svn commit: r289759 - in head/sys/arm: arm include

[Konstantin Belousov]

On Sun, Nov 01, 2015 at 09:11:51AM -0700, Ian Lepore wrote:
> It's almost certainly not related to sysinit ordering.  This exception
> is happening during mmc probing after interrupts are enabled.
> 
> It appears that the problem is the faulting code is running on one of
> the very early pre-allocated kernel stacks (perhaps in an interrupt
> handler on an idle thread stack), and these stacks are not in memory
> represented in the vm page arrays.  The mmc code is using a 64-byte
> buffer on the stack and mapping it for DMA.  Normally that causes a
> bounce for cacheline alignment, but unluckily in this case that buffer
> on the stack just happened to be aligned to a cacheline boundary and a
> multiple of the cacheline size, so no bounce.  That causes the new sync
> logic that is based on keeping vm_page_t pointers and offsets to get a
> NULL pointer back from PHYS_TO_VM_PAGE when mapping, then it dies at
> sync time trying to dereference that.  It used to work because the sync
> logic used to use the vaddr, not a page pointer.
> 
> Michal was working on a patch yesterday.

This actually is the same as the issue I had with the DMAR busdma on
x86.  You could see the trick used in the
sys/x86/iommu/busdma_dmar.c:dmar_bus_dmamap_load_buffer(),
the dumping case.  I just allocated fictitious pages to cope with
kernel core dump needing to DMA from memory which is not described
by vm_page_array.