Index: sys/fs/msdosfs/fat.h
===================================================================
--- sys/fs/msdosfs/fat.h
+++ sys/fs/msdosfs/fat.h
@@ -103,7 +103,13 @@
 int freeclusterchain(struct msdosfsmount *pmp, u_long startchain);
 int extendfile(struct denode *dep, u_long count, struct buf **bpp, u_long *ncp, int flags);
 void fc_purge(struct denode *dep, u_int frcn);
-int markvoldirty(struct msdosfsmount *pmp, int dirty);
+int markvoldirty_upgrade(struct msdosfsmount *pmp, bool dirty, bool rw_upgrade);
+
+static inline int
+markvoldirty(struct msdosfsmount *pmp, bool dirty)
+{
+	return (markvoldirty_upgrade(pmp, dirty, false));
+}
 
 #endif	/* _KERNEL || MAKEFS */
 #endif	/* !_FS_MSDOSFS_FAT_H_ */
Index: sys/fs/msdosfs/msdosfs_fat.c
===================================================================
--- sys/fs/msdosfs/msdosfs_fat.c
+++ sys/fs/msdosfs/msdosfs_fat.c
@@ -1121,7 +1121,7 @@
  *	?	(other errors from called routines)
  */
 int
-markvoldirty(struct msdosfsmount *pmp, int dirty)
+markvoldirty_upgrade(struct msdosfsmount *pmp, bool dirty, bool rw_upgrade)
 {
 	struct buf *bp;
 	u_long bn, bo, bsize, byteoffset, fatval;
@@ -1134,8 +1134,11 @@
 	if (FAT12(pmp))
 		return (0);
 
-	/* Can't change the bit on a read-only filesystem. */
-	if (pmp->pm_flags & MSDOSFSMNT_RONLY)
+	/*
+	 * Can't change the bit on a read-only filesystem, except as part of
+	 * ro->rw upgrade.
+	 */
+	if ((pmp->pm_flags & MSDOSFSMNT_RONLY) != 0 && !rw_upgrade)
 		return (EROFS);
 
 	/*
@@ -1145,10 +1148,8 @@
 	byteoffset = FATOFS(pmp, 1);
 	fatblock(pmp, byteoffset, &bn, &bsize, &bo);
 	error = bread(pmp->pm_devvp, bn, bsize, NOCRED, &bp);
-	if (error) {
-		brelse(bp);
+	if (error)
 		return (error);
-	}
 
 	/*
 	 * Get the current value of the FAT entry and set/clear the relevant
@@ -1174,5 +1175,24 @@
 	}
 
 	/* Write out the modified FAT block synchronously. */
-	return (bwrite(bp));
+	error = bwrite(bp);
+	if (error != 0) {
+		int terror;
+		/*
+		 * Get the block back and discard it without write to disk.
+		 *
+		 * If we're now discovering that this media is readonly (the
+		 * WriteProtect discovery earlier didn't discover this info) or
+		 * just gone, throw away our unimportant attempt to modify the
+		 * clean bit and bail higher up the stack (failing mount, or
+		 * failing RO -> RW transition, at least).  See PR 210316 for
+		 * more context.
+		 */
+		terror = getblkx(pmp->pm_devvp, bn, bsize, 0, 0, GB_UNMAPPED,
+		    &bp);
+		KASSERT(terror == 0, ("getblkx: %d for just written block %lu "
+		    "(handling failed write %d)\n", terror, bn, error));
+		bfinval(bp);
+	}
+	return (error);
 }
Index: sys/fs/msdosfs/msdosfs_vfsops.c
===================================================================
--- sys/fs/msdosfs/msdosfs_vfsops.c
+++ sys/fs/msdosfs/msdosfs_vfsops.c
@@ -311,16 +311,25 @@
 			if (error)
 				return (error);
 
+			/* Now that the volume is modifiable, mark it dirty. */
+			error = markvoldirty_upgrade(pmp, true, true);
+			if (error) {
+				/*
+				 * If dirtying the superblock failed, drop GEOM
+				 * 'w' refs (we're still RO).
+				 */
+				g_topology_lock();
+				(void)g_access(pmp->pm_cp, 0, -1, 0);
+				g_topology_unlock();
+
+				return (error);
+			}
+
 			pmp->pm_fmod = 1;
 			pmp->pm_flags &= ~MSDOSFSMNT_RONLY;
 			MNT_ILOCK(mp);
 			mp->mnt_flag &= ~MNT_RDONLY;
 			MNT_IUNLOCK(mp);
-
-			/* Now that the volume is modifiable, mark it dirty. */
-			error = markvoldirty(pmp, 1);
-			if (error)
-				return (error);
 		}
 	}
 	/*
@@ -701,10 +710,8 @@
 	if (ronly)
 		pmp->pm_flags |= MSDOSFSMNT_RONLY;
 	else {
-		if ((error = markvoldirty(pmp, 1)) != 0) {
-			(void)markvoldirty(pmp, 0);
+		if ((error = markvoldirty(pmp, 1)) != 0)
 			goto error_exit;
-		}
 		pmp->pm_fmod = 1;
 	}
 	mp->mnt_data =  pmp;
Index: sys/kern/vfs_bio.c
===================================================================
--- sys/kern/vfs_bio.c
+++ sys/kern/vfs_bio.c
@@ -2488,6 +2488,31 @@
 	bp->b_flags &= ~B_DEFERRED;
 }
 
+/*
+ *	bfinval:
+ *
+ *	Forced invalidation of dirty buffer contents.  The buffer is
+ *	invalidated and released.
+ */
+void
+bfinval(struct buf *bp)
+{
+	CTR3(KTR_BUF, "bfinval(%p) vp %p flags %X", bp, bp->b_vp, bp->b_flags);
+
+	/*
+	 * Remove from vnode's dirty list and adjust numdirtybuffers
+	 * accounting.
+	 */
+	bundirty(bp);
+
+	/*
+	 * Fire it into the sun.
+	 */
+	bp->b_flags |= B_INVAL | B_RELBUF | B_NOCACHE;
+	bp->b_flags &= ~(B_ASYNC | B_CACHE);
+	brelse(bp);
+}
+
 /*
  *	bawrite:
  *
Index: sys/sys/buf.h
===================================================================
--- sys/sys/buf.h
+++ sys/sys/buf.h
@@ -534,6 +534,7 @@
 int	bbarrierwrite(struct buf *);
 void	bdirty(struct buf *);
 void	bundirty(struct buf *);
+void	bfinval(struct buf *);
 void	bufstrategy(struct bufobj *, struct buf *);
 void	brelse(struct buf *);
 void	bqrelse(struct buf *);