block: Allow devices to indicate whether discarded blocks are zeroed

The discard ioctl is used by mkfs utilities to clear a block device
prior to putting metadata down.  However, not all devices return zeroed
blocks after a discard.  Some drives return stale data, potentially
containing old superblocks.  It is therefore important to know whether
discarded blocks are properly zeroed.

Both ATA and SCSI drives have configuration bits that indicate whether
zeroes are returned after a discard operation.  Implement a block level
interface that allows this information to be bubbled up the stack and
queried via a new block device ioctl.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index e727f6c44c44..784a919aa0d0 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -322,6 +322,7 @@ struct queue_limits {
         unsigned char           misaligned;
         unsigned char           discard_misaligned;
         unsigned char           no_cluster;
+        signed char             discard_zeroes_data;
 };
 
 struct request_queue
@@ -1150,6 +1151,19 @@ static inline int queue_sector_discard_alignment(struct request_queue *q,
                 & (q->limits.discard_granularity - 1);
 }
 
+static inline unsigned int queue_discard_zeroes_data(struct request_queue *q)
+{
+        if (q->limits.discard_zeroes_data == 1)
+                return 1;
+
+        return 0;
+}
+
+static inline unsigned int bdev_discard_zeroes_data(struct block_device *bdev)
+{
+        return queue_discard_zeroes_data(bdev_get_queue(bdev));
+}
+
 static inline int queue_dma_alignment(struct request_queue *q)
 {
         return q ? q->dma_alignment : 511;