mm/gup: fix wrongly calculated returned value in fault_in_safe_writeable()

Not like fault_in_readable() or fault_in_writeable(), in
fault_in_safe_writeable() local variable 'start' is increased page by page
to loop till the whole address range is handled.  However, it mistakenly
calculates the size of the handled range with 'uaddr - start'.

Fix it here.

Andreas said:

: In gfs2, fault_in_iov_iter_writeable() is used in
: gfs2_file_direct_read() and gfs2_file_read_iter(), so this potentially
: affects buffered as well as direct reads.  This bug could cause those
: gfs2 functions to spin in a loop.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Baoquan He <[email protected]>
Fixes: fe673d3 ("mm: gup: make fault_in_safe_writeable() use fixup_user_fault()")
Reviewed-by: Oscar Salvador <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Andreas Gruenbacher <[email protected]>
Cc: Yanjun.Zhu <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Baoquan He

mm/gup.c

@@ -2207,8 +2207,8 @@ size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
 	} while (start != end);
 	mmap_read_unlock(mm);
 
-	if (size > (unsigned long)uaddr - start)
-		return size - ((unsigned long)uaddr - start);
+	if (size > start - (unsigned long)uaddr)
+		return size - (start - (unsigned long)uaddr);
 	return 0;
 }
 EXPORT_SYMBOL(fault_in_safe_writeable);