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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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