[PATCH v4 21/22] netfs, afs: Fix write skipping in dir/link writepages

[David Howells <dhowells@redhat.com>]

Fix netfs_write_single() and afs_single_writepages() to better handle a
write that would be skipped due to lock contention and WB_SYNC_NONE by
returning 1 from netfs_write_single() if it skipped and making
afs_single_writepages() skip also.

This is really only theoretical for directories in netfs_write_single() as
the only path to that is through afs_single_writepages() that takes the
->validate_lock around it, thereby serialising it.

Fixes: 6dd80936618c ("afs: Use netfslib for directories")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
---
 fs/afs/dir.c           | 8 +++++++-
 fs/netfs/write_issue.c | 5 ++++-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/afs/dir.c b/fs/afs/dir.c
index aaaa55878ffd..3379d01c72fb 100644
--- a/fs/afs/dir.c
+++ b/fs/afs/dir.c
@@ -2206,7 +2206,11 @@ int afs_single_writepages(struct address_space *mapping,
 	/* Need to lock to prevent the folio queue and folios from being thrown
 	 * away.
 	 */
-	down_read(&dvnode->validate_lock);
+	if (!down_read_trylock(&dvnode->validate_lock)) {
+		if (wbc->sync_mode == WB_SYNC_NONE)
+			return 0;
+		down_read(&dvnode->validate_lock);
+	}
 
 	if (is_dir ?
 	    test_bit(AFS_VNODE_DIR_VALID, &dvnode->flags) :
@@ -2214,6 +2218,8 @@ int afs_single_writepages(struct address_space *mapping,
 		iov_iter_folio_queue(&iter, ITER_SOURCE, dvnode->directory, 0, 0,
 				     i_size_read(&dvnode->netfs.inode));
 		ret = netfs_writeback_single(mapping, wbc, &iter);
+		if (ret == 1)
+			ret = 0; /* Skipped write due to lock conflict. */
 	}
 
 	up_read(&dvnode->validate_lock);
diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index b0e9690bb90c..60525cf6e679 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -819,6 +819,9 @@ static int netfs_write_folio_single(struct netfs_io_request *wreq,
  *
  * Write a monolithic, non-pagecache object back to the server and/or
  * the cache.
+ *
+ * Return: 0 if successful; 1 if skipped due to lock conflict and WB_SYNC_NONE;
+ * or a negative error code.
  */
 int netfs_writeback_single(struct address_space *mapping,
 			   struct writeback_control *wbc,
@@ -836,7 +839,7 @@ int netfs_writeback_single(struct address_space *mapping,
 	if (!mutex_trylock(&ictx->wb_lock)) {
 		if (wbc->sync_mode == WB_SYNC_NONE) {
 			netfs_stat(&netfs_n_wb_lock_skip);
-			return 0;
+			return 1;
 		}
 		netfs_stat(&netfs_n_wb_lock_wait);
 		mutex_lock(&ictx->wb_lock);