powerpc/mm: Handle VM_FAULT_RETRY earlier

In do_page_fault() if handle_mm_fault() returns VM_FAULT_RETRY, retry
the page fault handling before anything else.

This would simplify the handling of the mmap_sem lock in this part of
the code.
Reviewed-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: NLaurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/fault.c

@@ -458,6 +458,26 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * the fault.
 	 */
 	fault = handle_mm_fault(vma, address, flags);
+
+	/*
+	 * Handle the retry right now, the mmap_sem has been released in that
+	 * case.
+	 */
+	if (unlikely(fault & VM_FAULT_RETRY)) {
+		/* We retry only once */
+		if (flags & FAULT_FLAG_ALLOW_RETRY) {
+			/*
+			 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
+			 * of starvation.
+			 */
+			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			flags |= FAULT_FLAG_TRIED;
+			if (!fatal_signal_pending(current))
+				goto retry;
+		}
+		/* We will enter mm_fault_error() below */
+	}
+
 	if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
 		if (fault & VM_FAULT_SIGSEGV)
 			goto bad_area;
@@ -469,11 +489,8 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	}
 
 	/*
-	 * 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.
+	 * Major/minor page fault accounting.
 	 */
-	if (flags & FAULT_FLAG_ALLOW_RETRY) {
 	if (fault & VM_FAULT_MAJOR) {
 		current->maj_flt++;
 		perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
@@ -494,14 +511,6 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 		perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
 			      regs, address);
 	}
-		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
-			flags |= FAULT_FLAG_TRIED;
-			goto retry;
-		}
-	}
 
 	up_read(&mm->mmap_sem);
 	goto bail;