mm/gup: fix fixup_user_fault() on multiple retries

This part was overlooked when reworking the gup code on multiple
retries.

When we get the 2nd+ retry, we'll be with TRIED flag set.  Current code
will bail out on the 2nd retry because the !TRIED check will fail so the
retry logic will be skipped.  What's worse is that, it will also return
zero which errornously hints the caller that the page is faulted in
while it's not.

The !TRIED flag check seems to not be needed even before the mutliple
retries change because if we get a VM_FAULT_RETRY, it must be the 1st
retry, and we should not have TRIED set for that.

Fix it by removing the !TRIED check, at the meantime check against fatal
signals properly before the page fault so we can still properly respond
to the user killing the process during retries.

Fixes: 4426e94 ("mm/gup: allow VM_FAULT_RETRY for multiple times")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Brian Geffon <bgeffon@google.com>
Link: http://lkml.kernel.org/r/20200502003523.8204-1-peterx@redhat.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup.c

@@ -1218,6 +1218,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 	if (!vma_permits_fault(vma, fault_flags))
 		return -EFAULT;
 
+	if ((fault_flags & FAULT_FLAG_KILLABLE) &&
+	    fatal_signal_pending(current))
+		return -EINTR;
+
 	ret = handle_mm_fault(vma, address, fault_flags);
 	major |= ret & VM_FAULT_MAJOR;
 	if (ret & VM_FAULT_ERROR) {
@@ -1230,11 +1234,9 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 
 	if (ret & VM_FAULT_RETRY) {
 		down_read(&mm->mmap_sem);
-		if (!(fault_flags & FAULT_FLAG_TRIED)) {
-			*unlocked = true;
-			fault_flags |= FAULT_FLAG_TRIED;
-			goto retry;
-		}
+		*unlocked = true;
+		fault_flags |= FAULT_FLAG_TRIED;
+		goto retry;
 	}
 
 	if (tsk) {