mm/khugepaged: fix iteration in collapse_file

Remove an unnecessary call to xas_set(index) when iterating over the
target range in collapse_file.  The extra call to xas_set reset the xas
cursor to the top of the tree, causing the xas_next call on the next
iteration to walk the tree to index instead of advancing to index+1.  This
returned the same page again, which would cause collapse_file to fail
because the page is already locked.

This bug was hidden when CONFIG_DEBUG_VM was set.  When that config was
used, the xas_load in a subsequent VM_BUG_ON assert would walk xas from
the top of the tree to index, causing the xas_next call on the next loop
iteration to advance the cursor as expected.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: a2e17cc ("mm/khugepaged: maintain page cache uptodate flag")
Signed-off-by: David Stevens <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jiaqi Yan <[email protected]>
Cc: Kirill A . Shutemov <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Mike Kravetz <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: David Stevens

mm/khugepaged.c

@@ -2070,7 +2070,6 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 					TTU_IGNORE_MLOCK | TTU_BATCH_FLUSH);
 
 		xas_lock_irq(&xas);
-		xas_set(&xas, index);
 
 		VM_BUG_ON_PAGE(page != xas_load(&xas), page);