m68k/mm: Port OOM changes to do_page_fault()

Commit d065bd8
(mm: retry page fault when blocking on disk transfer) and
commit 37b23e0
(x86,mm: make pagefault killable)

The above commits introduced changes into the x86 pagefault handler
for making the page fault handler retryable as well as killable.

These changes reduce the mmap_sem hold time, which is crucial
during OOM killer invocation.

Port these changes to m68k.

Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/mm/fault.c

@@ -72,7 +72,8 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct * vma;
-	int write, fault;
+	int fault;
+	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
 
 #ifdef DEBUG
 	printk ("do page fault:\nregs->sr=%#x, regs->pc=%#lx, address=%#lx, %ld, %p\n",
@@ -87,6 +88,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	if (in_atomic() || !mm)
 		goto no_context;
 
+retry:
 	down_read(&mm->mmap_sem);
 
 	vma = find_vma(mm, address);
@@ -117,14 +119,13 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 #ifdef DEBUG
 	printk("do_page_fault: good_area\n");
 #endif
-	write = 0;
 	switch (error_code & 3) {
 		default:	/* 3: write, present */
 			/* fall through */
 		case 2:		/* write, not present */
 			if (!(vma->vm_flags & VM_WRITE))
 				goto acc_err;
-			write++;
+			flags |= FAULT_FLAG_WRITE;
 			break;
 		case 1:		/* read, present */
 			goto acc_err;
@@ -139,21 +140,46 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * the fault.
 	 */
 
-	fault = handle_mm_fault(mm, vma, address, write ? FAULT_FLAG_WRITE : 0);
+	fault = handle_mm_fault(mm, vma, address, flags);
 #ifdef DEBUG
 	printk("handle_mm_fault returns %d\n",fault);
 #endif
+
+	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+		return 0;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;
 		else if (fault & VM_FAULT_SIGBUS)
 			goto bus_err;
 		BUG();
 	}
-	if (fault & VM_FAULT_MAJOR)
-		current->maj_flt++;
-	else
-		current->min_flt++;
+
+	/*
+	 * Major/minor page fault accounting is only done on the
+	 * initial attempt. If we go through a retry, it is extremely
+	 * likely that the page will be found in page cache at that point.
+	 */
+	if (flags & FAULT_FLAG_ALLOW_RETRY) {
+		if (fault & VM_FAULT_MAJOR)
+			current->maj_flt++;
+		else
+			current->min_flt++;
+		if (fault & VM_FAULT_RETRY) {
+			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
+			 * of starvation. */
+			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+
+			/*
+			 * No need to up_read(&mm->mmap_sem) as we would
+			 * have already released it in __lock_page_or_retry
+			 * in mm/filemap.c.
+			 */
+
+			goto retry;
+		}
+	}
 
 	up_read(&mm->mmap_sem);
 	return 0;