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

[Ruipeng Qi <ruipengqi3@gmail.com>]

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.


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) ?
>