12701 segspt_minfree needs right-sizing
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Patrick Mooney <pmooney@pfmooney.com>

@@ -18,11 +18,11 @@
  *
  * CDDL HEADER END
  */
 /*
  * Copyright (c) 1993, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright 2018 Joyent, Inc.
+ * Copyright 2019 Joyent, Inc.
  * Copyright (c) 2016 by Delphix. All rights reserved.
  */
 
 #include <sys/param.h>
 #include <sys/user.h>

@@ -58,21 +58,14 @@
  * # pages used for spt
  */
 size_t  spt_used;
 
 /*
- * segspt_minfree is the memory left for system after ISM
- * locked its pages; it is set up to 5% of availrmem in
- * sptcreate when ISM is created.  ISM should not use more
- * than ~90% of availrmem; if it does, then the performance
- * of the system may decrease. Machines with large memories may
- * be able to use up more memory for ISM so we set the default
- * segspt_minfree to 5% (which gives ISM max 95% of availrmem.
- * If somebody wants even more memory for ISM (risking hanging
- * the system) they can patch the segspt_minfree to smaller number.
+ * See spt_setminfree().
  */
 pgcnt_t segspt_minfree = 0;
+size_t segspt_minfree_clamp = (1UL << 30); /* 1GB in bytes */
 
 static int segspt_create(struct seg **segpp, void *argsp);
 static int segspt_unmap(struct seg *seg, caddr_t raddr, size_t ssize);
 static void segspt_free(struct seg *seg);
 static void segspt_free_pages(struct seg *seg, caddr_t addr, size_t len);

@@ -314,13 +307,37 @@
 static int segspt_reclaim(void *, caddr_t, size_t, struct page **,
                 enum seg_rw, int);
 static int spt_anon_getpages(struct seg *seg, caddr_t addr, size_t len,
                 page_t **ppa);
 
+/*
+ * This value corresponds to headroom in availrmem that ISM can never allocate
+ * (but others can).  The original intent here was to prevent ISM from locking
+ * all of the remaining availrmem into memory, making forward progress
+ * difficult. It's not clear how much this matters on modern systems.
+ *
+ * The traditional default value of 5% of total memory is used, except on
+ * systems where that quickly gets ridiculous: in that case we clamp at a rather
+ * arbitrary value of 1GB.
+ *
+ * Note that since this is called lazily on the first sptcreate(), in theory,
+ * this could represent a very small value if the system is heavily loaded
+ * already. In practice, the first ISM user is pretty likely to come along
+ * earlier during the system's operation.
+ *
+ * This never gets re-figured.
+ */
+static void
+spt_setminfree(void)
+{
+        segspt_minfree = availrmem / 20;
 
+        if (segspt_minfree_clamp != 0 &&
+            segspt_minfree > (segspt_minfree_clamp / PAGESIZE))
+                segspt_minfree = segspt_minfree_clamp / PAGESIZE;
+}
 
-/*ARGSUSED*/
 int
 sptcreate(size_t size, struct seg **sptseg, struct anon_map *amp,
     uint_t prot, uint_t flags, uint_t share_szc)
 {
         int     err;

@@ -329,12 +346,12 @@
 
 #ifdef DEBUG
         TNF_PROBE_1(sptcreate, "spt", /* CSTYLED */,
                         tnf_ulong, size, size );
 #endif
-        if (segspt_minfree == 0)        /* leave min 5% of availrmem for */
-                segspt_minfree = availrmem/20;  /* for the system */
+        if (segspt_minfree == 0)
+                spt_setminfree();
 
         if (!hat_supported(HAT_SHARED_PT, (void *)0))
                 return (EINVAL);
 
         /*