[PATCH 09/10] huge pagecache: mmap_sem is unlocked when truncation splits pmd

Hugh Dickins
2016-04-05T20:55:27Z

zap_pmd_range()'s CONFIG_DEBUG_VM !rwsem_is_locked(&mmap_sem) BUG()
will be invalid with huge pagecache, in whatever way it is implemented:
truncation of a hugely-mapped file to an unhugely-aligned size would
easily hit it.

(Although anon THP could in principle apply khugepaged to private file
mappings, which are not excluded by the MADV_HUGEPAGE restrictions, in
practice there's a vm_ops check which excludes them, so it never hits
this BUG() - there's no interface to "truncate" an anonymous mapping.)

We could complicate the test, to check i_mmap_rwsem also when there's a
vm_file; but my inclination was to make zap_pmd_range() more readable by
simply deleting this check. A search has shown no report of the issue in
the years since commit e0897d75f0b2 ("mm, thp: print useful information
when mmap_sem is unlocked in zap_pmd_range") expanded it from VM_BUG_ON()
- though I cannot point to what commit I would say then fixed the issue.

But there are a couple of other patches now floating around, neither
yet in the tree: let's agree to retain the check as a VM_BUG_ON_VMA(),
as Matthew Wilcox has done; but subject to a vma_is_anonymous() check,
as Kirill Shutemov has done. And let's get this in, without waiting
for any particular huge pagecache implementation to reach the tree.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
mm/memory.c | 11 ++---------
1 file changed, 2 insertions(+), 9 deletions(-)

--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1182,15 +1182,8 @@ static inline unsigned long zap_pmd_rang
next = pmd_addr_end(addr, end);
if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
if (next - addr != HPAGE_PMD_SIZE) {
-#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
+ VM_BUG_ON_VMA(vma_is_anonymous(vma) &&
+ !rwsem_is_locked(&tlb->mm->mmap_sem), vma);
split_huge_pmd(vma, pmd, addr);
} else if (zap_huge_pmd(tlb, vma, pmd, addr))
goto next;