writeback: factor writeback_get_batch() out of write_cache_pages()

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.

[hch@lst.de: heavily rebased, add helpers to get the tag and end index, don't keep the end index in struct writeback_control]
Link: https://lkml.kernel.org/r/20240215063649.2164017-9-hch@lst.de
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/writeback.h

@@ -11,6 +11,7 @@
 #include <linux/flex_proportions.h>
 #include <linux/backing-dev-defs.h>
 #include <linux/blk_types.h>
+#include <linux/pagevec.h>
 
 struct bio;
 
@@ -40,6 +41,7 @@ enum writeback_sync_modes {
  * in a manner such that unspecified fields are set to zero.
  */
 struct writeback_control {
+	/* public fields that can be set and/or consumed by the caller: */
 	long nr_to_write;		/* Write this many pages, and decrement
 					   this for each page written */
 	long pages_skipped;		/* Pages which were not written */
@@ -77,6 +79,10 @@ struct writeback_control {
 	 */
 	struct swap_iocb **swap_plug;
 
+	/* internal fields used by the ->writepages implementation: */
+	struct folio_batch fbatch;
+	pgoff_t index;
+
 #ifdef CONFIG_CGROUP_WRITEBACK
 	struct bdi_writeback *wb;	/* wb this writeback is issued under */
 	struct inode *inode;		/* inode being written out */

mm/page-writeback.c

@@ -2392,6 +2392,29 @@ static bool folio_prepare_writeback(struct address_space *mapping,
 	return true;
 }
 
+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
@@ -2429,38 +2452,32 @@ int write_cache_pages(struct address_space *mapping,
 {
 	int ret = 0;
 	int error;
-	struct folio_batch fbatch;
 	struct folio *folio;
-	int nr_folios;
-	pgoff_t index;
 	pgoff_t end;		/* Inclusive */
-	xa_mark_t tag;
 
-	folio_batch_init(&fbatch);
 	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) {
+	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
-		tag_pages_for_writeback(mapping, index, end);
+		tag_pages_for_writeback(mapping, wbc->index, end);
-		tag = PAGECACHE_TAG_TOWRITE;
+
-	} else {
+	folio_batch_init(&wbc->fbatch);
-		tag = PAGECACHE_TAG_DIRTY;
+
-	}
+	while (wbc->index <= end) {
-	while (index <= end) {
 		int i;
 
-		nr_folios = filemap_get_folios_tag(mapping, &index, end,
+		writeback_get_batch(mapping, wbc);
-				tag, &fbatch);
 
-		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++) {
-			folio = fbatch.folios[i];
+			folio = wbc->fbatch.folios[i];
+
 			folio_lock(folio);
 			if (!folio_prepare_writeback(mapping, wbc, folio)) {
 				folio_unlock(folio);
@@ -2498,8 +2515,6 @@ int write_cache_pages(struct address_space *mapping,
 					goto done;
 			}
 		}
-		folio_batch_release(&fbatch);
-		cond_resched();
 	}
 
 	/*
@@ -2512,6 +2527,7 @@ int write_cache_pages(struct address_space *mapping,
 	 * of the file if we are called again, which can only happen due to
 	 * -ENOMEM from the file system.
 	 */
+	folio_batch_release(&wbc->fbatch);
 	if (wbc->range_cyclic)
 		mapping->writeback_index = 0;
 	return ret;
@@ -2519,7 +2535,7 @@ int write_cache_pages(struct address_space *mapping,
 done:
 	if (wbc->range_cyclic)
 		mapping->writeback_index = folio->index + folio_nr_pages(folio);
-	folio_batch_release(&fbatch);
+	folio_batch_release(&wbc->fbatch);
 	return error;
 }
 EXPORT_SYMBOL(write_cache_pages);