bus_dmamem_alloc() can't handle large NOWAIT requests

[Scott Long]

Peter Jeremy wrote:

> I've just been tracking down a panic in 4.x and found that the problem
> is still present in 6.x.
> 
> According to bus_dma(9), bus_dmamem_alloc() can be invoked with a
> flag BUS_DMA_NOWAIT to indicate that sleep()ing is not allowed.
> 
> At least on the i386, if the requested size exceeds 1 page (or some
> other cases), the requested memory will be allocated via contigmalloc().
> 
> bus_dmamem_alloc() maps BUS_DMA_NOWAIT to M_NOWAIT but contigmalloc()
> does not support M_NOWAIT and will tsleep() under some conditions.
> 
> Unfortunately, I don't know enough about the VM code to be able to
> suggest a fix.
> 

Will contigmalloc() actually sleep?  If so, then this is something that
needs to be addressed in contigmalloc.

Scott