On ARM32 with HIGHMEM/HIGHPTE, break_ksm_pmd_entry() triggers a BUG during KSM unmerging because pte_unmap_unlock() is passed a pointer that may be beyond the mapped PTE page. The issue occurs when the PTE iteration loop completes without finding a KSM page. After the loop, 'ptep' has been incremented past the last PTE entry. On ARM32 LPAE with 512 PTEs per page (512 * 8 = 4096 bytes), this means ptep points to the next page, outside the kmap'd region. When pte_unmap_unlock(ptep, ptl) calls kunmap_local(ptep), it unmaps the wrong page address, leaving the original kmap slot still mapped. The next kmap_local then finds this slot unexpectedly occupied: WARNING: mm/highmem.c:622 kunmap_local_indexed (address mismatch) kernel BUG at mm/highmem.c:564 __kmap_local_pfn_prot (slot not empty) Fix this by passing start_ptep to pte_unmap_unlock(), which always points within the originally mapped PTE page. Reproducer: Run LTP ksm03 test on ARM32 with HIGHMEM enabled. The test triggers KSM merging followed by unmerging (writing 0 then 2 to /sys/kernel/mm/ksm/run), which exercises break_ksm_pmd_entry(). Fixes: 5d4939fc2258 ("ksm: perform a range-walk in break_ksm") Assisted-by: claude-opus-4-5-20251101 Signed-off-by: Sasha Levin --- mm/ksm.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/ksm.c b/mm/ksm.c index cfc182255c7b..2d89a7c8b4eb 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -650,7 +650,7 @@ static int break_ksm_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long en } } out_unlock: - pte_unmap_unlock(ptep, ptl); + pte_unmap_unlock(start_ptep, ptl); return found; } -- 2.51.0