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    SubjectRe: 2.6.23-rc1: BUG_ON in kmap_atomic_prot()
    On Tue, Jul 24 2007, Jens Axboe wrote:
    > On Mon, Jul 23 2007, Andrew Morton wrote:
    > > I worked out that the crash I saw was in
    > >
    > > BUG_ON(!pte_none(*(kmap_pte-idx)));
    > >
    > > in the read of kmap_pte[idx]. Which would be weird as the caller is using
    > > a literal KM_USER0.
    > >
    > > So maybe I goofed, and that BUG_ON is triggering (it scrolled off, and I am
    > > unable to reproduce it now).
    > >
    > > If that BUG_ON _is_ triggering then it might indicate that someone is doing
    > > a __GFP_HIGHMEM|__GFP_ZERO allocation while holding KM_USER0.
    >
    > Or doing double kunmaps, or doing a kunmap_atomic() on the page, not the
    > address. I've seen both of those end up triggering that BUG_ON() in a
    > later kmap.
    >
    > Looking over the 2.6.22..2.6.23-rc1 diff, I found one such error in
    > ocfs2 at least. But you are probably not using that, so I'll keep
    > looking...

    What about the new async crypto stuff? I've been looking, but is it
    guarenteed that async_memcpy() runs in process context with interrupts
    enabled always? If not, there's a km type bug there.

    In general, I think the highmem stuff could do with more safety checks:

    - People ALWAYS get the atomic unmaps wrong, passing in the page instead
    of the address. I've seen tons of these. And since kunmap_atomic()
    takes a void pointer, nobody notices until it goes boom.
    - People easily get the km type wrong - they use KM_USERx in interrupt
    context, or one of the irq variants without disabling interrupts.

    If we could just catch these two types of bugs, we've got a lot of these
    problems covered.


    --
    Jens Axboe

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