During pool export flush the ARC asynchronously

This also includes removing L2 vdevs asynchronously.

This commit also guarantees that spa_load_guid is unique.

The zpool reguid feature introduced the spa_load_guid, which is a
transient value used for runtime identification purposes in the ARC.
This value is not the same as the spa's persistent pool guid.

However, the value is seeded from spa_generate_load_guid() which
does not check for uniqueness against the spa_load_guid from other
pools.  Although extremely rare, you can end up with two different
pools sharing the same spa_load_guid value! So we guarantee that
the value is always unique and additionally not still in use by an
async arc flush task.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes openzfs#16215

Author: don-brady

include/sys/arc.h

@@ -63,8 +63,15 @@ extern "C" {
 	(hdr)->b_psize = ((x) >> SPA_MINBLOCKSHIFT); \
 } while (0)
 
+/* The l2size in the header is only used by L2 cache */
+#define	HDR_SET_L2SIZE(hdr, x) do { \
+	ASSERT(IS_P2ALIGNED((x), 1U << SPA_MINBLOCKSHIFT)); \
+	(hdr)->b_l2size = ((x) >> SPA_MINBLOCKSHIFT); \
+} while (0)
+
 #define	HDR_GET_LSIZE(hdr)	((hdr)->b_lsize << SPA_MINBLOCKSHIFT)
 #define	HDR_GET_PSIZE(hdr)	((hdr)->b_psize << SPA_MINBLOCKSHIFT)
+#define	HDR_GET_L2SIZE(hdr)	((hdr)->b_l2size << SPA_MINBLOCKSHIFT)
 
 typedef struct arc_buf_hdr arc_buf_hdr_t;
 typedef struct arc_buf arc_buf_t;
@@ -322,8 +329,10 @@ void arc_freed(spa_t *spa, const blkptr_t *bp);
 int arc_cached(spa_t *spa, const blkptr_t *bp);
 
 void arc_flush(spa_t *spa, boolean_t retry);
+void arc_flush_async(spa_t *spa);
 void arc_tempreserve_clear(uint64_t reserve);
 int arc_tempreserve_space(spa_t *spa, uint64_t reserve, uint64_t txg);
+boolean_t arc_async_flush_guid_inuse(uint64_t load_guid);
 
 uint64_t arc_all_memory(void);
 uint64_t arc_default_max(uint64_t min, uint64_t allmem);

include/sys/arc_impl.h

@@ -378,8 +378,8 @@ typedef struct l2arc_lb_ptr_buf {
  * L2ARC Internals
  */
 typedef struct l2arc_dev {
-	vdev_t			*l2ad_vdev;	/* vdev */
-	spa_t			*l2ad_spa;	/* spa */
+	vdev_t			*l2ad_vdev;	/* can be NULL during remove */
+	spa_t			*l2ad_spa;	/* can be NULL during remove */
 	uint64_t		l2ad_hand;	/* next write location */
 	uint64_t		l2ad_start;	/* first addr on device */
 	uint64_t		l2ad_end;	/* last addr on device */
@@ -475,8 +475,8 @@ struct arc_buf_hdr {
 
 	arc_buf_contents_t	b_type;
 	uint8_t			b_complevel;
-	uint8_t			b_reserved1; /* used for 4 byte alignment */
-	uint16_t		b_reserved2; /* used for 4 byte alignment */
+	uint8_t			b_reserved1;	/* used for 4 byte alignment */
+	uint16_t		b_l2size;	/* alignment or L2-only size */
 	arc_buf_hdr_t		*b_hash_next;
 	arc_flags_t		b_flags;
 

include/sys/spa.h

@@ -1106,6 +1106,7 @@ extern boolean_t spa_guid_exists(uint64_t pool_guid, uint64_t device_guid);
 extern char *spa_strdup(const char *);
 extern void spa_strfree(char *);
 extern uint64_t spa_generate_guid(spa_t *spa);
+extern uint64_t spa_generate_load_guid(void);
 extern void snprintf_blkptr(char *buf, size_t buflen, const blkptr_t *bp);
 extern void spa_freeze(spa_t *spa);
 extern int spa_change_guid(spa_t *spa, const uint64_t *guidp);