Increase spl_kmem_alloc_warn limit

In order to support ABD with large blocks the spl_kmem_alloc_warn
limit needs to be increased to 64K.

A 16M block requires that pointers be stored for 4096 4K-pages
on an x86_64 system.  Each of these pointers is 8 bytes requiring
an allocation of 8*4096=32,768 bytes.  The addition of a small
header to this structure pushes the allocation over the default
32K warning threshold.

In addition, fix a small bug where MAX was used instead of MIN
when setting the default.  This ensures a reasonable limit is
still set on systems with page sizes larger then 4K.

Reviewed-by: David Quigley <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#571

module/spl/spl-kmem.c

@@ -35,7 +35,7 @@
  * rate limited warning will be printed to the console for any kmem_alloc()
  * which exceeds a reasonable threshold.
  *
- * The default warning threshold is set to eight pages but capped at 32K to
+ * The default warning threshold is set to sixteen pages but capped at 64K to
  * accommodate systems using large pages. This value was selected to be small
  * enough to ensure the largest allocations are quickly noticed and fixed.
  * But large enough to avoid logging any warnings when a allocation size is
@@ -44,7 +44,7 @@
  * allocations are quickly caught. These warnings may be disabled by setting
  * the threshold to zero.
  */
-unsigned int spl_kmem_alloc_warn = MAX(8 * PAGE_SIZE, 32 * 1024);
+unsigned int spl_kmem_alloc_warn = MIN(16 * PAGE_SIZE, 64 * 1024);
 module_param(spl_kmem_alloc_warn, uint, 0644);
 MODULE_PARM_DESC(spl_kmem_alloc_warn,
  "Warning threshold in bytes for a kmem_alloc()");