arc_get_data_buf should be more aggressive in eviction when memory is unavailable

*** 219,228 ****
--- 219,242 ----
  
          return (kmem_cache_alloc(zio_buf_cache[c], KM_PUSHPAGE));
  }
  
  /*
+  * Same as zio_buf_alloc, but won't sleep in case memory cannot be allocated
+  * and will instead return immediately with a failure.
+  */
+ void *
+ zio_buf_alloc_canfail(size_t size)
+ {
+         size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
+ 
+         ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
+ 
+         return (kmem_cache_alloc(zio_buf_cache[c], KM_NOSLEEP | KM_NORMALPRI));
+ }
+ 
+ /*
   * Use zio_data_buf_alloc to allocate data.  The data will not appear in a
   * crashdump if the kernel panics.  This exists so that we will limit the amount
   * of ZFS data that shows up in a kernel crashdump.  (Thus reducing the amount
   * of kernel heap dumped to disk when the kernel panics)
   */
*** 234,243 ****
--- 248,272 ----
          ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
  
          return (kmem_cache_alloc(zio_data_buf_cache[c], KM_PUSHPAGE));
  }
  
+ /*
+  * Same as zio_data_buf_alloc, but won't sleep in case memory cannot be
+  * allocated and will instead return immediately with a failure.
+  */
+ void *
+ zio_data_buf_alloc_canfail(size_t size)
+ {
+         size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;
+ 
+         ASSERT(c < SPA_MAXBLOCKSIZE >> SPA_MINBLOCKSHIFT);
+ 
+         return (kmem_cache_alloc(zio_data_buf_cache[c],
+             KM_NOSLEEP | KM_NORMALPRI));
+ }
+ 
  void
  zio_buf_free(void *buf, size_t size)
  {
          size_t c = (size - 1) >> SPA_MINBLOCKSHIFT;