[PATCH] khugepaged: convert redundant check to WARN_ON

[Dev Jain]

Claim: folio_order(folio) == HPAGE_PMD_ORDER => folio->index == start.

Proof: Both loops in hpage_collapse_scan_file and collapse_file, which
iterate on the xarray, have the invariant that
start <= folio->index < start + HPAGE_PMD_NR ... (i)
A folio is always naturally aligned in the pagecache, therefore
folio_order == HPAGE_PMD_ORDER => IS_ALIGNED(folio->index, HPAGE_PMD_NR) == true ... (ii)
thp_vma_allowable_order -> thp_vma_suitable_order requires that the virtual
offsets in the VMA are aligned to the order,
=> IS_ALIGNED(start, HPAGE_PMD_NR) == true ... (iii)

Combining (i), (ii) and (iii), the claim is proven.

Therefore, convert this to a VM_WARN_ON.

Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
Based on mm-unstable (d9982f38eb6e). mm-selftests pass.

 mm/khugepaged.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index fa1e57fd2c469..f27cbb4d1f62c 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2000,8 +2000,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 		 * we locked the first folio, then a THP might be there already.
 		 * This will be discovered on the first iteration.
 		 */
-		if (folio_order(folio) == HPAGE_PMD_ORDER &&
-		    folio->index == start) {
+		if (folio_order(folio) == HPAGE_PMD_ORDER) {
+			VM_WARN_ON(folio->index != start);
+
 			/* Maybe PMD-mapped */
 			result = SCAN_PTE_MAPPED_HUGEPAGE;
 			goto out_unlock;
@@ -2329,8 +2330,9 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned
 			continue;
 		}
 
-		if (folio_order(folio) == HPAGE_PMD_ORDER &&
-		    folio->index == start) {
+		if (folio_order(folio) == HPAGE_PMD_ORDER) {
+			VM_WARN_ON(folio->index != start);
+
 			/* Maybe PMD-mapped */
 			result = SCAN_PTE_MAPPED_HUGEPAGE;
 			/*
-- 
2.34.1