linux-kernel - Re: [PATCH] mm/mremap: Fail map duplication attempts for private mappings

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Date:   Wed, 19 Jul 2017 09:39:50 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...e.com>, Aaron Lu <aaron.lu@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm/mremap: Fail map duplication attempts for private
 mappings

On 07/13/2017 12:11 PM, Vlastimil Babka wrote:
> [+CC linux-api]
> 
> On 07/13/2017 05:58 PM, Mike Kravetz wrote:
>> mremap will create a 'duplicate' mapping if old_size == 0 is
>> specified.  Such duplicate mappings make no sense for private
>> mappings.  If duplication is attempted for a private mapping,
>> mremap creates a separate private mapping unrelated to the
>> original mapping and makes no modifications to the original.
>> This is contrary to the purpose of mremap which should return
>> a mapping which is in some way related to the original.
>>
>> Therefore, return EINVAL in the case where if an attempt is
>> made to duplicate a private mapping.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

After considering Michal's concerns with follow on patch, it appears
this patch provides the most desired behavior.  Any other concerns
or issues with this patch?

If this moves forward, I will create man page updates to describe the
mremap(old_size == 0) behavior.

-- 
Mike Kravetz

> 
>> ---
>>  mm/mremap.c | 7 +++++++
>>  1 file changed, 7 insertions(+)
>>
>> diff --git a/mm/mremap.c b/mm/mremap.c
>> index cd8a1b1..076f506 100644
>> --- a/mm/mremap.c
>> +++ b/mm/mremap.c
>> @@ -383,6 +383,13 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
>>  	if (!vma || vma->vm_start > addr)
>>  		return ERR_PTR(-EFAULT);
>>  
>> +	/*
>> +	 * !old_len  is a special case where a mapping is 'duplicated'.
>> +	 * Do not allow this for private mappings.
>> +	 */
>> +	if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
>> +		return ERR_PTR(-EINVAL);
>> +
>>  	if (is_vm_hugetlb_page(vma))
>>  		return ERR_PTR(-EINVAL);
>>  
>>
>