dm: introduce num_flush_requests

Introduce num_flush_requests for a target to set to say how many flush
instructions (empty barriers) it wants to receive.  These are sent by
__clone_and_map_empty_barrier with map_info->flush_request going from 0
to (num_flush_requests - 1).

Old targets without flush support won't receive any flush requests.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
index 49c2362977fd..fc36a4d07723 100644
--- a/include/linux/device-mapper.h
+++ b/include/linux/device-mapper.h
@@ -21,6 +21,7 @@ typedef enum { STATUSTYPE_INFO, STATUSTYPE_TABLE } status_type_t;
 union map_info {
         void *ptr;
         unsigned long long ll;
+        unsigned flush_request;
 };
 
 /*
@@ -168,6 +169,16 @@ struct dm_target {
         sector_t split_io;
 
         /*
+         * A number of zero-length barrier requests that will be submitted
+         * to the target for the purpose of flushing cache.
+         *
+         * The request number will be placed in union map_info->flush_request.
+         * It is a responsibility of the target driver to remap these requests
+         * to the real underlying devices.
+         */
+        unsigned num_flush_requests;
+
+        /*
          * These are automatically filled in by
          * dm_table_get_device.
          */