arm64: hugetlb: remove the wrong pmd check in find_num_contig()

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.

This patch also removes the redundant checks for PGD/PUD in
the find_num_contig().

Acked-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Huang Shijie <shijie.huang@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
index 0a4c97b618eca5b2c3ca58bcf89750a2ae02c1a0..9a1069dd889f9dc57d2c6510ffb5bdc82b28bf84 100644 (file)
--- a/arch/arm64/mm/hugetlbpage.c
+++ b/arch/arm64/mm/hugetlbpage.c
@@ -51,20 +51,8 @@ static int find_num_contig(struct mm_struct *mm, unsigned long addr,
        *pgsize = PAGE_SIZE;
        if (!pte_cont(pte))
                return 1;
-       if (!pgd_present(*pgd)) {
-               VM_BUG_ON(!pgd_present(*pgd));
-               return 1;
-       }
        pud = pud_offset(pgd, addr);
-       if (!pud_present(*pud)) {
-               VM_BUG_ON(!pud_present(*pud));
-               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;