Re: [PATCH] f2fs: optimize f2fs DIO overwrites

[Chao Yu <chao@kernel.org>]

On 3/7/25 22:56, Yohan Joung wrote:
> this is unnecessary when we know we are overwriting already allocated
> blocks and the overhead of starting a transaction can be significant
> especially for multithreaded workloads doing small writes.

Hi Yohan,

So you're trying to avoid f2fs_map_lock() in dio write path, right?

Thanks,

> 
> Signed-off-by: Yohan Joung <yohan.joung@sk.com>
> ---
>  fs/f2fs/data.c | 20 ++++++++++++++++++++
>  fs/f2fs/f2fs.h |  1 +
>  fs/f2fs/file.c |  5 ++++-
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 09437dbd1b42..728630037b74 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -4267,6 +4267,26 @@ static int f2fs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  	return 0;
>  }
>  
> +static int f2fs_iomap_overwrite_begin(struct inode *inode, loff_t offset,
> +		loff_t length, unsigned flags, struct iomap *iomap,
> +		struct iomap *srcmap)
> +{
> +	int ret;
> +
> +	/*
> +	 * Even for writes we don't need to allocate blocks, so just pretend
> +	 * we are reading to save overhead of starting a transaction.
> +	 */
> +	flags &= ~IOMAP_WRITE;
> +	ret = f2fs_iomap_begin(inode, offset, length, flags, iomap, srcmap);
> +	WARN_ON_ONCE(!ret && iomap->type != IOMAP_MAPPED);
> +	return ret;
> +}
> +
>  const struct iomap_ops f2fs_iomap_ops = {
>  	.iomap_begin	= f2fs_iomap_begin,
>  };
> +
> +const struct iomap_ops f2fs_iomap_overwrite_ops = {
> +	.iomap_begin	= f2fs_iomap_overwrite_begin,
> +};
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index c6cc2694f9ac..0511ab5ed42a 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -3936,6 +3936,7 @@ void f2fs_destroy_post_read_processing(void);
>  int f2fs_init_post_read_wq(struct f2fs_sb_info *sbi);
>  void f2fs_destroy_post_read_wq(struct f2fs_sb_info *sbi);
>  extern const struct iomap_ops f2fs_iomap_ops;
> +extern const struct iomap_ops f2fs_iomap_overwrite_ops;
>  
>  static inline struct page *f2fs_find_data_page(struct inode *inode,
>  		pgoff_t index, pgoff_t *next_pgofs)
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 82b21baf5628..bb2fe6dac9b6 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -4985,6 +4985,7 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
>  	const ssize_t count = iov_iter_count(from);
>  	unsigned int dio_flags;
>  	struct iomap_dio *dio;
> +	const struct iomap_ops *iomap_ops = &f2fs_iomap_ops;
>  	ssize_t ret;
>  
>  	trace_f2fs_direct_IO_enter(inode, iocb, count, WRITE);
> @@ -5025,7 +5026,9 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
>  	dio_flags = 0;
>  	if (pos + count > inode->i_size)
>  		dio_flags |= IOMAP_DIO_FORCE_WAIT;
> -	dio = __iomap_dio_rw(iocb, from, &f2fs_iomap_ops,
> +	else if (f2fs_overwrite_io(inode, pos, count))
> +		iomap_ops = &f2fs_iomap_overwrite_ops;
> +	dio = __iomap_dio_rw(iocb, from, iomap_ops,
>  			     &f2fs_iomap_dio_write_ops, dio_flags, NULL, 0);
>  	if (IS_ERR_OR_NULL(dio)) {
>  		ret = PTR_ERR_OR_ZERO(dio);