Re: [PATCH v3] f2fs: fix potential deadlock in f2fs_balance_fs()

[Ruipeng Qi <ruipengqi3@gmail.com>]

On 5/3/2026 5:47 PM, Chao Yu wrote:
> On 5/2/26 20:41, 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
>>   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.
>>
>> v3:
>> - Fixed minor grammatical issues
>> - Add comment on lockless list_empty() to explain why it is safe
>>    without holding bio_list_lock
>>
>> Suggested-by: Chao <chao@kernel.org>
>
> Chao Yu <chao@kernel.org>, :)
>
>> Signed-off-by: Ruipeng Qi <ruipengqi3@gmail.com>
>
> Reviewed-by: Chao Yu <chao@kernel.org>
>
> Thanks,

Hi Jaegeuk, Chao,

Gentle ping.

This patch has been discussed for about a month and
already got Reviewed-by from Chao.

Could you please pick up this patch when you have time?

Thanks,
Ruipeng