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

Jan Kara
2025-10-20T08:56:53Z

On Fri 17-10-25 05:45:48, 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.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> 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 */
> --
> 2.47.3
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR