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

Christoph Hellwig
2025-10-15T06:27:44Z

The relatively low minimal writeback size of 4MiB leads 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>
---
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..23f1f10646b7 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 */
+ unsigned int 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