On 11/26/2015 12:02 AM, Naoya Horiguchi wrote:
> Currently at the beginning of hugetlb_fault(), we call huge_pte_offset()
> and check whether the obtained *ptep is a migration/hwpoison entry or not.
> And if not, then we get to call huge_pte_alloc(). This is racy because the
> *ptep could turn into migration/hwpoison entry after the huge_pte_offset()
> check. This race results in BUG_ON in huge_pte_alloc().
I assume the BUG_ON you hit in huge_pte_alloc is:
BUG_ON(pte && !pte_none(*pte) && !pte_huge(*pte));
Correct?
This means either:
1) The pte was present when entering hugetlb_fault() and not marked
for migration or hwpoisoned.
2) The pte was added to the page table after the call to huge_pte_offset()
and before the call to huge_pte_alloc().
Your patch will take care of case # 1. I am not sure case # 2 is possible,
but your patch would not address this situation.
--
Mike Kravetz
>
> We don't have to call huge_pte_alloc() when the huge_pte_offset() returns
> non-NULL, so let's fix this bug with moving the code into else block.
>
> Note that the *ptep could turn into a migration/hwpoison entry after
> this block, but that's not a problem because we have another !pte_present
> check later (we never go into hugetlb_no_page() in that case.)
>
> Fixes: 290408d4a250 ("hugetlb: hugepage migration core")
> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> [2.6.36+]
> ---
> mm/hugetlb.c | 8 ++++----
> 1 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git next-20151123/mm/hugetlb.c next-20151123_patched/mm/hugetlb.c
> index 1101ccd..6ad5e91 100644
> --- next-20151123/mm/hugetlb.c
> +++ next-20151123_patched/mm/hugetlb.c
> @@ -3696,12 +3696,12 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> } else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
> return VM_FAULT_HWPOISON_LARGE |
> VM_FAULT_SET_HINDEX(hstate_index(h));
> + } else {
> + ptep = huge_pte_alloc(mm, address, huge_page_size(h));
> + if (!ptep)
> + return VM_FAULT_OOM;
> }
>
> - ptep = huge_pte_alloc(mm, address, huge_page_size(h));
> - if (!ptep)
> - return VM_FAULT_OOM;
> -
> mapping = vma->vm_file->f_mapping;
> idx = vma_hugecache_offset(h, vma, address);
>
>
--
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