[PATCH 02/12] writeback: Factor writeback_get_batch() out of write_cache_pages()

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

This simple helper will be the basis of the writeback iterator.
To make this work, we need to remember the current index
and end positions in writeback_control.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 include/linux/writeback.h |  2 ++
 mm/page-writeback.c       | 49 +++++++++++++++++++++------------------
 2 files changed, 29 insertions(+), 22 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 5b7d11f54013..7dd050b40e4b 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -54,6 +54,8 @@ struct writeback_control {
 	loff_t range_end;
 
 	struct folio_batch fbatch;
+	pgoff_t index;
+	pgoff_t end;		/* Inclusive */
 	pgoff_t done_index;
 	int err;
 
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index abd7c0eebc72..67c7f1564727 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2378,6 +2378,22 @@ static int writeback_finish(struct address_space *mapping,
 	return wbc->err;
 }
 
+static void writeback_get_batch(struct address_space *mapping,
+		struct writeback_control *wbc)
+{
+	xa_mark_t tag;
+
+	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
+		tag = PAGECACHE_TAG_TOWRITE;
+	else
+		tag = PAGECACHE_TAG_DIRTY;
+
+	folio_batch_release(&wbc->fbatch);
+	cond_resched();
+	filemap_get_folios_tag(mapping, &wbc->index, wbc->end, tag,
+			&wbc->fbatch);
+}
+
 /**
  * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
  * @mapping: address space structure to write
@@ -2414,41 +2430,32 @@ int write_cache_pages(struct address_space *mapping,
 		      void *data)
 {
 	int error;
-	int nr_folios;
-	pgoff_t index;
-	pgoff_t end;		/* Inclusive */
-	xa_mark_t tag;
 
 	if (wbc->range_cyclic) {
-		index = mapping->writeback_index; /* prev offset */
-		end = -1;
+		wbc->index = mapping->writeback_index; /* prev offset */
+		wbc->end = -1;
 	} else {
-		index = wbc->range_start >> PAGE_SHIFT;
-		end = wbc->range_end >> PAGE_SHIFT;
+		wbc->index = wbc->range_start >> PAGE_SHIFT;
+		wbc->end = wbc->range_end >> PAGE_SHIFT;
 		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
 			wbc->range_whole = 1;
 	}
-	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages) {
-		tag_pages_for_writeback(mapping, index, end);
-		tag = PAGECACHE_TAG_TOWRITE;
-	} else {
-		tag = PAGECACHE_TAG_DIRTY;
-	}
+	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
+		tag_pages_for_writeback(mapping, wbc->index, wbc->end);
 
-	wbc->done_index = index;
+	wbc->done_index = wbc->index;
 	folio_batch_init(&wbc->fbatch);
 	wbc->err = 0;
 
-	while (index <= end) {
+	while (wbc->index <= wbc->end) {
 		int i;
 
-		nr_folios = filemap_get_folios_tag(mapping, &index, end,
-				tag, &wbc->fbatch);
+		writeback_get_batch(mapping, wbc);
 
-		if (nr_folios == 0)
+		if (wbc->fbatch.nr == 0)
 			break;
 
-		for (i = 0; i < nr_folios; i++) {
+		for (i = 0; i < wbc->fbatch.nr; i++) {
 			struct folio *folio = wbc->fbatch.folios[i];
 
 			wbc->done_index = folio->index;
@@ -2524,8 +2531,6 @@ int write_cache_pages(struct address_space *mapping,
 			    wbc->sync_mode == WB_SYNC_NONE)
 				return writeback_finish(mapping, wbc, true);
 		}
-		folio_batch_release(&wbc->fbatch);
-		cond_resched();
 	}
 
 	return writeback_finish(mapping, wbc, false);
-- 
2.39.2