[PATCH v2 33/34] xfs: allow remounting read-only after a shutdown

[Christian Brauner <brauner@kernel.org>]

xfs_remount_ro() starts with sync_filesystem() which fails with EIO
after a shutdown. So a shut down xfs can't be remounted read-only and
emergency remount (sysrq u) WARNs in reconfigure_super(), a panic with
panic_on_warn:

  forced remount of a xfs fs returned -5

Going read-only doesn't need to write anything. So ignore the errors
from flushing and from freeing the COW and post-EOF blocks once xfs is
shut down, also if the flush shut it down itself. The rest of
xfs_remount_ro() copes with a shutdown like unmount does. And refuse
going back to read-write with EIO, xfs_remount_rw() didn't check for a
shutdown at all.

Fixes: b97cca3ba909 ("xfs: only bother with sync_filesystem during readonly remount")
Cc: stable@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/xfs/xfs_super.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 5a06132aa384..59c83438d402 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -2041,6 +2041,10 @@ xfs_remount_rw(
 	struct xfs_sb		*sbp = &mp->m_sb;
 	int error;
 
+	/* Nothing can be written anymore after a shutdown. */
+	if (xfs_is_shutdown(mp))
+		return -EIO;
+
 	if (mp->m_logdev_targp && mp->m_logdev_targp != mp->m_ddev_targp &&
 	    xfs_readonly_buftarg(mp->m_logdev_targp)) {
 		xfs_warn(mp,
@@ -2115,9 +2119,12 @@ xfs_remount_ro(
 	};
 	int			error;
 
-	/* Flush all the dirty data to disk. */
+	/*
+	 * Flush all the dirty data to disk. That fails after a shutdown, also
+	 * one the flush itself caused, but going read-only still has to work.
+	 */
 	error = sync_filesystem(mp->m_super);
-	if (error)
+	if (error && !xfs_is_shutdown(mp))
 		return error;
 
 	/*
@@ -2133,7 +2140,7 @@ xfs_remount_ro(
 	 * cached inode, so this requires a synchronous cache scan.
 	 */
 	error = xfs_blockgc_free_space(mp, &icw);
-	if (error) {
+	if (error && !xfs_is_shutdown(mp)) {
 		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
 		return error;
 	}

-- 
2.53.0