linux-kernel - RE: [Q] mm/memblock.c: cast truncates bits from RED

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Date:	Mon, 20 Jun 2011 19:31:26 -0500
From:	H Hartley Sweeten <hartleys@...ionengravers.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	"hpa@...ux.intel.com" <hpa@...ux.intel.com>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: RE: [Q] mm/memblock.c: cast truncates bits from RED_INACTIVE

On Monday, June 20, 2011 5:03 PM, Andrew Morton wrote:
> On Tue, 14 Jun 2011 19:47:19 -0500 H Hartley Sweeten wrote:
>
>> Hello all,
>> 
>> Sparse is reporting a couple warnings in mm/memblock.c:
>> 
>> 	warning: cast truncates bits from constant value (9f911029d74e35b becomes 9d74e35b)
>> 
>> The warnings are due to the cast of RED_INACTIVE in memblock_analyze():
>> 
>> 	/* Check marker in the unused last array entry */
>> 	WARN_ON(memblock_memory_init_regions[INIT_MEMBLOCK_REGIONS].base
>> 		!= (phys_addr_t)RED_INACTIVE);
>> 	WARN_ON(memblock_reserved_init_regions[INIT_MEMBLOCK_REGIONS].base
>> 		!= (phys_addr_t)RED_INACTIVE);
>> 
>> And in memblock_init():
>> 
>> 	/* Write a marker in the unused last array entry */
>> 	memblock.memory.regions[INIT_MEMBLOCK_REGIONS].base = (phys_addr_t)RED_INACTIVE;
>> 	memblock.reserved.regions[INIT_MEMBLOCK_REGIONS].base = (phys_addr_t)RED_INACTIVE;
>> 
>> Could this cause any problems?  If not, is there anyway to quiet the sparse noise?
>> 
>
> It's all just a debugging check and that check will continue to work OK
> despite this bug.
>
> But yes, it's ugly and should be fixed.
>
> I don't think that mm/memblock.c should have reused RED_INACTIVE. 
> That's a slab thing and wedging it into a phys_addr_t was
> inappropriate.
>
> In fact I don't think RED_INACTIVE should exist.  It's just inviting
> other subsystems to (ab)use it.  It should be replaced by a
> slab-specific SLAB_RED_INACTIVE, as slub did with SLUB_RED_INACTIVE.
>
>
> I'd suggest something like the below, which I didn't test.  Feel free to
> send it back at me, or ignore it ;)
>
>
> diff -puN include/linux/poison.h~a include/linux/poison.h
> --- a/include/linux/poison.h~a
> +++ a/include/linux/poison.h
> @@ -40,6 +40,12 @@
>  #define	RED_INACTIVE	0x09F911029D74E35BULL	/* when obj is inactive */
>  #define	RED_ACTIVE	0xD84156C5635688C0ULL	/* when obj is active */
>  
> +#ifdef CONFIG_PHYS_ADDR_T_64BIT
> +#define MEMBLOCK_INACTIVE	0x3a84fb0144c9e71bULL
> +#else
> +#define MEMBLOCK_INACTIVE	0x44c9e71bUL
> +#endif
> +
>  #define SLUB_RED_INACTIVE	0xbb
>  #define SLUB_RED_ACTIVE		0xcc
>  
> diff -puN mm/memblock.c~a mm/memblock.c
> --- a/mm/memblock.c~a
> +++ a/mm/memblock.c
> @@ -758,9 +758,9 @@ void __init memblock_analyze(void)
>  
>  	/* Check marker in the unused last array entry */
>  	WARN_ON(memblock_memory_init_regions[INIT_MEMBLOCK_REGIONS].base
> -		!= (phys_addr_t)RED_INACTIVE);
> +		!= MEMBLOCK_INACTIVE);
>  	WARN_ON(memblock_reserved_init_regions[INIT_MEMBLOCK_REGIONS].base
> -		!= (phys_addr_t)RED_INACTIVE);
> +		!= MEMBLOCK_INACTIVE);
>  
>  	memblock.memory_size = 0;
>  
> @@ -786,8 +786,8 @@ void __init memblock_init(void)
>  	memblock.reserved.max	= INIT_MEMBLOCK_REGIONS;
>  
>  	/* Write a marker in the unused last array entry */
> -	memblock.memory.regions[INIT_MEMBLOCK_REGIONS].base = (phys_addr_t)RED_INACTIVE;
> -	memblock.reserved.regions[INIT_MEMBLOCK_REGIONS].base = (phys_addr_t)RED_INACTIVE;
> +	memblock.memory.regions[INIT_MEMBLOCK_REGIONS].base = MEMBLOCK_INACTIVE;
> +	memblock.reserved.regions[INIT_MEMBLOCK_REGIONS].base = MEMBLOCK_INACTIVE;
>  
>  	/* Create a dummy zero size MEMBLOCK which will get coalesced away later.
>  	 * This simplifies the memblock_add() code below...

FWIW, your patch above quiet's the sparse warnings on my system (arm ep93xx) and
the system boots and runs fine.

If you want it..

Tested-by: H Hartley Sweeten <hsweeten@...ionengravers.com>
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