Disable direct reclaim for z_wr_* threads

The direct reclaim path in the z_wr_* threads must be disabled
to ensure forward progress is always maintained for txg processing.
This ensures that a txg will never get stuck waiting on itself
because it entered the following memory reclaim callpath.

  ->prune_icache()->dispose_list()->zpl_clear_inode()->zfs_inactive()
  ->dmu_tx_assign()->dmu_tx_wait()->tgx_wait_open()

It would be preferable to target this exact code path but the
kernel offers no way to do this without custom patches.  To avoid
this we are forced to disable all reclaim for these threads.  It
should not be necessary to do this for other other z_* threads
because they will not hold a txg open.

Closes openzfs#232

include/sys/zfs_context.h

@@ -368,6 +368,7 @@ typedef void (task_func_t)(void *);
 #define	TASKQ_DYNAMIC		0x0004	/* Use dynamic thread scheduling */
 #define	TASKQ_THREADS_CPU_PCT	0x0008	/* Scale # threads by # cpus */
 #define	TASKQ_DC_BATCH		0x0010	/* Mark threads as batch */
+#define	TASKQ_NORECLAIM		0x0020	/* Disable direct memory reclaim */
 
 #define	TQ_SLEEP	KM_SLEEP	/* Can block for memory */
 #define	TQ_NOSLEEP	KM_NOSLEEP	/* cannot block for memory; may fail */

module/zfs/spa.c

@@ -614,9 +614,8 @@ spa_get_errlists(spa_t *spa, avl_tree_t *last, avl_tree_t *scrub)
 
 static taskq_t *
 spa_taskq_create(spa_t *spa, const char *name, enum zti_modes mode,
-    uint_t value)
+    uint_t value, uint_t flags)
 {
-	uint_t flags = TASKQ_PREPOPULATE;
 	boolean_t batch = B_FALSE;
 
 	switch (mode) {
@@ -666,13 +665,17 @@ spa_create_zio_taskqs(spa_t *spa)
 			const zio_taskq_info_t *ztip = &zio_taskqs[t][q];
 			enum zti_modes mode = ztip->zti_mode;
 			uint_t value = ztip->zti_value;
+			uint_t flags = TASKQ_PREPOPULATE;
 			char name[32];
 
+			if (t == ZIO_TYPE_WRITE)
+				flags |= TASKQ_NORECLAIM;
+
 			(void) snprintf(name, sizeof (name),
 			    "%s_%s", zio_type_name[t], zio_taskq_types[q]);
 
 			spa->spa_zio_taskq[t][q] =
-			    spa_taskq_create(spa, name, mode, value);
+			    spa_taskq_create(spa, name, mode, value, flags);
 		}
 	}
 }