microblaze: Port OOM changes to do_page_fault

Commit d065bd81
(mm: retry page fault when blocking on disk transfer) and
commit 37b23e05
(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 microblaze.
Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>

arch/microblaze/mm/fault.c

@@ -92,6 +92,8 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	int code = SEGV_MAPERR;
 	int is_write = error_code & ESR_S;
 	int fault;
+	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE |
+					 (is_write ? FAULT_FLAG_WRITE : 0);
 
 	regs->ear = address;
 	regs->esr = error_code;
@@ -138,6 +140,7 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 		if (kernel_mode(regs) && !search_exception_tables(regs->pc))
 			goto bad_area_nosemaphore;
 
+retry:
 		down_read(&mm->mmap_sem);
 	}
 
@@ -210,7 +213,11 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * make sure we exit gracefully rather than endlessly redo
 	 * the fault.
 	 */
-	fault = handle_mm_fault(mm, vma, address, is_write ? FAULT_FLAG_WRITE : 0);
+	fault = handle_mm_fault(mm, vma, address, flags);
+
+	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;
@@ -218,11 +225,27 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 			goto do_sigbus;
 		BUG();
 	}
-	if (unlikely(fault & VM_FAULT_MAJOR))
-		current->maj_flt++;
-	else
-		current->min_flt++;
+
+	if (flags & FAULT_FLAG_ALLOW_RETRY) {
+		if (unlikely(fault & VM_FAULT_MAJOR))
+			current->maj_flt++;
+		else
+			current->min_flt++;
+		if (fault & VM_FAULT_RETRY) {
+			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);
+
 	/*
 	 * keep track of tlb+htab misses that are good addrs but
 	 * just need pte's created via handle_mm_fault()