[PATCH 18/24] huge tmpfs: mmap_sem is unlocked when truncation splits huge pmd

[Hugh Dickins]

zap_pmd_range()'s CONFIG_DEBUG_VM !rwsem_is_locked(&mmap_sem) BUG()
is invalid with huge tmpfs, where truncation of a hugely-mapped file
to an unhugely-aligned size easily hits 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 I'm inclined to make zap_pmd_range() more readable by
simply deleting this check.  A search has shown no report of the issue
in the 2.5 years since 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.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
 mm/memory.c |   13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

--- thpfs.orig/mm/memory.c	2015-02-20 19:34:48.083909034 -0800
+++ thpfs/mm/memory.c	2015-02-20 19:34:53.467896724 -0800
@@ -1219,18 +1219,9 @@ static inline unsigned long zap_pmd_rang
 	do {
 		next = pmd_addr_end(addr, end);
 		if (pmd_trans_huge(*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
+			if (next - addr != HPAGE_PMD_SIZE)
 				split_huge_page_pmd(vma, addr, pmd);
-			} else if (zap_huge_pmd(tlb, vma, pmd, addr))
+			else if (zap_huge_pmd(tlb, vma, pmd, addr))
 				goto next;
 			/* fall through */
 		}
--
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