Re: [Q] mm/memblock.c: cast truncates bits from RED_INACTIVE

On 6/21/11 3:31 AM, H Hartley Sweeten wrote:

On Monday, June 20, 2011 5:03 PM, Andrew Morton wrote:

quoted

On Tue, 14 Jun 2011 19:47:19 -0500 H Hartley Sweeten wrote:

quoted

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@visionengravers.com>

Acked-by: Pekka Enberg <penberg@kernel.org>

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