Re: [PATCH 19/19] writeback: simplify writeback iteration

[Christoph Hellwig <hch@lst.de>]

On Tue, Jan 30, 2024 at 03:22:05PM +0100, Christoph Hellwig wrote:
> And now for real:

Another slight variant of this would be to move the nr_to_write || err
check for WB_SYNC_NONE out of the branch.  This adds extra tests for
the initial iteration, but unindents a huge comment, and moves it closer
to the other branch that it also describes.  The downside at least to
me is that the normal non-termination path is not the straight line
through function.  Thoughs?

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 973f57ad9ee548..ff6e73453aa8c4 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2461,24 +2461,6 @@ struct folio *writeback_iter(struct address_space *mapping,
 		wbc->nr_to_write -= folio_nr_pages(folio);
 		if (*error && !wbc->err)
 			wbc->err = *error;
-
-		/*
-		 * For integrity sync  we have to keep going until we have
-		 * written all the folios we tagged for writeback prior to
-		 * entering the writeback loop, even if we run past
-		 * wbc->nr_to_write or encounter errors.
-		 *
-		 * This is because the file system may still have state to clear
-		 * for each folio.  We'll eventually return the first error
-		 * encountered.
-		 *
-		 * For background writeback just push done_index past this folio
-		 * so that we can just restart where we left off and media
-		 * errors won't choke writeout for the entire file.
-		 */
-		if (wbc->sync_mode == WB_SYNC_NONE &&
-		    (wbc->err || wbc->nr_to_write <= 0))
-			goto finish;
 	} else {
 		if (wbc->range_cyclic)
 			wbc->index = mapping->writeback_index; /* prev offset */
@@ -2491,17 +2473,20 @@ struct folio *writeback_iter(struct address_space *mapping,
 		wbc->err = 0;
 	}
 
-	folio = writeback_get_folio(mapping, wbc);
-	if (!folio)
-		goto finish;
-	return folio;
-
-finish:
-	folio_batch_release(&wbc->fbatch);
-
 	/*
+	 * For integrity sync  we have to keep going until we have written all
+	 * the folios we tagged for writeback prior to entering the writeback
+	 * loop, even if we run past wbc->nr_to_write or encounter errors.
+	 *
+	 * This is because the file system may still have state to clear for
+	 * each folio.  We'll eventually return the first error encountered.
+	 *
+	 * For background writeback just push done_index past this folio so that
+	 * we can just restart where we left off and media errors won't choke
+	 * writeout for the entire file.
+	 *
 	 * For range cyclic writeback we need to remember where we stopped so
-	 * that we can continue there next time we are called.  If  we hit the
+	 * that we can continue there next time we are called.  If we hit the
 	 * last page and there is more work to be done, wrap back to the start
 	 * of the file.
 	 *
@@ -2509,14 +2494,21 @@ struct folio *writeback_iter(struct address_space *mapping,
 	 * of the file if we are called again, which can only happen due to
 	 * -ENOMEM from the file system.
 	 */
-	if (wbc->range_cyclic) {
-		WARN_ON_ONCE(wbc->sync_mode != WB_SYNC_NONE);
-		if (wbc->err || wbc->nr_to_write <= 0)
+	if (wbc->sync_mode == WB_SYNC_NONE &&
+	    (wbc->err || wbc->nr_to_write <= 0)) {
+		if (wbc->range_cyclic)
 			mapping->writeback_index =
 				folio->index + folio_nr_pages(folio);
-		else
+	} else {
+		folio = writeback_get_folio(mapping, wbc);
+		if (folio)
+			return folio;
+
+		if (wbc->range_cyclic)
 			mapping->writeback_index = 0;
 	}
+
+	folio_batch_release(&wbc->fbatch);
 	*error = wbc->err;
 	return NULL;
 }