writeback: add a writeback iterator

Refactor the code left in write_cache_pages into an iterator that the file
system can call to get the next folio for a writeback operation:

	struct folio *folio = NULL;

	while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
		error = <do per-folio writeback>;
	}

The twist here is that the error value is passed by reference, so that the
iterator can restore it when breaking out of the loop.

Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
iterator and needs is just kept in the write_cache_pages legacy wrapper. 
in preparation for eventually killing it off.

Heavily based on a for_each* based iterator from Matthew Wilcox.

Link: https://lkml.kernel.org/r/20240215063649.2164017-14-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index f67b3ea866a0..9845cb62e40b 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -82,6 +82,7 @@ struct writeback_control {
 	/* internal fields used by the ->writepages implementation: */
 	struct folio_batch fbatch;
 	pgoff_t index;
+	int saved_err;
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 	struct bdi_writeback *wb;	/* wb this writeback is issued under */
@@ -366,6 +367,9 @@ int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
 
 bool wb_over_bg_thresh(struct bdi_writeback *wb);
 
+struct folio *writeback_iter(struct address_space *mapping,
+		struct writeback_control *wbc, struct folio *folio, int *error);
+
 typedef int (*writepage_t)(struct folio *folio, struct writeback_control *wbc,
 				void *data);