writeback: allow the file system to override MIN_WRITEBACK_PAGES

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>
Link: https://patch.msgid.link/20251017034611.651385-3-hch@lst.de
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Christoph Hellwig

fs/fs-writeback.c

@@ -32,11 +32,6 @@
 #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
  */
@@ -1889,8 +1884,8 @@ 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;
 
@@ -1913,7 +1908,8 @@ 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);
 }
 
 /*
@@ -2015,7 +2011,7 @@ 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;
 

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:

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)

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 */