Re: [f2fs-dev] [PATCH v3 04/12] f2fs: cache: introduce writeback thread

[Daeho Jeong]

On Tue, Aug 25, 2026 at 6:06 AM Chao Yu via Linux-f2fs-devel
<linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx> wrote:
>
> This patch introduces a background writeback kthread (f2fs_writeback-x:y)
> to periodically flush dirty metadata cache entries with a default
> interval of 5 seconds.
>
> It manages thread lifecycle across mount, unmount, and remount (rw/ro)
> transitions, and hooks synchronous flushing into checkpoint commits.

What do you mean by the above line saying "hooks.."?

>
> Signed-off-by: Chao Yu <chao@xxxxxxxxxx>
> ---
>  fs/f2fs/cache.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/f2fs/cache.h | 13 ++++++++++++
>  fs/f2fs/f2fs.h  |  3 +++
>  fs/f2fs/super.c | 31 ++++++++++++++++++++++++++-
>  4 files changed, 102 insertions(+), 1 deletion(-)
>
> diff --git a/fs/f2fs/cache.c b/fs/f2fs/cache.c
> index 3cee33c69880..dcfea6c2e1bb 100644
> --- a/fs/f2fs/cache.c
> +++ b/fs/f2fs/cache.c
> @@ -615,3 +615,59 @@ unsigned long f2fs_shrink_cache(struct f2fs_sb_info *sbi,
>  {
>         return f2fs_do_shrink_cache(META_CACHE(sbi), nr_to_scan);
>  }
> +
> +static int f2fs_cache_writeback_kthread(void *data)
> +{
> +       struct f2fs_sb_info *sbi = data;
> +       struct f2fs_cache_kthread *cache_thread = &sbi->cache_thread;
> +       wait_queue_head_t *wq = &cache_thread->cache_wb_wq;
> +       unsigned int interval = DEF_DIRTY_CACHE_TIMEOUT;
> +
> +       set_freezable();
> +
> +       while (!kthread_should_stop()) {
> +               wait_event_freezable_timeout(*wq,
> +                               kthread_should_stop() ||
> +                               cache_thread->cache_wb_task == NULL,

Maybe a redundant check?

> +                               msecs_to_jiffies(interval));
> +
> +               if (kthread_should_stop())
> +                       break;
> +               if (f2fs_cp_error(sbi))
> +                       continue;
> +       }
> +       return 0;
> +}
> +
> +int f2fs_start_cache_wb_thread(struct f2fs_sb_info *sbi)
> +{
> +       struct f2fs_cache_kthread *cache_thread = &sbi->cache_thread;
> +       struct task_struct *task;
> +       dev_t dev = sbi->sb->s_dev;
> +       char name[36];
> +
> +       if (cache_thread->cache_wb_task)
> +               return 0;
> +
> +       init_waitqueue_head(&cache_thread->cache_wb_wq);
> +       snprintf(name, sizeof(name), "f2fs_writeback-%u:%u",
> +                       MAJOR(dev), MINOR(dev));
> +
> +       task = kthread_run(f2fs_cache_writeback_kthread, sbi, "%s", name);
> +       if (IS_ERR(task))
> +               return PTR_ERR(task);
> +
> +       cache_thread->cache_wb_task = task;
> +       return 0;
> +}
> +
> +void f2fs_stop_cache_wb_thread(struct f2fs_sb_info *sbi)
> +{
> +       struct f2fs_cache_kthread *cache_thread = &sbi->cache_thread;
> +
> +       if (!cache_thread->cache_wb_task)
> +               return;
> +
> +       kthread_stop(cache_thread->cache_wb_task);
> +       cache_thread->cache_wb_task = NULL;
> +}
> diff --git a/fs/f2fs/cache.h b/fs/f2fs/cache.h
> index 618b377590da..ff3d8781119c 100644
> --- a/fs/f2fs/cache.h
> +++ b/fs/f2fs/cache.h
> @@ -187,4 +187,17 @@ void f2fs_stop_cache_wb_thread(struct f2fs_sb_info *sbi);
>  unsigned long f2fs_shrink_cache(struct f2fs_sb_info *sbi,
>                                 unsigned long nr_to_scan);
>
> +#define DEF_DIRTY_CACHE_TIMEOUT 5000

Maybe this one should be tunnable, right?

> +
> +struct f2fs_cache_kthread {
> +       struct task_struct *cache_wb_task;
> +       wait_queue_head_t cache_wb_wq;

There is no wakeup all around the patchset. Is this for future usage?

