[PATCH v2] mm: don't break integrity writeback on ->writepage() error

[Brian Foster <bfoster@redhat.com>]

The write_cache_pages() function is used in both background and
integrity writeback scenarios by various filesystems. Background
writeback is mostly concerned with cleaning a certain number of
dirty pages based on various mm heuristics. It may not write the
full set of dirty pages or wait for I/O to complete. Integrity
writeback is responsible for persisting a set of dirty pages before
the writeback job completes. For example, an fsync() call must
perform integrity writeback to ensure data is on disk before the
call returns.

write_cache_pages() unconditionally breaks out of its processing
loop in the event of a ->writepage() error. This is fine for
background writeback, which had no strict requirements and will
eventually come around again. This can cause problems for integrity
writeback on filesystems that might need to clean up state
associated with failed page writeouts. For example, XFS performs
internal delayed allocation accounting before returning a
->writepage() error, where applicable. If the current writeback
happens to be associated with an unmount and write_cache_pages()
completes the writeback prematurely due to error, the filesystem is
unmounted in an inconsistent state if dirty+delalloc pages still
exist.

To handle this problem, update write_cache_pages() to always process
the full set of pages for integrity writeback regardless of
->writepage() errors. Save the first encountered error and return it
to the caller once complete. This facilitates XFS (or any other fs
that expects integrity writeback to process the entire set of dirty
pages) to clean up its internal state completely in the event of
persistent mapping errors. Background writeback continues to exit on
the first error encountered.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---

Here's a v2 with minor enhancenents based on Andrew Morton's feedback. I
combined the additional comments with the existing one to avoid having
too many multi-indent/multi-line comments in this area.

Brian

v2:
- Dropped unnecessary ->for_sync check.
- Added comment and updated commit log description.
v1: https://marc.info/?l=linux-fsdevel&m=154143578027082&w=2

 mm/page-writeback.c | 35 +++++++++++++++++++++--------------
 1 file changed, 21 insertions(+), 14 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 3f690bae6b78..59b4b56d3762 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2154,6 +2154,7 @@ int write_cache_pages(struct address_space *mapping,
 {
 	int ret = 0;
 	int done = 0;
+	int error;
 	struct pagevec pvec;
 	int nr_pages;
 	pgoff_t uninitialized_var(writeback_index);
@@ -2227,25 +2228,31 @@ int write_cache_pages(struct address_space *mapping,
 				goto continue_unlock;
 
 			trace_wbc_writepage(wbc, inode_to_bdi(mapping->host));
-			ret = (*writepage)(page, wbc, data);
-			if (unlikely(ret)) {
-				if (ret == AOP_WRITEPAGE_ACTIVATE) {
+			error = (*writepage)(page, wbc, data);
+			if (unlikely(error)) {
+				/*
+				 * Handle errors according to the type of
+				 * writeback. There's no need to continue to 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) {
 					unlock_page(page);
-					ret = 0;
-				} else {
-					/*
-					 * done_index is set past this page,
-					 * so media errors will not choke
-					 * background writeout for the entire
-					 * file. This has consequences for
-					 * range_cyclic semantics (ie. it may
-					 * not be suitable for data integrity
-					 * writeout).
-					 */
+					error = 0;
+				} else if (wbc->sync_mode != WB_SYNC_ALL) {
+					ret = error;
 					done_index = page->index + 1;
 					done = 1;
 					break;
 				}
+				if (!ret)
+					ret = error;
 			}
 
 			/*
-- 
2.17.2