[PATCH] vfs, syncfs: Do not ignore return code from ->sync_fs()

[Vivek Goyal <vgoyal@redhat.com>]

I see that current implementation of __sync_filesystem() ignores the
return code from ->sync_fs(). I am not sure why that's the case.

Ignoring ->sync_fs() return code is problematic for overlayfs where
it can return error if sync_filesystem() on upper super block failed.
That error will simply be lost and sycnfs(overlay_fd), will get
success (despite the fact it failed).

I am assuming that we want to continue to call __sync_blockdev()
despite the fact that there have been errors reported from
->sync_fs(). So I wrote this simple patch which captures the
error from ->sync_fs() but continues to call __sync_blockdev()
and returns error from sync_fs() if there is one.

There might be some very good reasons to not capture ->sync_fs()
return code, I don't know. Hence thought of proposing this patch.
Atleast I will get to know the reason. I still need to figure
a way out how to propagate overlay sync_fs() errors to user
space.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 fs/sync.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

Index: redhat-linux/fs/sync.c
===================================================================
--- redhat-linux.orig/fs/sync.c	2020-12-16 09:15:49.831565653 -0500
+++ redhat-linux/fs/sync.c	2020-12-16 09:23:42.499853207 -0500
@@ -30,14 +30,18 @@
  */
 static int __sync_filesystem(struct super_block *sb, int wait)
 {
+	int ret, ret2;
+
 	if (wait)
 		sync_inodes_sb(sb);
 	else
 		writeback_inodes_sb(sb, WB_REASON_SYNC);
 
 	if (sb->s_op->sync_fs)
-		sb->s_op->sync_fs(sb, wait);
-	return __sync_blockdev(sb->s_bdev, wait);
+		ret = sb->s_op->sync_fs(sb, wait);
+	ret2 = __sync_blockdev(sb->s_bdev, wait);
+
+	return ret ? ret : ret2;
 }
 
 /*