Clean up OS-specific ARC and kmem code

OS-specific code (e.g. under `module/os/linux`) does not need to share
its code structure with any other operating systems.  In particular, the
ARC and kmem code need not be similar to the code in illumos, because we
won't be syncing this OS-specific code between operating systems.  For
example, if/when illumos support is added to the common repo, we would
add a file `module/os/illumos/zfs/arc_os.c` for the illumos versions of
this code.

Therefore, we can simplify the code in the OS-specific ARC and kmem
routines.

These changes do not impact system behavior, they are purely code
cleanup.  The changes are:

Arenas are not used on Linux or FreeBSD (they are always `NULL`), so
`heap_arena`, `zio_arena`, and `zio_alloc_arena` can be removed, along
with code that uses them.

In `arc_available_memory()`:
 * `desfree` is unused, remove it
 * rename `freemem` to avoid conflict with pre-existing `#define`
 * remove checks related to arenas
 * use units of bytes, rather than converting from bytes to pages and
   then back to bytes

`SPL_KMEM_CACHE_REAP` is unused, remove it.

`skc_reap` is unused, remove it.

The `count` argument to `spl_kmem_cache_reap_now()` is unused, remove
it.

`vmem_size()` and associated type and macros are unused, remove them.

In `arc_memory_throttle()`, use a less confusing variable name to store
the result of `arc_free_memory()`.

Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10499

include/os/freebsd/spl/sys/kmem.h

@@ -90,13 +90,10 @@ void *calloc(size_t n, size_t s);
 #define	kmem_cache_reap_now kmem_cache_reap_soon
 #define	freemem				vm_free_count()
 #define	minfree				vm_cnt.v_free_min
-#define	heap_arena			kernel_arena
-#define	zio_arena			NULL
 #define	kmem_alloc(size, kmflags)	zfs_kmem_alloc((size), (kmflags))
 #define	kmem_zalloc(size, kmflags)				\
 	zfs_kmem_alloc((size), (kmflags) | M_ZERO)
 #define	kmem_free(buf, size)		zfs_kmem_free((buf), (size))
-#define	vmem_qcache_reap(ptr)	((void)0)
 
 
 #endif	/* _OPENSOLARIS_SYS_KMEM_H_ */

include/os/linux/spl/sys/kmem_cache.h

@@ -100,7 +100,6 @@ extern struct rw_semaphore spl_kmem_cache_sem;
 #define	SKC_MAGIC			0x2c2c2c2c
 
 #define	SPL_KMEM_CACHE_DELAY		15	/* Minimum slab release age */
-#define	SPL_KMEM_CACHE_REAP		0	/* Default reap everything */
 #define	SPL_KMEM_CACHE_OBJ_PER_SLAB	8	/* Target objects per slab */
 #define	SPL_KMEM_CACHE_OBJ_PER_SLAB_MIN	1	/* Minimum objects per slab */
 #define	SPL_KMEM_CACHE_ALIGN		8	/* Default object alignment */
@@ -185,7 +184,6 @@ typedef struct spl_kmem_cache {
 	uint32_t		skc_slab_objs;	/* Objects per slab */
 	uint32_t		skc_slab_size;	/* Slab size */
 	uint32_t		skc_delay;	/* Slab reclaim interval */
-	uint32_t		skc_reap;	/* Slab reclaim count */
 	atomic_t		skc_ref;	/* Ref count callers */
 	taskqid_t		skc_taskqid;	/* Slab reclaim task */
 	struct list_head	skc_list;	/* List of caches linkage */
@@ -219,7 +217,7 @@ extern void spl_kmem_cache_destroy(spl_kmem_cache_t *skc);
 extern void *spl_kmem_cache_alloc(spl_kmem_cache_t *skc, int flags);
 extern void spl_kmem_cache_free(spl_kmem_cache_t *skc, void *obj);
 extern void spl_kmem_cache_set_allocflags(spl_kmem_cache_t *skc, gfp_t flags);
-extern void spl_kmem_cache_reap_now(spl_kmem_cache_t *skc, int count);
+extern void spl_kmem_cache_reap_now(spl_kmem_cache_t *skc);
 extern void spl_kmem_reap(void);
 extern uint64_t spl_kmem_cache_inuse(kmem_cache_t *cache);
 extern uint64_t spl_kmem_cache_entry_size(kmem_cache_t *cache);
@@ -230,8 +228,7 @@ extern uint64_t spl_kmem_cache_entry_size(kmem_cache_t *cache);
 #define	kmem_cache_destroy(skc)		spl_kmem_cache_destroy(skc)
 #define	kmem_cache_alloc(skc, flags)	spl_kmem_cache_alloc(skc, flags)
 #define	kmem_cache_free(skc, obj)	spl_kmem_cache_free(skc, obj)
-#define	kmem_cache_reap_now(skc)	\
-    spl_kmem_cache_reap_now(skc, skc->skc_reap)
+#define	kmem_cache_reap_now(skc)	spl_kmem_cache_reap_now(skc)
 #define	kmem_reap()			spl_kmem_reap()
 
 /*

include/os/linux/spl/sys/vmem.h

@@ -31,12 +31,6 @@
 
 typedef struct vmem { } vmem_t;
 
-extern vmem_t *heap_arena;
-extern vmem_t *zio_alloc_arena;
-extern vmem_t *zio_arena;
-
-extern size_t vmem_size(vmem_t *vmp, int typemask);
-
 /*
  * Memory allocation interfaces
  */
@@ -97,7 +91,6 @@ extern size_t vmem_size(vmem_t *vmp, int typemask);
 #define	vmem_alloc(sz, fl)	spl_vmem_alloc((sz), (fl), __func__, __LINE__)
 #define	vmem_zalloc(sz, fl)	spl_vmem_zalloc((sz), (fl), __func__, __LINE__)
 #define	vmem_free(ptr, sz)	spl_vmem_free((ptr), (sz))
-#define	vmem_qcache_reap(ptr)	((void)0)
 
 extern void *spl_vmem_alloc(size_t sz, int fl, const char *func, int line);
 extern void *spl_vmem_zalloc(size_t sz, int fl, const char *func, int line);