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

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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>
This commit is contained in:
Huang Shijie 2016-11-08 13:44:38 +08:00 committed by Catalin Marinas
parent 6ed0038d5e
commit 20156ce236
1 changed files with 0 additions and 12 deletions
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12
arch/arm64/mm/hugetlbpage.c
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@ -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;