[PATCH 07/17] writeback: Factor writeback_get_batch() out of write_cache_pages()

[Christoph Hellwig <hch@xxxxxx>]

From: "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>
[hch: heavily rebased, add helpers to get the tag and end index,
      don't keep the end index in struct writeback_control]
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 include/linux/writeback.h |  1 +
 mm/page-writeback.c       | 49 +++++++++++++++++++++++++--------------
 2 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 390f2dd03cf27e..195393981ccb5c 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -81,6 +81,7 @@ struct writeback_control {
 
 	/* internal fields used by the ->writepages implementation: */
 	struct folio_batch fbatch;
+	pgoff_t index;
 	int err;
 
 #ifdef CONFIG_CGROUP_WRITEBACK
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 564d5faf562ba7..798e5264dc0353 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2383,6 +2383,29 @@ static void writeback_finish(struct address_space *mapping,
 	}
 }
 
+static xa_mark_t wbc_to_tag(struct writeback_control *wbc)
+{
+	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
+		return PAGECACHE_TAG_TOWRITE;
+	return PAGECACHE_TAG_DIRTY;
+}
+
+static pgoff_t wbc_end(struct writeback_control *wbc)
+{
+	if (wbc->range_cyclic)
+		return -1;
+	return wbc->range_end >> PAGE_SHIFT;
+}
+
+static void writeback_get_batch(struct address_space *mapping,
+		struct writeback_control *wbc)
+{
+	folio_batch_release(&wbc->fbatch);
+	cond_resched();
+	filemap_get_folios_tag(mapping, &wbc->index, wbc_end(wbc),
+			wbc_to_tag(wbc), &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
@@ -2419,38 +2442,30 @@ 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 */
+		wbc->index = mapping->writeback_index; /* prev offset */
 		end = -1;
 	} else {
-		index = wbc->range_start >> PAGE_SHIFT;
+		wbc->index = wbc->range_start >> PAGE_SHIFT;
 		end = wbc->range_end >> PAGE_SHIFT;
 	}
-	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, end);
 
 	folio_batch_init(&wbc->fbatch);
 	wbc->err = 0;
 
-	while (index <= end) {
+	while (wbc->index <= 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];
 			unsigned long nr;
 
@@ -2525,8 +2540,6 @@ int write_cache_pages(struct address_space *mapping,
 				return error;
 			}
 		}
-		folio_batch_release(&wbc->fbatch);
-		cond_resched();
 	}
 
 	writeback_finish(mapping, wbc, 0);
-- 
2.39.2