[PATCH 3/8] iomap: treat a write through cache the same as FUA

Jens Axboe
2023-07-24T22:55:22Z

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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
fs/iomap/direct-io.c | 34 ++++++++++++++++++++--------------
1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 7d627d43d10b..6b690fc22365 100644
--- a/fs/iomap/direct-io.c
+++ b/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);
--
2.40.1