dm thin metadata: Add support for a pre-commit callback

Add support for one pre-commit callback which is run right before the
metadata are committed.

This allows the thin provisioning target to run a callback before the
metadata are committed and is required by the next commit.

Cc: [e-mail]
Signed-off-by: Nikos Tsironis <[e-mail]>
Acked-by: Joe Thornber <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-thin-metadata.c

···
 	sector_t data_block_size;
 
 	/*
+	 * Pre-commit callback.
+	 *
+	 * This allows the thin provisioning target to run a callback before
+	 * the metadata are committed.
+	 */
+	dm_pool_pre_commit_fn pre_commit_fn;
+	void *pre_commit_context;
+
+	/*
 	 * We reserve a section of the metadata for commit overhead.
 	 * All reported space does *not* include this.
 	 */
···
 	if (unlikely(!pmd->in_service))
 		return 0;
 
+	if (pmd->pre_commit_fn) {
+		r = pmd->pre_commit_fn(pmd->pre_commit_context);
+		if (r < 0) {
+			DMERR("pre-commit callback failed");
+			return r;
+		}
+	}
+
 	r = __write_changed_details(pmd);
 	if (r < 0)
 		return r;
···
 	pmd->in_service = false;
 	pmd->bdev = bdev;
 	pmd->data_block_size = data_block_size;
+	pmd->pre_commit_fn = NULL;
+	pmd->pre_commit_context = NULL;
 
 	r = __create_persistent_data_objects(pmd, format_device);
 	if (r) {
···
 	pmd_write_unlock(pmd);
 
 	return r;
+}
+
+void dm_pool_register_pre_commit_callback(struct dm_pool_metadata *pmd,
+					  dm_pool_pre_commit_fn fn,
+					  void *context)
+{
+	pmd_write_lock_in_core(pmd);
+	pmd->pre_commit_fn = fn;
+	pmd->pre_commit_context = context;
+	pmd_write_unlock(pmd);
 }
 
 int dm_pool_metadata_set_needs_check(struct dm_pool_metadata *pmd)

drivers/md/dm-thin-metadata.h

···
  */
 void dm_pool_issue_prefetches(struct dm_pool_metadata *pmd);
 
+/* Pre-commit callback */
+typedef int (*dm_pool_pre_commit_fn)(void *context);
+
+void dm_pool_register_pre_commit_callback(struct dm_pool_metadata *pmd,
+					  dm_pool_pre_commit_fn fn,
+					  void *context);
+
 /*----------------------------------------------------------------*/
 
 #endif