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

[Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

Hi Ruipeng,

Sorry, I missed your patch.

On 3/25/2026 9:37 PM, 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.
> 
> To avoid this potential deadlock, always call f2fs_submit_merged_write
> before triggering f2fs_gc in f2fs_balance_fs.
> 
> Signed-off-by: Ruipeng Qi <ruipengqi3@gmail.com>
> ---
>   fs/f2fs/segment.c | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 6a97fe76712b..b58299e49c23 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -454,6 +454,20 @@ void f2fs_balance_fs(struct f2fs_sb_info *sbi, bool need)
>   		io_schedule();
>   		finish_wait(&sbi->gc_thread->fggc_wq, &wait);
>   	} else {
> +
> +		/*
> +		 * Before triggering foreground GC, submit all cached DATA
> +		 * write bios. During writeback, pages may be added to
> +		 * write_io[DATA].bio with PG_writeback set but the bio not
> +		 * yet submitted. If GC's move_data_page() blocks on
> +		 * __folio_lock() for such a folio, and the lock holder waits
> +		 * for PG_writeback to clear via VFS folio_wait_writeback()
> +		 * neither thread can make progress. Flushing here ensures
> +		 * the bio completion callback can clear PG_writeback.
> +		 */
> +
> +		f2fs_submit_merged_write(sbi, DATA);

Do we need to call f2fs_submit_merged_ipu_write(sbi, bio, NULL) to commit
cached IPU folios as well?

Not sure, this race condition will happen for node folio.

Thanks,

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



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