              .init_gc_type = f2fs_sb_has_blkzoned(sbi) ?