khugepaged: drain LRU add pagevec after swapin

collapse_huge_page() tries to swap in pages that are part of the PMD
range.  Just swapped in page goes though LRU add cache.  The cache gets
extra reference on the page.

The extra reference can lead to the collapse fail: the following
__collapse_huge_page_isolate() would check refcount and abort collapse
seeing unexpected refcount.

The fix is to drain local LRU add cache in
__collapse_huge_page_swapin() if we successfully swapped in any pages.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Zi Yan <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Acked-by: Yang Shi <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Ralph Campbell <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/khugepaged.c

@@ -931,6 +931,11 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
 	}
 	vmf.pte--;
 	pte_unmap(vmf.pte);
+
+	/* Drain LRU add pagevec to remove extra pin on the swapped in pages */
+	if (swapped_in)
+		lru_add_drain();
+
 	trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 1);
 	return true;
 }