A couple more MIPS fixes for 4.18:

- Use async IPIs for arch_trigger_cpumask_backtrace() in order to
     avoid warnings & deadlocks, fixing a problem introduced in v3.19
     with the fix trivial to backport as far as v4.9.
 
   - Fix ioremap()'s MMU/TLB backed path to avoid spuriously rejecting
     valid requests due to an incorrect belief that the memory region is
     backed by potentially-in-use RAM. This fixes a regression in v4.2.
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Merge tag 'mips_fixes_4.18_3' of git://git.kernel.org/pub/scm/linux/kernel/git/mips/linux

Pull MIPS fixes from Paul Burton:
 "A couple more MIPS fixes for 4.18:

   - Use async IPIs for arch_trigger_cpumask_backtrace() in order to
     avoid warnings & deadlocks, fixing a problem introduced in v3.19
     with the fix trivial to backport as far as v4.9.

   - Fix ioremap()'s MMU/TLB backed path to avoid spuriously rejecting
     valid requests due to an incorrect belief that the memory region is
     backed by potentially-in-use RAM. This fixes a regression in v4.2"

* tag 'mips_fixes_4.18_3' of git://git.kernel.org/pub/scm/linux/kernel/git/mips/linux:
  MIPS: Fix ioremap() RAM check
  MIPS: Use async IPIs for arch_trigger_cpumask_backtrace()
  MIPS: Call dump_stack() from show_regs()

arch/mips/kernel/process.c

@@ -29,6 +29,7 @@
 #include <linux/kallsyms.h>
 #include <linux/random.h>
 #include <linux/prctl.h>
+#include <linux/nmi.h>
 
 #include <asm/asm.h>
 #include <asm/bootinfo.h>
@@ -655,28 +656,42 @@ unsigned long arch_align_stack(unsigned long sp)
 	return sp & ALMASK;
 }
 
-static void arch_dump_stack(void *info)
+static DEFINE_PER_CPU(call_single_data_t, backtrace_csd);
+static struct cpumask backtrace_csd_busy;
+
+static void handle_backtrace(void *info)
 {
-	struct pt_regs *regs;
+	nmi_cpu_backtrace(get_irq_regs());
+	cpumask_clear_cpu(smp_processor_id(), &backtrace_csd_busy);
+}
 
-	regs = get_irq_regs();
+static void raise_backtrace(cpumask_t *mask)
+{
+	call_single_data_t *csd;
+	int cpu;
 
-	if (regs)
-		show_regs(regs);
+	for_each_cpu(cpu, mask) {
+		/*
+		 * If we previously sent an IPI to the target CPU & it hasn't
+		 * cleared its bit in the busy cpumask then it didn't handle
+		 * our previous IPI & it's not safe for us to reuse the
+		 * call_single_data_t.
+		 */
+		if (cpumask_test_and_set_cpu(cpu, &backtrace_csd_busy)) {
+			pr_warn("Unable to send backtrace IPI to CPU%u - perhaps it hung?\n",
+				cpu);
+			continue;
+		}
 
-	dump_stack();
+		csd = &per_cpu(backtrace_csd, cpu);
+		csd->func = handle_backtrace;
+		smp_call_function_single_async(cpu, csd);
+	}
 }
 
 void arch_trigger_cpumask_backtrace(const cpumask_t *mask, bool exclude_self)
 {
-	long this_cpu = get_cpu();
-
-	if (cpumask_test_cpu(this_cpu, mask) && !exclude_self)
-		dump_stack();
-
-	smp_call_function_many(mask, arch_dump_stack, NULL, 1);
-
-	put_cpu();
+	nmi_trigger_cpumask_backtrace(mask, exclude_self, raise_backtrace);
 }
 
 int mips_get_process_fp_mode(struct task_struct *task)

arch/mips/kernel/traps.c

@@ -351,6 +351,7 @@ static void __show_regs(const struct pt_regs *regs)
 void show_regs(struct pt_regs *regs)
 {
 	__show_regs((struct pt_regs *)regs);
+	dump_stack();
 }
 
 void show_registers(struct pt_regs *regs)

arch/mips/mm/ioremap.c

@@ -9,6 +9,7 @@
 #include <linux/export.h>
 #include <asm/addrspace.h>
 #include <asm/byteorder.h>
+#include <linux/ioport.h>
 #include <linux/sched.h>
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
@@ -98,6 +99,20 @@ static int remap_area_pages(unsigned long address, phys_addr_t phys_addr,
 	return error;
 }
 
+static int __ioremap_check_ram(unsigned long start_pfn, unsigned long nr_pages,
+			       void *arg)
+{
+	unsigned long i;
+
+	for (i = 0; i < nr_pages; i++) {
+		if (pfn_valid(start_pfn + i) &&
+		    !PageReserved(pfn_to_page(start_pfn + i)))
+			return 1;
+	}
+
+	return 0;
+}
+
 /*
  * Generic mapping function (not visible outside):
  */
@@ -116,8 +131,8 @@ static int remap_area_pages(unsigned long address, phys_addr_t phys_addr,
 
 void __iomem * __ioremap(phys_addr_t phys_addr, phys_addr_t size, unsigned long flags)
 {
+	unsigned long offset, pfn, last_pfn;
 	struct vm_struct * area;
-	unsigned long offset;
 	phys_addr_t last_addr;
 	void * addr;
 
@@ -137,18 +152,16 @@ void __iomem * __ioremap(phys_addr_t phys_addr, phys_addr_t size, unsigned long
 		return (void __iomem *) CKSEG1ADDR(phys_addr);
 
 	/*
-	 * Don't allow anybody to remap normal RAM that we're using..
+	 * Don't allow anybody to remap RAM that may be allocated by the page
+	 * allocator, since that could lead to races & data clobbering.
 	 */
-	if (phys_addr < virt_to_phys(high_memory)) {
-		char *t_addr, *t_end;
-		struct page *page;
-
-		t_addr = __va(phys_addr);
-		t_end = t_addr + (size - 1);
-
-		for(page = virt_to_page(t_addr); page <= virt_to_page(t_end); page++)
-			if(!PageReserved(page))
-				return NULL;
+	pfn = PFN_DOWN(phys_addr);
+	last_pfn = PFN_DOWN(last_addr);
+	if (walk_system_ram_range(pfn, last_pfn - pfn + 1, NULL,
+				  __ioremap_check_ram) == 1) {
+		WARN_ONCE(1, "ioremap on RAM at %pa - %pa\n",
+			  &phys_addr, &last_addr);
+		return NULL;
 	}
 
 	/*