Commits on Jul 25, 2021

1. Split deep vmem_alloc()/vmem_xalloc() stacks …

   In openzfsonosx#90
   a user reported panics on an M1 with the message
   
   "Invalid kernel stack pointer (probable overflow)."
   
   In at least several of these a deep multi-arena allocation
   was in progress (several vmem_alloc/vmem_xalloc reaching
   all the way down through vmem_bucket_alloc,
   xnu_alloc_throttled, and ultimately to osif_malloc).
   
   The stack frames above the first vmem_alloc were also fairly large.
   
   This commit sets a dynamically sysctl-tunable threshold
   (8k default) for remaining stack size as reported by xnu.
   If we do not have more bytes than that when vmem_alloc()
   is called, then the actual allocation will be done in a
   separate worker thread which will start with a nearly
   empty stack that is much more likely to hold the various
   frames all the way through our code boundary with the
   kernel and beyond.
   
   The xnu / mach thread_call API (osfmk/kern/thread_call.h)
   is used to avoid circular dependencies with taskq, and the
   mechanism is per-arena costing a quick stack-depth check
   per vmem_alloc() but allowing for wildly varying stack
   depths above the first vmem_alloc() call.
   
   Vmem arenas now have two further kstats: the lowest amount
   of available stack space seen at a vmem_alloc() into it,
   and the number of times the allocation work has been done
   in a thread_call worker.
   
   * some spl_vmem.c functions are given inline hints
   
   These are small functions with no or very few automatic
   variables that were good candidates for clang/llvm's
   inlining heuristics before we switched to building
   the kext with -finline-hint-functions.
   
   * remove some (unrelated) unused variables which escaped
   previous commits, eliminating a couple compile-time
   warnings.

   

   rottegift committed Jul 25, 2021
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