linux-kernel - [PATCH 08/13] writeback: Simplify the loops in write_cache

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Message-Id: <20240203071147.862076-9-hch@lst.de>
Date: Sat,  3 Feb 2024 08:11:42 +0100
From: Christoph Hellwig <hch@....de>
To: linux-mm@...ck.org
Cc: Matthew Wilcox <willy@...radead.org>,
	Jan Kara <jack@...e.com>,
	David Howells <dhowells@...hat.com>,
	Brian Foster <bfoster@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	"Darrick J. Wong" <djwong@...nel.org>,
	linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jan Kara <jack@...e.cz>,
	Dave Chinner <dchinner@...hat.com>
Subject: [PATCH 08/13] writeback: Simplify the loops in write_cache_pages()

From: "Matthew Wilcox (Oracle)" <willy@...radead.org>

Collapse the two nested loops into one.  This is needed as a step
towards turning this into an iterator.

Note that this drops the "index <= end" check in the previous outer loop
and just relies on filemap_get_folios_tag() to return 0 entries when
index > end.  This actually has a subtle implication when end == -1
because then the returned index will be -1 as well and thus if there is
page present on index -1, we could be looping indefinitely. But as the
comment in filemap_get_folios_tag documents this as already broken anyway
we should not worry about it here either.  The fix for that would
probably a change to the filemap_get_folios_tag() calling convention.

Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
[hch: updated the commit log based on feedback from Jan Kara]
Signed-off-by: Christoph Hellwig <hch@....de>
Reviewed-by: Brian Foster <bfoster@...hat.com>
Reviewed-by: Jan Kara <jack@...e.cz>
Acked-by: Dave Chinner <dchinner@...hat.com>
---
 mm/page-writeback.c | 75 ++++++++++++++++++++++-----------------------
 1 file changed, 36 insertions(+), 39 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 23363ed712f646..d7ab42def43035 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2454,6 +2454,7 @@ int write_cache_pages(struct address_space *mapping,
 	int error;
 	struct folio *folio;
 	pgoff_t end;		/* Inclusive */
+	int i = 0;
 
 	if (wbc->range_cyclic) {
 		wbc->index = mapping->writeback_index; /* prev offset */
@@ -2467,53 +2468,49 @@ int write_cache_pages(struct address_space *mapping,
 
 	folio_batch_init(&wbc->fbatch);
 
-	while (wbc->index <= end) {
-		int i;
-
-		writeback_get_batch(mapping, wbc);
-
+	for (;;) {
+		if (i == wbc->fbatch.nr) {
+			writeback_get_batch(mapping, wbc);
+			i = 0;
+		}
 		if (wbc->fbatch.nr == 0)
 			break;
 
-		for (i = 0; i < wbc->fbatch.nr; i++) {
-			folio = wbc->fbatch.folios[i];
+		folio = wbc->fbatch.folios[i++];
 
-			folio_lock(folio);
-			if (!folio_prepare_writeback(mapping, wbc, folio)) {
-				folio_unlock(folio);
-				continue;
-			}
+		folio_lock(folio);
+		if (!folio_prepare_writeback(mapping, wbc, folio)) {
+			folio_unlock(folio);
+			continue;
+		}
 
-			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
+		trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
 
-			error = writepage(folio, wbc, data);
-			wbc->nr_to_write -= folio_nr_pages(folio);
+		error = writepage(folio, wbc, data);
+		wbc->nr_to_write -= folio_nr_pages(folio);
 
-			if (error == AOP_WRITEPAGE_ACTIVATE) {
-				folio_unlock(folio);
-				error = 0;
-			}
+		if (error == AOP_WRITEPAGE_ACTIVATE) {
+			folio_unlock(folio);
+			error = 0;
+		}
 
-			/*
-			 * For integrity writeback we have to keep going until
-			 * we have written all the folios we tagged for
-			 * writeback above, even if we run past wbc->nr_to_write
-			 * or encounter errors.
-			 * We stash away the first error we encounter in
-			 * wbc->saved_err so that it can be retrieved when we're
-			 * done.  This is because the file system may still have
-			 * state to clear for each folio.
-			 *
-			 * For background writeback we exit as soon as we run
-			 * past wbc->nr_to_write or encounter the first error.
-			 */
-			if (wbc->sync_mode == WB_SYNC_ALL) {
-				if (error && !ret)
-					ret = error;
-			} else {
-				if (error || wbc->nr_to_write <= 0)
-					goto done;
-			}
+		/*
+		 * For integrity writeback we have to keep going until we have
+		 * written all the folios we tagged for writeback above, even if
+		 * we run past wbc->nr_to_write or encounter errors.
+		 * We stash away the first error we encounter in wbc->saved_err
+		 * so that it can be retrieved when we're done.  This is because
+		 * the file system may still have state to clear for each folio.
+		 *
+		 * For background writeback we exit as soon as we run past
+		 * wbc->nr_to_write or encounter the first error.
+		 */
+		if (wbc->sync_mode == WB_SYNC_ALL) {
+			if (error && !ret)
+				ret = error;
+		} else {
+			if (error || wbc->nr_to_write <= 0)
+				goto done;
 		}
 	}
 
-- 
2.39.2