Re: [PATCH 09/12] writeback: Factor writeback_iter_next() out of write_cache_pages()

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

On Tue, Jun 27, 2023 at 04:31:17PM +0100, Matthew Wilcox wrote:
> It makes the callers neater.  Compare:
> 
>                if (!folio)
>                         return writeback_finish(mapping, wbc, false);
> vs
> 		if (!folio) {
> 			writeback_finish(mapping, wbc, false);
> 			return NULL;
> 		}
> 
> Similarly for the other two callers.

Not sure I agree.  See my quickly cooked up patch below.  But in the
end this completely superficial and I won't complain, do it the way
your prefer.

> 
> > > +		if (error == AOP_WRITEPAGE_ACTIVATE) {
> > > +			folio_unlock(folio);
> > > +			error = 0;
> > 
> > Note there really shouldn't be any need for this in outside of the
> > legacy >writepage case.  But it might make sense to delay the removal
> > until after ->writepage is gone to avoid bugs in conversions.
> 
> ext4_journalled_writepage_callback() still returns
> AOP_WRITEPAGE_ACTIVATE, and that's used by a direct call to
> write_cache_pages().

Yeah.  But that could trivially do the open coded unlock_page.
But probably not worth mixing into this series.

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 55832679af2194..07bbbc0dec4d00 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2360,7 +2360,7 @@ void tag_pages_for_writeback(struct address_space *mapping,
 }
 EXPORT_SYMBOL(tag_pages_for_writeback);
 
-static struct folio *writeback_finish(struct address_space *mapping,
+static void writeback_finish(struct address_space *mapping,
 		struct writeback_control *wbc, bool done)
 {
 	folio_batch_release(&wbc->fbatch);
@@ -2374,8 +2374,6 @@ static struct folio *writeback_finish(struct address_space *mapping,
 		wbc->done_index = 0;
 	if (wbc->range_cyclic || (wbc->range_whole && wbc->nr_to_write > 0))
 		mapping->writeback_index = wbc->done_index;
-
-	return NULL;
 }
 
 static struct folio *writeback_get_next(struct address_space *mapping,
@@ -2435,20 +2433,19 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
 {
 	struct folio *folio;
 
-	for (;;) {
-		folio = writeback_get_next(mapping, wbc);
-		if (!folio)
-			return writeback_finish(mapping, wbc, false);
+	while ((folio = writeback_get_next(mapping, wbc))) {
 		wbc->done_index = folio->index;
 
 		folio_lock(folio);
-		if (likely(should_writeback_folio(mapping, wbc, folio)))
-			break;
+		if (likely(should_writeback_folio(mapping, wbc, folio))) {
+			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
+			return folio;
+		}
 		folio_unlock(folio);
 	}
 
-	trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
-	return folio;
+	writeback_finish(mapping, wbc, false);
+	return NULL;
 }
 
 struct folio *writeback_iter_init(struct address_space *mapping,
@@ -2494,7 +2491,7 @@ struct folio *writeback_iter_next(struct address_space *mapping,
 			wbc->err = error;
 			wbc->done_index = folio->index +
 					folio_nr_pages(folio);
-			return writeback_finish(mapping, wbc, true);
+			goto done;
 		}
 		if (!wbc->err)
 			wbc->err = error;
@@ -2507,9 +2504,12 @@ struct folio *writeback_iter_next(struct address_space *mapping,
 	 * to entering this loop.
 	 */
 	if (--wbc->nr_to_write <= 0 && wbc->sync_mode == WB_SYNC_NONE)
-		return writeback_finish(mapping, wbc, true);
+		goto done;
 
 	return writeback_get_folio(mapping, wbc);
+done:
+	writeback_finish(mapping, wbc, true);
+	return NULL;
 }
 
 /**