iomap: treat a write through cache the same as FUA

Whether we have a write back cache and are using FUA or don't have
a write back cache at all is the same situation. Treat them the same.

This makes the IOMAP_DIO_WRITE_FUA name a bit misleading, as we have
two cases that provide stable writes:

1) Volatile write cache with FUA writes
2) Normal write without a volatile write cache

Rename that flag to IOMAP_DIO_STABLE_WRITE to make that clearer, and
update some of the FUA comments as well.
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/iomap/direct-io.c

@@ -20,7 +20,7 @@
  * Private flags for iomap_dio, must not overlap with the public ones in
  * iomap.h:
  */
-#define IOMAP_DIO_WRITE_FUA	(1U << 28)
+#define IOMAP_DIO_WRITE_THROUGH	(1U << 28)
 #define IOMAP_DIO_NEED_SYNC	(1U << 29)
 #define IOMAP_DIO_WRITE		(1U << 30)
 #define IOMAP_DIO_DIRTY		(1U << 31)
@@ -219,7 +219,7 @@ static void iomap_dio_zero(const struct iomap_iter *iter, struct iomap_dio *dio,
 /*
  * Figure out the bio's operation flags from the dio request, the
  * mapping, and whether or not we want FUA.  Note that we can end up
- * clearing the WRITE_FUA flag in the dio request.
+ * clearing the WRITE_THROUGH flag in the dio request.
  */
 static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
 		const struct iomap *iomap, bool use_fua)
@@ -233,7 +233,7 @@ static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
 	if (use_fua)
 		opflags |= REQ_FUA;
 	else
-		dio->flags &= ~IOMAP_DIO_WRITE_FUA;
+		dio->flags &= ~IOMAP_DIO_WRITE_THROUGH;
 
 	return opflags;
 }
@@ -273,11 +273,13 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
 		 * Use a FUA write if we need datasync semantics, this is a pure
 		 * data IO that doesn't require any metadata updates (including
 		 * after IO completion such as unwritten extent conversion) and
-		 * the underlying device supports FUA. This allows us to avoid
-		 * cache flushes on IO completion.
+		 * the underlying device either supports FUA or doesn't have
+		 * a volatile write cache. This allows us to avoid cache flushes
+		 * on IO completion.
 		 */
 		if (!(iomap->flags & (IOMAP_F_SHARED|IOMAP_F_DIRTY)) &&
-		    (dio->flags & IOMAP_DIO_WRITE_FUA) && bdev_fua(iomap->bdev))
+		    (dio->flags & IOMAP_DIO_WRITE_THROUGH) &&
+		    (bdev_fua(iomap->bdev) || !bdev_write_cache(iomap->bdev)))
 			use_fua = true;
 	}
 
@@ -553,13 +555,16 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 			dio->flags |= IOMAP_DIO_NEED_SYNC;
 
 		       /*
-			* For datasync only writes, we optimistically try
-			* using FUA for this IO.  Any non-FUA write that
-			* occurs will clear this flag, hence we know before
-			* completion whether a cache flush is necessary.
+			* For datasync only writes, we optimistically try using
+			* WRITE_THROUGH for this IO. This flag requires either
+			* FUA writes through the device's write cache, or a
+			* normal write to a device without a volatile write
+			* cache. For the former, Any non-FUA write that occurs
+			* will clear this flag, hence we know before completion
+			* whether a cache flush is necessary.
 			*/
 			if (!(iocb->ki_flags & IOCB_SYNC))
-				dio->flags |= IOMAP_DIO_WRITE_FUA;
+				dio->flags |= IOMAP_DIO_WRITE_THROUGH;
 		}
 
 		/*
@@ -621,10 +626,11 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 		iomap_dio_set_error(dio, ret);
 
 	/*
-	 * If all the writes we issued were FUA, we don't need to flush the
-	 * cache on IO completion. Clear the sync flag for this case.
+	 * If all the writes we issued were already written through to the
+	 * media, we don't need to flush the cache on IO completion. Clear the
+	 * sync flag for this case.
 	 */
-	if (dio->flags & IOMAP_DIO_WRITE_FUA)
+	if (dio->flags & IOMAP_DIO_WRITE_THROUGH)
 		dio->flags &= ~IOMAP_DIO_NEED_SYNC;
 
 	WRITE_ONCE(iocb->private, dio->submit.poll_bio);