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    SubjectRe: mtd: kernel BUG at arch/x86/mm/pat.c:279!
    On Fri, Sep 7, 2012 at 3:42 PM, Suresh Siddha <> wrote:
    > - unsigned long start;
    > - unsigned long off;
    > - u32 len;
    > + resource_size_t start, off;
    > + unsigned long len;

    So since the oops is on x86-64, I don't think it's the "unsigned long"
    -> "resource_size_t" part (which can be an issue on 32-bit
    architectures, though).

    The "u32 len" -> "unsigned long len" thing *might* make a difference, though.

    I also think your patch is incomplete even on 32-bit, because this:

    > if (mtd->type == MTD_RAM || mtd->type == MTD_ROM) {
    > off = vma->vm_pgoff << PAGE_SHIFT;

    is still wrong. It probably should be

    off = vma->vm_pgoff;
    off <<= PAGE_SHIFT;

    because vm_pgoff may be a 32-bit type, while "resource_size_t" may be
    64-bit. Shifting the 32-bit type without a cast (implicit or explicit)
    isn't going to help.

    That said, we have absolutely *tons* of bugs with this particular
    pattern. Just do

    git grep 'vm_pgoff.*<<.*PAGE_SHIFT'

    and there are distressingly few casts in there (there's a few, mainly
    in fs/proc).

    Now, I suspect many of them are fine just because most users probably
    are size-limited anyway, but it's a bit distressing stuff. And I
    suspect it means we might want to introduce a helper function like

    static inline u64 vm_offset(struct vm_area_struct *vma)
    return (u64)vma->vm_pgoff << PAGE_SHIFT;

    or something. Maybe add the "vm_length()" helper while at it too,
    since the whole "vma->vm_end - vma->vm_start" thing is so common.

    Anyway, since Sasha's oops is clearly not 32-bit, the above issues
    don't matter, and it would be interesting to hear if it's the 32-bit
    'len' thing that triggers this problem. Still, I can't see how it
    would - as far as I can tell, a truncated 'len' would at most result
    in spurious early "return -EINVAL", not any real problem.

    What are we missing?

    Sasha, since you can apparently reproduce it, can you replace the
    "BUG_ON()" with just a

    if (start >= end) {
    printf("bogus range %llx - %llx\n", start, end);
    return -EINVAL;

    or something.

    I'm starting to suspect that maybe it's actually that the length is
    *zero*, and start == end, and that we should just return zero for that
    case. But let's see what Sasha finds..


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      Last update: 2012-09-08 02:02    [W:0.024 / U:107.000 seconds]
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