linux-kernel - Re: [PATCH] mm/mremap: Document MREMAP_FIXED dependency on MREMAP

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Message-ID: <40c61daf-da2c-bab9-99d0-a7d7147f4514@oracle.com>
Date:   Mon, 10 Jul 2017 10:31:29 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/mremap: Document MREMAP_FIXED dependency on
 MREMAP_MAYMOVE

On 07/10/2017 06:41 AM, Michal Hocko wrote:
> On Mon 10-07-17 17:02:11, Anshuman Khandual wrote:
>> In the header file, just specify the dependency of MREMAP_FIXED
>> on MREMAP_MAYMOVE and make it explicit for the user space.
> 
> I really fail to see a point of this patch. The depency belongs to the
> code and it seems that we already enforce it
> 	if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
> 		return ret;
> 
> So what is the point here?

Agree, I am not sure of your reasoning.

If to assist the programmer, there is no need as this is clearly specified
in the man page:

"If  MREMAP_FIXED  is  specified,  then MREMAP_MAYMOVE must also be
 specified."

-- 
Mike Kravetz

> 
>> Signed-off-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
>> ---
>>  include/uapi/linux/mman.h | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/uapi/linux/mman.h b/include/uapi/linux/mman.h
>> index ade4acd..8cae3f6 100644
>> --- a/include/uapi/linux/mman.h
>> +++ b/include/uapi/linux/mman.h
>> @@ -3,8 +3,10 @@
>>  
>>  #include <asm/mman.h>
>>  
>> -#define MREMAP_MAYMOVE	1
>> -#define MREMAP_FIXED	2
>> +#define MREMAP_MAYMOVE	1 /* VMA can move after remap and resize */
>> +#define MREMAP_FIXED	2 /* VMA can remap at particular address */
>> +
>> +/* NOTE: MREMAP_FIXED must be set with MREMAP_MAYMOVE, not alone */
>>  
>>  #define OVERCOMMIT_GUESS		0
>>  #define OVERCOMMIT_ALWAYS		1
>> -- 
>> 1.8.5.2
>>
>> --
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>