MIPS: Avoid mcheck by flushing page range in huge_ptep_set_access_flags()

Problem:

1) Huge page mapping of anonymous memory is initially invalid.  Will be
   faulted in by copy-on-write mechanism.

2) Userspace attempts store at the end of the huge mapping.

3) TLB Refill exception handler fill TLB with a normal (4K sized)
   invalid page at the end of the huge mapping virtual address range.

4) Userspace restarted, and re-attempts the store at the end of the
   huge mapping.

5) Page from #3 is invalid, we get a fault and go to the hugepage
   fault handler.  This tries to map a huge page and calls
   huge_ptep_set_access_flags() to install the mapping.

6) We just call the generic ptep_set_access_flags() to set up the page
   tables, but the flush there assumes a normal (4K sized) page and
   only tries to flush the first part of the huge page virtual address
   out of the TLB, since the existing entry from step #3 doesn't
   conflict, nothing is flushed.

7) We attempt to load the mapping into the TLB, but because it
   conflicts with the entry from step #3, we get a Machine Check
   exception.

The fix: Flush the entire rage covered by the huge page in
huge_ptep_set_access_flags(), and remove the optimization in
local_flush_tlb_range() so that the flush actually does the correct
thing.
Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: Hillf Danton <dhillf@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/4661/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
(cherry picked from commit dd617f258cc39d36be26afee9912624a2d23112c)

arch/mips/include/asm/hugetlb.h

@@ -95,7 +95,17 @@ static inline int huge_ptep_set_access_flags(struct vm_area_struct *vma,
 					     pte_t *ptep, pte_t pte,
 					     int dirty)
 {
-	return ptep_set_access_flags(vma, addr, ptep, pte, dirty);
+	int changed = !pte_same(*ptep, pte);
+
+	if (changed) {
+		set_pte_at(vma->vm_mm, addr, ptep, pte);
+		/*
+		 * There could be some standard sized pages in there,
+		 * get them all.
+		 */
+		flush_tlb_range(vma, addr, addr + HPAGE_SIZE);
+	}
+	return changed;
 }
 
 static inline pte_t huge_ptep_get(pte_t *ptep)

arch/mips/mm/tlb-r4k.c

@@ -120,18 +120,11 @@ void local_flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 
 	if (cpu_context(cpu, mm) != 0) {
 		unsigned long size, flags;
-		int huge = is_vm_hugetlb_page(vma);
 
 		ENTER_CRITICAL(flags);
-		if (huge) {
-			start = round_down(start, HPAGE_SIZE);
-			end = round_up(end, HPAGE_SIZE);
-			size = (end - start) >> HPAGE_SHIFT;
-		} else {
-			start = round_down(start, PAGE_SIZE << 1);
-			end = round_up(end, PAGE_SIZE << 1);
-			size = (end - start) >> (PAGE_SHIFT + 1);
-		}
+		start = round_down(start, PAGE_SIZE << 1);
+		end = round_up(end, PAGE_SIZE << 1);
+		size = (end - start) >> (PAGE_SHIFT + 1);
 		if (size <= current_cpu_data.tlbsize/2) {
 			int oldpid = read_c0_entryhi();
 			int newpid = cpu_asid(cpu, mm);
@@ -140,10 +133,7 @@ void local_flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
 				int idx;
 
 				write_c0_entryhi(start | newpid);
-				if (huge)
-					start += HPAGE_SIZE;
-				else
-					start += (PAGE_SIZE << 1);
+				start += (PAGE_SIZE << 1);
 				mtc0_tlbw_hazard();
 				tlb_probe();
 				tlb_probe_hazard();