mm, thp: print useful information when mmap_sem is unlocked in zap_pmd_range

Andrea asked for addr, end, vma->vm_start, and vma->vm_end to be emitted
when !rwsem_is_locked(&tlb->mm->mmap_sem).  Otherwise, debugging the
underlying issue is more difficult.

Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 1 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 1b7dc662bf9..8762c4f915f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1225,7 +1225,15 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
                 next = pmd_addr_end(addr, end);
                 if (pmd_trans_huge(*pmd)) {
                         if (next - addr != HPAGE_PMD_SIZE) {
-                                VM_BUG_ON(!rwsem_is_locked(&tlb->mm->mmap_sem));
+#ifdef CONFIG_DEBUG_VM
+                                if (!rwsem_is_locked(&tlb->mm->mmap_sem)) {
+                                        pr_err("%s: mmap_sem is unlocked! addr=0x%lx end=0x%lx vma->vm_start=0x%lx vma->vm_end=0x%lx\n",
+                                                __func__, addr, end,
+                                                vma->vm_start,
+                                                vma->vm_end);
+                                        BUG();
+                                }
+#endif
                                 split_huge_page_pmd(vma->vm_mm, pmd);
                         } else if (zap_huge_pmd(tlb, vma, pmd, addr))
                                 goto next;