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Date: Sat, 08 Jan 2005 22:25:03 -0700
From: Scott Long <scottl@freebsd.org>
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To: Peter Jeremy <PeterJeremy@optushome.com.au>
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cc: freebsd-current@freebsd.org
Subject: Re: bus_dmamem_alloc() can't handle large NOWAIT requests
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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