Messages in this thread | | | Date | Tue, 24 Jul 2007 10:22:07 +0200 | From | Jens Axboe <> | Subject | Re: 2.6.23-rc1: BUG_ON in kmap_atomic_prot() |
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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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