Re: [PATCH 2/3] writeback: allow the file system to override MIN_WRITEBACK_PAGES

["Nirjhar Roy (IBM)" <nirjhar.roy.lists@gmail.com>]

On Fri, 2025-10-17 at 05:45 +0200, Christoph Hellwig wrote:
> The relatively low minimal writeback size of 4MiB means that written back
> inodes on rotational media are switched a lot.  Besides introducing
> additional seeks, this also can lead to extreme file fragmentation on
> zoned devices when a lot of files are cached relative to the available
> writeback bandwidth.
> 
> Add a superblock field that allows the file system to override the
> default size.
So this patch doesn't really explicitly set s_min_writeback_pages to a non-default/overridden value,
right? That is being done in the next patch, isn't it?
--NR
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/fs-writeback.c         | 14 +++++---------
>  fs/super.c                |  1 +
>  include/linux/fs.h        |  1 +
>  include/linux/writeback.h |  5 +++++
>  4 files changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 11fd08a0efb8..6d50b02cdab6 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -31,11 +31,6 @@
>  #include <linux/memcontrol.h>
>  #include "internal.h"
>  
> -/*
> - * 4MB minimal write chunk size
> - */
> -#define MIN_WRITEBACK_PAGES	(4096UL >> (PAGE_SHIFT - 10))
> -
>  /*
>   * Passed into wb_writeback(), essentially a subset of writeback_control
>   */
> @@ -1874,8 +1869,8 @@ static int writeback_single_inode(struct inode *inode,
>  	return ret;
>  }
>  
> -static long writeback_chunk_size(struct bdi_writeback *wb,
> -				 struct wb_writeback_work *work)
> +static long writeback_chunk_size(struct super_block *sb,
> +		struct bdi_writeback *wb, struct wb_writeback_work *work)
>  {
>  	long pages;
>  
> @@ -1898,7 +1893,8 @@ static long writeback_chunk_size(struct bdi_writeback *wb,
>  	pages = min(wb->avg_write_bandwidth / 2,
>  		    global_wb_domain.dirty_limit / DIRTY_SCOPE);
>  	pages = min(pages, work->nr_pages);
> -	return round_down(pages + MIN_WRITEBACK_PAGES, MIN_WRITEBACK_PAGES);
> +	return round_down(pages + sb->s_min_writeback_pages,
> +			sb->s_min_writeback_pages);
>  }
>  
>  /*
> @@ -2000,7 +1996,7 @@ static long writeback_sb_inodes(struct super_block *sb,
>  		inode->i_state |= I_SYNC;
>  		wbc_attach_and_unlock_inode(&wbc, inode);
>  
> -		write_chunk = writeback_chunk_size(wb, work);
> +		write_chunk = writeback_chunk_size(inode->i_sb, wb, work);
>  		wbc.nr_to_write = write_chunk;
>  		wbc.pages_skipped = 0;
>  
> diff --git a/fs/super.c b/fs/super.c
> index 5bab94fb7e03..599c1d2641fe 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -389,6 +389,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
>  		goto fail;
>  	if (list_lru_init_memcg(&s->s_inode_lru, s->s_shrink))
>  		goto fail;
> +	s->s_min_writeback_pages = MIN_WRITEBACK_PAGES;
>  	return s;
>  
>  fail:
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index c895146c1444..ae6f37c6eaa4 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1583,6 +1583,7 @@ struct super_block {
>  
>  	spinlock_t		s_inode_wblist_lock;
>  	struct list_head	s_inodes_wb;	/* writeback inodes */
> +	long			s_min_writeback_pages;
>  } __randomize_layout;
>  
>  static inline struct user_namespace *i_user_ns(const struct inode *inode)
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index 22dd4adc5667..49e1dd96f43e 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -374,4 +374,9 @@ bool redirty_page_for_writepage(struct writeback_control *, struct page *);
>  void sb_mark_inode_writeback(struct inode *inode);
>  void sb_clear_inode_writeback(struct inode *inode);
>  
> +/*
> + * 4MB minimal write chunk size
> + */
> +#define MIN_WRITEBACK_PAGES	(4096UL >> (PAGE_SHIFT - 10))
> +
>  #endif		/* WRITEBACK_H */