Re: [PATCH] f2fs: skip clean inode update during fsync

[Chao Yu <chao@kernel.org>]

On 5/28/26 22:09, Wenjie Qi wrote:
> f2fs_do_sync_file() calls f2fs_skip_inode_update() before deciding
> whether it has to write an inode block and continue into the recovery
> info/flush path.
> 
> For a full fsync, f2fs_skip_inode_update() currently returns false when
> FI_AUTO_RECOVER is not set.  That makes fsync on an already clean file
> call f2fs_write_inode().  f2fs_write_inode() then returns immediately if
> the in-memory timestamps match the inode block and FI_DIRTY_INODE is not
> set, but f2fs_do_sync_file() still continues through go_write and may end
> at f2fs_issue_flush().
> 
> Avoid that unnecessary path for clean, time-consistent inodes without
> FI_AUTO_RECOVER.  Keep the existing conservative checks for keep-size
> files and non-block-aligned i_size before allowing the skip, and leave the
> FI_AUTO_RECOVER path unchanged.
> 
> On a QEMU/KASAN test VM, repeated fsync() on an existing clean F2FS file
> improved from about 35.7 us/fsync to about 1.13 us/fsync.  The baseline
> issued one flush per fsync, while the patched kernel kept the F2FS flush
> count unchanged over 140000 clean fsync calls.
> 
> Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
> ---
>  fs/f2fs/f2fs.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 9f24287de4c3..ebd485abecb4 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -3668,11 +3668,13 @@ static inline bool f2fs_skip_inode_update(struct inode *inode, int dsync)
>  		spin_unlock(&sbi->inode_lock[DIRTY_META]);
>  		return ret;
>  	}
> -	if (!is_inode_flag_set(inode, FI_AUTO_RECOVER) ||
> -			file_keep_isize(inode) ||
> -			i_size_read(inode) & ~PAGE_MASK)
> +	if (file_keep_isize(inode) || i_size_read(inode) & ~PAGE_MASK)
>  		return false;
>  
> +	if (!is_inode_flag_set(inode, FI_AUTO_RECOVER))
> +		return f2fs_is_time_consistent(inode) &&
> +			!is_inode_flag_set(inode, FI_DIRTY_INODE);

IIUC, without this additional check condition, f2fs_write_inode() will skip to update
inode page as well according to the same check condition, so, do you know why will we
have so many benefits on fsync?

int f2fs_write_inode(struct inode *inode, struct writeback_control *wbc)
{
...
	/*
	 * atime could be updated without dirtying f2fs inode in lazytime mode
	 */
	if (f2fs_is_time_consistent(inode) &&
		!is_inode_flag_set(inode, FI_DIRTY_INODE))
		return 0;

Thanks,

> +
>  	if (!f2fs_is_time_consistent(inode))
>  		return false;
>