linux-kernel - [PATCH 03/17] writeback: rework the loop termination condition in write_cache

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Message-Id: <20231218153553.807799-4-hch@lst.de>
Date: Mon, 18 Dec 2023 16:35:39 +0100
From: Christoph Hellwig <hch@....de>
To: linux-mm@...ck.org
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Jan Kara <jack@...e.com>,
	David Howells <dhowells@...hat.com>,
	Brian Foster <bfoster@...hat.com>,
	linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 03/17] writeback: rework the loop termination condition in write_cache_pages

Rework we deal with the cleanup after the writepage call.  First handle
the magic AOP_WRITEPAGE_ACTIVATE separately from real error returns to
get it out of the way of the actual error handling.  Then merge the
code to set ret for integrity vs non-integrity writeback.  For
non-integrity writeback the loop is terminated on the first error, so
ret will never be non-zero.  Then use a single block to check for
non-integrity writewack to consolidate the cases where it returns for
either an error or running off the end of nr_to_write.

Signed-off-by: Christoph Hellwig <hch@....de>
---
 mm/page-writeback.c | 62 +++++++++++++++++++++------------------------
 1 file changed, 29 insertions(+), 33 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 8e312d73475646..7ed6c2bc8dd51c 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2474,43 +2474,39 @@ int write_cache_pages(struct address_space *mapping,
 			error = writepage(folio, wbc, data);
 			nr = folio_nr_pages(folio);
 			wbc->nr_to_write -= nr;
-			if (unlikely(error)) {
-				/*
-				 * Handle errors according to the type of
-				 * writeback. There's no need to continue for
-				 * background writeback. Just push done_index
-				 * past this page so media errors won't choke
-				 * writeout for the entire file. For integrity
-				 * writeback, we must process the entire dirty
-				 * set regardless of errors because the fs may
-				 * still have state to clear for each page. In
-				 * that case we continue processing and return
-				 * the first error.
-				 */
-				if (error == AOP_WRITEPAGE_ACTIVATE) {
-					folio_unlock(folio);
-					error = 0;
-				} else if (wbc->sync_mode != WB_SYNC_ALL) {
-					ret = error;
-					done_index = folio->index + nr;
-					done = 1;
-					break;
-				}
-				if (!ret)
-					ret = error;
+
+			/*
+			 * Handle the legacy AOP_WRITEPAGE_ACTIVATE magic return
+			 * value.  Eventually all instances should just unlock
+			 * the folio themselves and return 0;
+			 */
+			if (error == AOP_WRITEPAGE_ACTIVATE) {
+				folio_unlock(folio);
+				error = 0;
 			}
 
 			/*
-			 * We stop writing back only if we are not doing
-			 * integrity sync. In case of integrity sync we have to
-			 * keep going until we have written all the pages
-			 * we tagged for writeback prior to entering this loop.
+			 * For integrity sync  we have to keep going until we
+			 * have written all the folios we tagged for writeback
+			 * prior to entering this 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.
 			 */
-			done_index = folio->index + nr;
-			if (wbc->nr_to_write <= 0 &&
-			    wbc->sync_mode == WB_SYNC_NONE) {
-				done = 1;
-				break;
+			if (error && !ret)
+				ret = error;
+			if (wbc->sync_mode == WB_SYNC_NONE) {
+				if (ret || wbc->nr_to_write <= 0) {
+					done_index = folio->index + nr;
+					done = 1;
+					break;
+				}
 			}
 		}
 		folio_batch_release(&fbatch);
-- 
2.39.2