dm crypt: always disable discard_zeroes_data

If optional discard support in dm-crypt is enabled, discards requests
bypass the crypt queue and blocks of the underlying device are discarded.
For the read path, discarded blocks are handled the same as normal
ciphertext blocks, thus decrypted.

So if the underlying device announces discarded regions return zeroes,
dm-crypt must disable this flag because after decryption there is just
random noise instead of zeroes.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

2 files changed, 21 insertions, 0 deletions

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 49da55c1528a..8c2a000cf3f5 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -1698,6 +1698,8 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
         }
 
         ti->num_flush_requests = 1;
+        ti->discard_zeroes_data_unsupported = 1;
+
         return 0;
 
 bad:
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 322669807077..bc04518e9d8b 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -1283,6 +1283,22 @@ static bool dm_table_supports_flush(struct dm_table *t, unsigned flush)
         return 0;
 }
 
+static bool dm_table_discard_zeroes_data(struct dm_table *t)
+{
+        struct dm_target *ti;
+        unsigned i = 0;
+
+        /* Ensure that all targets supports discard_zeroes_data. */
+        while (i < dm_table_get_num_targets(t)) {
+                ti = dm_table_get_target(t, i++);
+
+                if (ti->discard_zeroes_data_unsupported)
+                        return 0;
+        }
+
+        return 1;
+}
+
 void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
                                struct queue_limits *limits)
 {
@@ -1305,6 +1321,9 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
         }
         blk_queue_flush(q, flush);
 
+        if (!dm_table_discard_zeroes_data(t))
+                q->limits.discard_zeroes_data = 0;
+
         dm_table_set_integrity(t);
 
         /*