linux-kernel - Re: [PATCH] mm/hugetlb.c: check the failure case for find

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Date:   Thu, 25 Jul 2019 10:53:46 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Navid Emamdoost <navid.emamdoost@...il.com>, emamd001@....edu
Cc:     kjlu@....edu, smccaman@....edu, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb.c: check the failure case for find_vma

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@...il.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;
>