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, 2 insertions, 0 deletions

diff --git a/block/blk-settings.c b/block/blk-settings.c
index 1ebc1fdb9144..dd1f1e0e196f 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -101,6 +101,7 @@ void blk_set_default_limits(struct queue_limits *lim)
         lim->discard_granularity = 0;
         lim->discard_alignment = 0;
         lim->discard_misaligned = 0;
+        lim->discard_zeroes_data = -1;
         lim->logical_block_size = lim->physical_block_size = lim->io_min = 512;
         lim->bounce_pfn = (unsigned long)(BLK_BOUNCE_ANY >> PAGE_SHIFT);
         lim->alignment_offset = 0;
@@ -544,6 +545,7 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 
         t->io_min = max(t->io_min, b->io_min);
         t->no_cluster |= b->no_cluster;
+        t->discard_zeroes_data &= b->discard_zeroes_data;
 
         /* Bottom device offset aligned? */
         if (offset &&