mm,btrfs: add a filemap_flush_nr helper

Abstract out the btrfs-specific behavior of kicking off I/O on a number
of pages on an address_space into a well-defined helper.

Note: there is no kerneldoc comment for the new function because it is
not part of the public API.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20251024080431.324236-7-hch@lst.de
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 22 insertions, 0 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 99d6919af60d..e344b79a012d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -474,6 +474,28 @@ int filemap_flush(struct address_space *mapping)
 }
 EXPORT_SYMBOL(filemap_flush);
 
+/*
+ * Start writeback on @nr_to_write pages from @mapping.  No one but the existing
+ * btrfs caller should be using this.  Talk to linux-mm if you think adding a
+ * new caller is a good idea.
+ */
+int filemap_flush_nr(struct address_space *mapping, long *nr_to_write)
+{
+	struct writeback_control wbc = {
+		.nr_to_write = *nr_to_write,
+		.sync_mode = WB_SYNC_NONE,
+		.range_start = 0,
+		.range_end = LLONG_MAX,
+	};
+	int ret;
+
+	ret = filemap_fdatawrite_wbc(mapping, &wbc);
+	if (!ret)
+		*nr_to_write = wbc.nr_to_write;
+	return ret;
+}
+EXPORT_SYMBOL_FOR_MODULES(filemap_flush_nr, "btrfs");
+
 /**
  * filemap_range_has_page - check if a page exists in range.
  * @mapping:           address space within which to check