Linux 6.10: work harder to avoid kmem_cache_alloc reuse

Linux 6.10 change kmem_cache_alloc to be a macro, rather than a
function, such that the old #undef for it in spl-kmem-cache.c would
remove its definition completely, breaking the build.

This inverts the model used before. Rather than always defining the
kmem_cache_* macro, then undefining then inside spl-kmem-cache.c,
instead we make a special tag to indicate we're currently inside
spl-kmem-cache.c, and not defining those in macros in the first place,
so we can use the kernel-supplied kmem_cache_* functions to implement
spl_kmem_cache_*, as we expect.

For all other callers, we create the macros as normal and remove access
to the kernel's own conflicting names.

Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>

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

@@ -192,22 +192,25 @@ 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);
 
+#ifndef	SPL_KMEM_CACHE_IMPLEMENTING
+/*
+ * Macros for the kmem_cache_* API expected by ZFS and SPL clients. We don't
+ * define them inside spl-kmem-cache.c, as that uses the kernel's incompatible
+ * kmem_cache_* facilities to implement ours.
+ */
+
+/* Avoid conflicts with kernel names that might be implemented as macros. */
+#undef	kmem_cache_alloc
+
 #define	kmem_cache_create(name, size, align, ctor, dtor, rclm, priv, vmp, fl) \
     spl_kmem_cache_create(name, size, align, ctor, dtor, rclm, priv, vmp, fl)
 #define	kmem_cache_set_move(skc, move)	spl_kmem_cache_set_move(skc, move)
 #define	kmem_cache_destroy(skc)		spl_kmem_cache_destroy(skc)
-/*
- * This is necessary to be compatible with other kernel modules
- * or in-tree filesystem that may define kmem_cache_alloc,
- * like bcachefs does it now.
- */
-#ifdef kmem_cache_alloc
-#undef kmem_cache_alloc
-#endif
 #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)
 #define	kmem_reap()			spl_kmem_reap()
+#endif
 
 /*
  * The following functions are only available for internal use.

module/os/linux/spl/spl-kmem-cache.c

@@ -21,6 +21,8 @@
  *  with the SPL.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#define	SPL_KMEM_CACHE_IMPLEMENTING
+
 #include <linux/percpu_compat.h>
 #include <sys/kmem.h>
 #include <sys/kmem_cache.h>
@@ -33,16 +35,6 @@
 #include <linux/swap.h>
 #include <linux/prefetch.h>
 
-/*
- * Within the scope of spl-kmem.c file the kmem_cache_* definitions
- * are removed to allow access to the real Linux slab allocator.
- */
-#undef kmem_cache_destroy
-#undef kmem_cache_create
-#undef kmem_cache_alloc
-#undef kmem_cache_free
-
-
 /*
  * Linux 3.16 replaced smp_mb__{before,after}_{atomic,clear}_{dec,inc,bit}()
  * with smp_mb__{before,after}_atomic() because they were redundant. This is