We need to commit dirty metadata and pages to disk
before remounting exfat as read-only.

This fixes a failure in xfstests generic/452

generic/452 does the following:
cp something <exfat>/
mount -o remount,ro <exfat>

the <exfat>/something is corrupted. because while
exfat is remounted as read-only, exfat doesn't
have a chance to commit metadata and
vfs invalidates page caches in a block device.

Signed-off-by: Hyunchul Lee <hyc....@gmail.com>
---
Changes from v1:
- Does not check the return value of sync_filesystem to
  allow to change from "rw" to "ro" even when this function
  fails.
- Add the detailed explanation why generic/452 fails

 fs/exfat/super.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/exfat/super.c b/fs/exfat/super.c
index e650e65536f8..253a92460d52 100644
--- a/fs/exfat/super.c
+++ b/fs/exfat/super.c
@@ -693,10 +693,20 @@ static void exfat_free(struct fs_context *fc)
        }
 }
 
+static int exfat_reconfigure(struct fs_context *fc)
+{
+       fc->sb_flags |= SB_NODIRATIME;
+
+       /* volume flag will be updated in exfat_sync_fs */
+       sync_filesystem(fc->root->d_sb);
+       return 0;
+}
+
 static const struct fs_context_operations exfat_context_ops = {
        .parse_param    = exfat_parse_param,
        .get_tree       = exfat_get_tree,
        .free           = exfat_free,
+       .reconfigure    = exfat_reconfigure,
 };
 
 static int exfat_init_fs_context(struct fs_context *fc)
-- 
2.17.1

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