linux-kernel - Re: [PATCH] Revert "mm: remove __GFP_NO

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Date:	Wed, 21 Nov 2012 12:30:18 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Mel Gorman <mgorman@...e.de>, Zdenek Kabelac <zkabelac@...hat.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Jiri Slaby <jslaby@...e.cz>, <Valdis.Kletnieks@...edu>,
	Jiri Slaby <jirislaby@...il.com>, <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>
Subject: Re: [PATCH] Revert "mm: remove __GFP_NO_KSWAPD"

On 11/21/2012 12:18 AM, Andrew Morton wrote:
> On Tue, 20 Nov 2012 13:18:19 +0400
> Glauber Costa <glommer@...allels.com> wrote:
> 
>> On 11/12/2012 03:37 PM, Mel Gorman wrote:
>>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
>>> index 02c1c971..d0a7967 100644
>>> --- a/include/linux/gfp.h
>>> +++ b/include/linux/gfp.h
>>> @@ -31,6 +31,7 @@ struct vm_area_struct;
>>>  #define ___GFP_THISNODE		0x40000u
>>>  #define ___GFP_RECLAIMABLE	0x80000u
>>>  #define ___GFP_NOTRACK		0x200000u
>>> +#define ___GFP_NO_KSWAPD	0x400000u
>>>  #define ___GFP_OTHER_NODE	0x800000u
>>>  #define ___GFP_WRITE		0x1000000u
>>
>> Keep in mind that this bit has been reused in -mm.
>> If this patch needs to be reverted, we'll need to first change
>> the definition of __GFP_KMEMCG (and __GFP_BITS_SHIFT as a result), or it
>> would break things.
> 
> I presently have
> 
> /* Plain integer GFP bitmasks. Do not use this directly. */
> #define ___GFP_DMA		0x01u
> #define ___GFP_HIGHMEM		0x02u
> #define ___GFP_DMA32		0x04u
> #define ___GFP_MOVABLE		0x08u
> #define ___GFP_WAIT		0x10u
> #define ___GFP_HIGH		0x20u
> #define ___GFP_IO		0x40u
> #define ___GFP_FS		0x80u
> #define ___GFP_COLD		0x100u
> #define ___GFP_NOWARN		0x200u
> #define ___GFP_REPEAT		0x400u
> #define ___GFP_NOFAIL		0x800u
> #define ___GFP_NORETRY		0x1000u
> #define ___GFP_MEMALLOC		0x2000u
> #define ___GFP_COMP		0x4000u
> #define ___GFP_ZERO		0x8000u
> #define ___GFP_NOMEMALLOC	0x10000u
> #define ___GFP_HARDWALL		0x20000u
> #define ___GFP_THISNODE		0x40000u
> #define ___GFP_RECLAIMABLE	0x80000u
> #define ___GFP_KMEMCG		0x100000u
> #define ___GFP_NOTRACK		0x200000u
> #define ___GFP_NO_KSWAPD	0x400000u
> #define ___GFP_OTHER_NODE	0x800000u
> #define ___GFP_WRITE		0x1000000u
> 
> and
> 

Humm, I didn't realize there were also another free space at 0x100000u.
This seems fine.

> #define __GFP_BITS_SHIFT 25	/* Room for N __GFP_FOO bits */
> #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
> 
> Which I think is OK?
Yes, if we haven't increased the size of the flag-space, no need to
change it.

> 
> I'd forgotten about __GFP_BITS_SHIFT.  Should we do this?
> 
> --- a/include/linux/gfp.h~a
> +++ a/include/linux/gfp.h
> @@ -35,6 +35,7 @@ struct vm_area_struct;
>  #define ___GFP_NO_KSWAPD	0x400000u
>  #define ___GFP_OTHER_NODE	0x800000u
>  #define ___GFP_WRITE		0x1000000u
> +/* If the above are modified, __GFP_BITS_SHIFT may need updating */
>  
This is a very helpful comment.
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