linux-kernel - Re: [PATCH] mm/hugetlbfs: Unmap pages if page fault raced with hole punch

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Message-ID: <568F3D73.60901@oracle.com>
Date:	Thu, 7 Jan 2016 20:39:15 -0800
From:	Mike Kravetz <mike.kravetz@...cle.com>
To:	Hillf Danton <hillf.zj@...baba-inc.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:	"'Hugh Dickins'" <hughd@...gle.com>,
	"'Naoya Horiguchi'" <n-horiguchi@...jp.nec.com>,
	"'Davidlohr Bueso'" <dave@...olabs.net>,
	"'Dave Hansen'" <dave.hansen@...ux.intel.com>,
	"'Andrew Morton'" <akpm@...ux-foundation.org>,
	"'Michel Lespinasse'" <walken@...gle.com>
Subject: Re: [PATCH] mm/hugetlbfs: Unmap pages if page fault raced with hole
 punch

On 01/07/2016 12:06 AM, Hillf Danton wrote:
>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>> index 0444760..0871d70 100644
>> --- a/fs/hugetlbfs/inode.c
>> +++ b/fs/hugetlbfs/inode.c
>> @@ -324,11 +324,46 @@ static void remove_huge_page(struct page *page)
>>  	delete_from_page_cache(page);
>>  }
>>
>> +static inline void
>> +hugetlb_vmdelete_list(struct rb_root *root, pgoff_t start, pgoff_t end)
>> +{
>> +	struct vm_area_struct *vma;
>> +
>> +	/*
>> +	 * end == 0 indicates that the entire range after
>> +	 * start should be unmapped.
>> +	 */
>> +	vma_interval_tree_foreach(vma, root, start, end ? end : ULONG_MAX) {
> 
> [1] perhaps end can be reused.
> 
>> +		unsigned long v_offset;
>> +
>> +		/*
>> +		 * Can the expression below overflow on 32-bit arches?
>> +		 * No, because the interval tree returns us only those vmas
>> +		 * which overlap the truncated area starting at pgoff,
>> +		 * and no vma on a 32-bit arch can span beyond the 4GB.
>> +		 */
>> +		if (vma->vm_pgoff < start)
>> +			v_offset = (start - vma->vm_pgoff) << PAGE_SHIFT;
>> +		else
>> +			v_offset = 0;
>> +
>> +		if (end) {
>> +			end = ((end - start) << PAGE_SHIFT) +
>> +			       vma->vm_start + v_offset;
> 
> [2] end is input to be pgoff_t, but changed to be the type of v_offset.
> Further we cannot handle the case that end is input to be zero.
> See the diff below please.
>
<snip>
> 
> --- a/fs/hugetlbfs/inode.c	Thu Jan  7 15:04:35 2016
> +++ b/fs/hugetlbfs/inode.c	Thu Jan  7 15:31:03 2016
> @@ -461,8 +461,11 @@ hugetlb_vmdelete_list(struct rb_root *ro
>  	 * end == 0 indicates that the entire range after
>  	 * start should be unmapped.
>  	 */
> -	vma_interval_tree_foreach(vma, root, start, end ? end : ULONG_MAX) {
> +	if (!end)
> +		end = ULONG_MAX;
> +	vma_interval_tree_foreach(vma, root, start, end) {
>  		unsigned long v_offset;
> +		unsigned long v_end;
>  
>  		/*
>  		 * Can the expression below overflow on 32-bit arches?
> @@ -475,15 +478,12 @@ hugetlb_vmdelete_list(struct rb_root *ro
>  		else
>  			v_offset = 0;
>  
> -		if (end) {
> -			end = ((end - start) << PAGE_SHIFT) +
> +		v_end = ((end - start) << PAGE_SHIFT) +
>  			       vma->vm_start + v_offset;
> -			if (end > vma->vm_end)
> -				end = vma->vm_end;
> -		} else
> -			end = vma->vm_end;
> +		if (v_end > vma->vm_end)
> +			v_end = vma->vm_end;
>  
> -		unmap_hugepage_range(vma, vma->vm_start + v_offset, end, NULL);
> +		unmap_hugepage_range(vma, vma->vm_start + v_offset, v_end, NULL);
>  	}
>  }
>  
> --

Unfortunately, that calculation of v_end is not correct.  I know it
is based on the existing code, but the existing code it not correct.

I attempted to fix in a patch earlier today, but that was not correct
either.  Below is a proposed new version of hugetlb_vmdelete_list.
Let me know what you think.

static inline void
hugetlb_vmdelete_list(struct rb_root *root, pgoff_t start, pgoff_t end)
{
	struct vm_area_struct *vma;

	/*
	 * end == 0 indicates that the entire range after
	 * start should be unmapped.
	 */
	vma_interval_tree_foreach(vma, root, start, end ? end : ULONG_MAX) {
		unsigned long v_offset;
		unsigned long v_end;

		/*
		 * Can the expression below overflow on 32-bit arches?
		 * No, because the interval tree returns us only those vmas
		 * which overlap the truncated area starting at pgoff,
		 * and no vma on a 32-bit arch can span beyond the 4GB.
		 */
		if (vma->vm_pgoff < start)
			v_offset = (start - vma->vm_pgoff) << PAGE_SHIFT;
		else
			v_offset = 0;

		if (!end)
			v_end = vma->vm_end;
		else {
			v_end = ((end - vma->vm_pgoff) << PAGE_SHIFT)
							+ vma->vm_start;
			if (v_end > vma->vm_end)
				v_end = vma->vm_end;
		}

		unmap_hugepage_range(vma, vma->vm_start + v_offset, v_end,
									NULL);
	}
}

-- 
Mike Kravetz