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

[Chao Yu <chao@kernel.org>]

On 4/29/26 11:39, Ruipeng Qi wrote:
> 
> On 2026/4/27 16:38, Chao Yu wrote:
>> On 4/26/26 17:32, ruipengqi wrote:
>>> From: Ruipeng Qi <ruipengqi3@gmail.com>
>>>
>>> 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@kernel.org>
>>> Signed-off-by: Ruipeng Qi <ruipengqi3@gmail.com>
>>> ---
>>>   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.

Ah, right, we only need to submit the folios cached by local thread.

> 
> 
> 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

Seems fine.

> 
> 
> 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);

Can we relocate above code to below the variable definitions?

Thanks,

>>> +
>>>           struct f2fs_gc_control gc_control = {
>>>               .victim_segno = NULL_SEGNO,
>>>               .init_gc_type = f2fs_sb_has_blkzoned(sbi) ?
>>