[PATCH 1/2] arm64: hugetlb: remove the wrong pmd check in find_num_contig()

From: Huang Shijie
Date: Thu Nov 03 2016 - 02:03:14 EST


The find_num_contig() will return 1 when the pmd is not present.
It will cause a kernel dead loop in the following scenaro:

1.) pmd entry is not present.

2.) the page fault occurs:
... hugetlb_fault() --> hugetlb_no_page() --> set_huge_pte_at()

3.) set_huge_pte_at() will only set the first PMD entry, since the
find_num_contig just return 1 in this case. So the PMD entries
are all empty except the first one.

4.) when kernel accesses the address mapped by the second PMD entry,
a new page fault occurs:
... hugetlb_fault() --> huge_ptep_set_access_flags()

The second PMD entry is still empty now.

5.) When the kernel returns, the access will cause a page fault again.
The kernel will run like the "4)" above.
We will see a dead loop since here.

The dead loop is caught in the 32M hugetlb page (2M PMD + Contiguous bit).

This patch removes wrong pmd check, and fixes this dead loop.

Acked-by: Steve Capper <steve.capper@xxxxxxx>
Signed-off-by: Huang Shijie <shijie.huang@xxxxxxx>
---
arch/arm64/mm/hugetlbpage.c | 4 ----
1 file changed, 4 deletions(-)

diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
index 2e49bd2..4811ef1 100644
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -61,10 +61,6 @@ static int find_num_contig(struct mm_struct *mm, unsigned long addr,
return 1;
}
pmd = pmd_offset(pud, addr);
- if (!pmd_present(*pmd)) {
- VM_BUG_ON(!pmd_present(*pmd));
- return 1;
- }
if ((pte_t *)pmd == ptep) {
*pgsize = PMD_SIZE;
return CONT_PMDS;
--
2.5.5