[PATCH v2] khugepaged: remove redundant index check for pmd-folios

[Dev Jain <dev.jain@arm.com>]

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, remove this check.
While at it, simplify the comments.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
v1->v2:
 - Remove the check instead of converting to VM_WARN_ON
 - While at it, simplify the comments

Based on mm-new (8982358e1c87).

 mm/khugepaged.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5f668c1dd0fe4..b7b4680d27ab1 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2015,9 +2015,7 @@ 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) {
-			/* Maybe PMD-mapped */
+		if (folio_order(folio) == HPAGE_PMD_ORDER) {
 			result = SCAN_PTE_MAPPED_HUGEPAGE;
 			goto out_unlock;
 		}
@@ -2345,15 +2343,11 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm,
 			continue;
 		}
 
-		if (folio_order(folio) == HPAGE_PMD_ORDER &&
-		    folio->index == start) {
-			/* Maybe PMD-mapped */
+		if (folio_order(folio) == HPAGE_PMD_ORDER) {
 			result = SCAN_PTE_MAPPED_HUGEPAGE;
 			/*
-			 * For SCAN_PTE_MAPPED_HUGEPAGE, further processing
-			 * by the caller won't touch the page cache, and so
-			 * it's safe to skip LRU and refcount checks before
-			 * returning.
+			 * PMD-sized THP implies that we can only try
+			 * retracting the PTE table.
 			 */
 			folio_put(folio);
 			break;
-- 
2.34.1