writeback: rework the loop termination condition in write_cache_pages

Rework the way 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 path.

The split the handling on intgrity vs non-integrity branches first, and
return early using a goto for the non-ingegrity early loop condition to
remove the need for the done and done_index local variables, and for
assigning the error to ret when we can just return error directly.

Link: https://lkml.kernel.org/r/20240215063649.2164017-7-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page-writeback.c

@@ -2396,13 +2396,12 @@ int write_cache_pages(struct address_space *mapping,
 		      void *data)
 {
 	int ret = 0;
-	int done = 0;
 	int error;
 	struct folio_batch fbatch;
+	struct folio *folio;
 	int nr_folios;
 	pgoff_t index;
 	pgoff_t end;		/* Inclusive */
-	pgoff_t done_index;
 	xa_mark_t tag;
 
 	folio_batch_init(&fbatch);
@@ -2419,8 +2418,7 @@ int write_cache_pages(struct address_space *mapping,
 	} else {
 		tag = PAGECACHE_TAG_DIRTY;
 	}
-	done_index = index;
-	while (!done && (index <= end)) {
+	while (index <= end) {
 		int i;
 
 		nr_folios = filemap_get_folios_tag(mapping, &index, end,
@@ -2430,11 +2428,7 @@ int write_cache_pages(struct address_space *mapping,
 			break;
 
 		for (i = 0; i < nr_folios; i++) {
-			struct folio *folio = fbatch.folios[i];
-			unsigned long nr;
-
-			done_index = folio->index;
-
+			folio = fbatch.folios[i];
 			folio_lock(folio);
 
 			/*
@@ -2469,45 +2463,32 @@ int write_cache_pages(struct address_space *mapping,
 
 			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
 			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;
+			wbc->nr_to_write -= folio_nr_pages(folio);
+
+			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 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.
 			 */
-			done_index = folio->index + nr;
-			if (wbc->nr_to_write <= 0 &&
-			    wbc->sync_mode == WB_SYNC_NONE) {
-				done = 1;
-				break;
+			if (wbc->sync_mode == WB_SYNC_ALL) {
+				if (error && !ret)
+					ret = error;
+			} else {
+				if (error || wbc->nr_to_write <= 0)
+					goto done;
 			}
 		}
 		folio_batch_release(&fbatch);
@@ -2524,14 +2505,15 @@ 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.
 	 */
-	if (wbc->range_cyclic) {
-		if (done)
-			mapping->writeback_index = done_index;
-		else
-			mapping->writeback_index = 0;
-	}
-
+	if (wbc->range_cyclic)
+		mapping->writeback_index = 0;
 	return ret;
+
+done:
+	if (wbc->range_cyclic)
+		mapping->writeback_index = folio->index + folio_nr_pages(folio);
+	folio_batch_release(&fbatch);
+	return error;
 }
 EXPORT_SYMBOL(write_cache_pages);