Re: [PATCH] mm/hugetlb.c: check the failure case for find_vma

[Navid Emamdoost <navid.emamdoost@gmail.com>]

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Hi Mike,

Thanks for your detailed explanation. I see that for huge_pte_alloc the
address is checked to be within vma.

Best regards,

On Thu, Jul 25, 2019 at 12:53 PM Mike Kravetz <mike.kravetz@oracle.com>
wrote:

> On 7/24/19 6:39 PM, Navid Emamdoost wrote:
> > find_vma may fail and return NULL. The null check is added.
> >
> > Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
> > ---
> >  mm/hugetlb.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index ede7e7f5d1ab..9c5e8b7a6476 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -4743,6 +4743,9 @@ void adjust_range_if_pmd_sharing_possible(struct
> vm_area_struct *vma,
> >  pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t
> *pud)
> >  {
> >       struct vm_area_struct *vma = find_vma(mm, addr);
> > +     if (!vma)
> > +             return (pte_t *)pmd_alloc(mm, pud, addr);
> > +
>
> Hello Navid,
>
> You should not mix declarations and code like this.  I am surprised that
> your
> compiler did not issue a warning such as:
>
> mm/hugetlb.c: In function ‘huge_pmd_share’:
> mm/hugetlb.c:4815:2: warning: ISO C90 forbids mixed declarations and code
> [-Wdeclaration-after-statement]
>   struct address_space *mapping = vma->vm_file->f_mapping;
>   ^~~~~~
>
> While it is true that the routine find_vma can return NULL.  I do not
> believe it is possible here within the context of huge_pmd_share.  Why?
>
> huge_pmd_share is called from huge_pte_alloc to allocate a page table
> entry for a huge page.  So, the calling code is attempting to populate
> page tables.  There are three callers of huge_pte_alloc: hugetlb_fault,
> copy_hugetlb_page_range and __mcopy_atomic_hugetlb.  In each of these
> routines (or their callers) it has been verified that address is within
> a vma.  In addition, mmap_sem is held so that vmas can not change.
> Therefore, there should be no way for find_vma to return NULL here.
>
> Please let me know if there is something I have overlooked.  Otherwise,
> there is no need for such a modification.
> --
> Mike Kravetz
>
> >       struct address_space *mapping = vma->vm_file->f_mapping;
> >       pgoff_t idx = ((addr - vma->vm_start) >> PAGE_SHIFT) +
> >                       vma->vm_pgoff;
> >
>


-- 
Navid.

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