[PATCH] mm/mm_slot: make sure slot is the first element of its wrapper structure

[Wei Yang <richard.weiyang@gmail.com>]

When using mm_slot in ksm/khugepaged, there is code snip like:

	slot = mm_slot_lookup(mm_slots_hash, mm);
	mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot);
	if (mm_slot && ..) {
	}

This is only valid when mm_slot is the first element of its wrapper
structure, otherwise a NULL slot would converted to a mm_slot with
negative value. And current code thinks it is valid and continue.

Current code works since mm_slot is the first element, but make sure it
won't be disturbed.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
---
 mm/khugepaged.c | 5 ++++-
 mm/ksm.c        | 5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index af5f5c80fe4e..668e74ad33b7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -108,7 +108,7 @@ struct collapse_control {
  * @slot: hash lookup from mm to mm_slot
  */
 struct khugepaged_mm_slot {
-	struct mm_slot slot;
+	struct mm_slot slot;	/* keep it the first element */
 };
 
 /**
@@ -2382,6 +2382,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
 	struct vm_area_struct *vma;
 	int progress = 0;
 
+	BUILD_BUG_ON_MSG(mm_slot_entry(NULL, struct khugepaged_mm_slot, slot),
+			"slot should be the first element");
+
 	VM_BUG_ON(!pages);
 	lockdep_assert_held(&khugepaged_mm_lock);
 	*result = SCAN_FAIL;
diff --git a/mm/ksm.c b/mm/ksm.c
index 2ef29802a49b..0d486dbdf7d3 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -124,7 +124,7 @@ typedef u8 rmap_age_t;
  * @rmap_list: head for this mm_slot's singly-linked list of rmap_items
  */
 struct ksm_mm_slot {
-	struct mm_slot slot;
+	struct mm_slot slot;	/* keep it the first element */
 	struct ksm_rmap_item *rmap_list;
 };
 
@@ -3842,6 +3842,9 @@ static int __init ksm_init(void)
 	struct task_struct *ksm_thread;
 	int err;
 
+	BUILD_BUG_ON_MSG(mm_slot_entry(NULL, struct ksm_mm_slot, slot),
+			"slot should be the first element");
+
 	/* The correct value depends on page size and endianness */
 	zero_checksum = calc_checksum(ZERO_PAGE(0));
 	/* Default to false for backwards compatibility */
-- 
2.34.1