linux-kernel - Re: [RFC V1 02/31] mm/mmap: Clarify protection

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Date:   Mon, 31 Jan 2022 08:55:12 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        hch@...radead.org, akpm@...ux-foundation.org
Subject: Re: [RFC V1 02/31] mm/mmap: Clarify protection_map[] indices



On 1/27/22 6:09 PM, Mike Rapoport wrote:
> On Mon, Jan 24, 2022 at 06:26:39PM +0530, Anshuman Khandual wrote:
>> protection_map[] maps vm_flags access combinations into page protection
>> value as defined by the platform via __PXXX and __SXXX macros. The array
>> indices in protection_map[], represents vm_flags access combinations but
>> it's not very intuitive to derive. This makes it clear and explicit.
> 
> The protection_map is going to be removed in one of the next patches, why
> bother with this patch at all?
This makes the transition from protection_map[] into __vm_get_page_prot()
more intuitive, where protection_map[] gets dropped. This helps platforms
(first ones subscribing ARCH_HAS_VM_GET_PAGE_PROT before this drop) create
/formulate the required switch case elements in their vm_get_page_prot().

The existing protection_map[] is not clear in demonstrating how exactly
the vm_flags combination is mapped into page protection map. This helps
clarify the underlying switch before we move on defining it on platforms.

>  
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: linux-mm@...ck.org
>> Cc: linux-kernel@...r.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>>  mm/mmap.c | 18 ++++++++++++++++--
>>  1 file changed, 16 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 1e8fdb0b51ed..254d716220df 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -102,8 +102,22 @@ static void unmap_region(struct mm_struct *mm,
>>   *								x: (yes) yes
>>   */
>>  pgprot_t protection_map[16] __ro_after_init = {
>> -	__P000, __P001, __P010, __P011, __P100, __P101, __P110, __P111,
>> -	__S000, __S001, __S010, __S011, __S100, __S101, __S110, __S111
>> +	[VM_NONE]				= __P000,
>> +	[VM_READ]				= __P001,
>> +	[VM_WRITE]				= __P010,
>> +	[VM_READ|VM_WRITE]			= __P011,
>> +	[VM_EXEC]				= __P100,
>> +	[VM_EXEC|VM_READ]			= __P101,
>> +	[VM_EXEC|VM_WRITE]			= __P110,
>> +	[VM_EXEC|VM_READ|VM_WRITE]		= __P111,
>> +	[VM_SHARED]				= __S000,
>> +	[VM_SHARED|VM_READ]			= __S001,
>> +	[VM_SHARED|VM_WRITE]			= __S010,
>> +	[VM_SHARED|VM_READ|VM_WRITE]		= __S011,
>> +	[VM_SHARED|VM_EXEC]			= __S100,
>> +	[VM_SHARED|VM_READ|VM_EXEC]		= __S101,
>> +	[VM_SHARED|VM_WRITE|VM_EXEC]		= __S110,
>> +	[VM_SHARED|VM_READ|VM_WRITE|VM_EXEC]	= __S111
>>  };
>>  
>>  #ifndef CONFIG_ARCH_HAS_FILTER_PGPROT
>> -- 
>> 2.25.1
>>
>>
>