xtensa/mm/fault.c: port OOM changes to do_page_fault

Commits d065bd81 ("mm: retry page fault when blocking on disk
transfer") and 37b23e05 ("x86,mm: make pagefault killable")
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 xtensa.
Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Acked-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/xtensa/mm/fault.c

@@ -44,6 +44,7 @@ void do_page_fault(struct pt_regs *regs)
 
 	int is_write, is_exec;
 	int fault;
+	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
 
 	info.si_code = SEGV_MAPERR;
 
@@ -71,6 +72,7 @@ void do_page_fault(struct pt_regs *regs)
 	       address, exccause, regs->pc, is_write? "w":"", is_exec? "x":"");
 #endif
 
+retry:
 	down_read(&mm->mmap_sem);
 	vma = find_vma(mm, address);
 
@@ -93,6 +95,7 @@ void do_page_fault(struct pt_regs *regs)
 	if (is_write) {
 		if (!(vma->vm_flags & VM_WRITE))
 			goto bad_area;
+		flags |= FAULT_FLAG_WRITE;
 	} else if (is_exec) {
 		if (!(vma->vm_flags & VM_EXEC))
 			goto bad_area;
@@ -104,7 +107,11 @@ void do_page_fault(struct pt_regs *regs)
 	 * 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;
@@ -112,10 +119,22 @@ void do_page_fault(struct pt_regs *regs)
 			goto do_sigbus;
 		BUG();
 	}
+	if (flags & FAULT_FLAG_ALLOW_RETRY) {
 		if (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);
 	return;