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SubjectRe: [Q] mm/memblock.c: cast truncates bits from RED_INACTIVE
On Tue, 14 Jun 2011 19:47:19 -0500
H Hartley Sweeten <hartleys@visionengravers.com> 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...
_


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