> +       atomic_t cache_wb_trigger;
> +       unsigned int cache_wb_interval_ms;

Plz, remove unused fields from above struct.

> +};
> +
> +int f2fs_start_cache_wb_thread(struct f2fs_sb_info *sbi);
> +void f2fs_stop_cache_wb_thread(struct f2fs_sb_info *sbi);
> +void f2fs_sync_cache_wb(struct f2fs_sb_info *sbi);

dead code?

Thanks,

> +
>  #endif /* _LINUX_F2FS_CACHE_H */
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 0f855ac7d1e8..9fe9fadbb810 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -2111,6 +2111,9 @@ struct f2fs_sb_info {
>
>         /* f2fs internal cache */
>         struct f2fs_cached_block_list meta_blocks;
> +
> +       /* internal cache flush thread */
> +       struct f2fs_cache_kthread cache_thread;
>  };
>
>  /* Definitions to access f2fs_sb_info */
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index 32e80c70e141..9bb19f4c5fd2 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -2020,6 +2020,7 @@ static void f2fs_put_super(struct super_block *sb)
>          * flush all issued checkpoints and stop checkpoint issue thread.
>          * after then, all checkpoints should be done by each process context.
>          */
> +       f2fs_stop_cache_wb_thread(sbi);
>         f2fs_stop_ckpt_thread(sbi);
>
>         /*
> @@ -2826,6 +2827,7 @@ static int __f2fs_remount(struct fs_context *fc, struct super_block *sb)
>         unsigned int flags = fc->sb_flags;
>         int err;
>         bool need_restart_gc = false, need_stop_gc = false;
> +       bool need_restart_wb = false, need_stop_wb = false;
>         bool need_restart_flush = false, need_stop_flush = false;
>         bool need_restart_discard = false, need_stop_discard = false;
>         bool need_enable_checkpoint = false, need_disable_checkpoint = false;
> @@ -2984,6 +2986,18 @@ static int __f2fs_remount(struct fs_context *fc, struct super_block *sb)
>                 need_stop_gc = true;
>         }
>
> +       if (flags & SB_RDONLY) {
> +               if (sbi->cache_thread.cache_wb_task) {
> +                       f2fs_stop_cache_wb_thread(sbi);
> +                       need_restart_wb = true;
> +               }
> +       } else if (!sbi->cache_thread.cache_wb_task) {
> +               err = f2fs_start_cache_wb_thread(sbi);
> +               if (err)
> +                       goto restore_gc;
> +               need_stop_wb = true;
> +       }
> +
>         if (flags & SB_RDONLY) {
>                 sync_inodes_sb(sb);
>
> @@ -3006,7 +3020,7 @@ static int __f2fs_remount(struct fs_context *fc, struct super_block *sb)
>         } else {
>                 err = f2fs_create_flush_cmd_control(sbi);
>                 if (err)
> -                       goto restore_gc;
> +                       goto restore_wb;
>                 need_stop_flush = true;
>         }
>
> @@ -3103,6 +3117,13 @@ static int __f2fs_remount(struct fs_context *fc, struct super_block *sb)
>                 clear_opt(sbi, FLUSH_MERGE);
>                 f2fs_destroy_flush_cmd_control(sbi, false);
>         }
> +restore_wb:
> +       if (need_restart_wb) {
> +               if (f2fs_start_cache_wb_thread(sbi))
> +                       f2fs_warn(sbi, "background cache writeback thread has stopped");
> +       } else if (need_stop_wb) {
> +               f2fs_stop_cache_wb_thread(sbi);
> +       }
>  restore_gc:
>         if (need_restart_gc) {
>                 if (f2fs_start_gc_thread(sbi))
> @@ -5497,6 +5518,12 @@ static int f2fs_fill_super(struct super_block *sb, struct fs_context *fc)
>                         goto sync_free_meta;
>         }
>
> +       if (!f2fs_readonly(sb)) {
> +               err = f2fs_start_cache_wb_thread(sbi);
> +               if (err)
> +                       goto stop_gc_thread;
> +       }
> +
>         /* recover broken superblock */
>         if (recovery) {
>                 err = f2fs_commit_super(sbi, true);
> @@ -5519,6 +5546,8 @@ static int f2fs_fill_super(struct super_block *sb, struct fs_context *fc)
>         sbi->umount_lock_holder = NULL;
>         return 0;
>
> +stop_gc_thread:
> +       f2fs_stop_gc_thread(sbi);
>  sync_free_meta:
>         /* safe to flush all the data */
>         sync_filesystem(sbi->sb);
> --
> 2.49.0
>
>
>
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