msdosfs & ffs: flush cache if updating mount to r/o

[Stefan Fritsch]

Other file systems can be changed later.

diff --git sys/msdosfs/msdosfs_vfsops.c sys/msdosfs/msdosfs_vfsops.c
index e13b0b1ea0b..5cd273c4087 100644
--- sys/msdosfs/msdosfs_vfsops.c
+++ sys/msdosfs/msdosfs_vfsops.c
@@ -64,6 +64,7 @@
 #include <sys/malloc.h>
 #include <sys/dirent.h>
 #include <sys/disk.h>
+#include <sys/dkio.h>
 #include <sys/stdint.h>
 
 #include <msdosfs/bpb.h>
@@ -128,8 +129,14 @@ msdosfs_mount(struct mount *mp, const char *path, void 
*data,
                        if (mp->mnt_flag & MNT_FORCE)
                                flags |= FORCECLOSE;
                        error = vflush(mp, NULLVP, flags);
-                       if (!error)
+                       if (!error) {
+                               int force = 0;
+
                                pmp->pm_flags |= MSDOSFSMNT_RONLY;
+                               /* may be not supported, ignore error */
+                               VOP_IOCTL(pmp->pm_devvp, DIOCCACHESYNC,
+                                   &force, FWRITE, FSCRED, p);
+                       }
                }
                if (!error && (mp->mnt_flag & MNT_RELOAD))
                        /* not yet implemented */
diff --git sys/ufs/ffs/ffs_vfsops.c sys/ufs/ffs/ffs_vfsops.c
index 0770e9aa45d..3714c0c09cc 100644
--- sys/ufs/ffs/ffs_vfsops.c
+++ sys/ufs/ffs/ffs_vfsops.c
@@ -456,6 +456,16 @@ success:
                                fs->fs_flags &= ~FS_DOSOFTDEP;
                }
                ffs_sbupdate(ump, MNT_WAIT);
+               if (ronly) {
+                       int force = 0;
+
+                       /*
+                        * Updating mount to readonly. Try a cache flush.
+                        * Ignore error because the ioctl may not be supported.
+                        */
+                       VOP_IOCTL(ump->um_devvp, DIOCCACHESYNC, &force,
+                           FWRITE, FSCRED, p);
+               }
        }
        return (0);