Re: [PATCH 0/4] kdump: crashkernel reservation from CMA

[Jiri Bohac]

Hi Tao, 

On Sat, Nov 25, 2023 at 09:51:54AM +0800, Tao Liu wrote:
> Thanks for the idea of using CMA as part of memory for the 2nd kernel.
> However I have a question:
> 
> What if there is on-going DMA/RDMA access on the CMA range when 1st
> kernel crash? There might be data corruption when 2nd kernel and
> DMA/RDMA write to the same place, how to address such an issue?

The crash kernel CMA area(s) registered via
cma_declare_contiguous() are distinct from the
dma_contiguous_default_area or device-specific CMA areas that
dma_alloc_contiguous() would use to reserve memory for DMA.

Kernel pages will not be allocated from the crash kernel CMA
area(s), because they are not GFP_MOVABLE. The CMA area will only
be used for user pages. 

User pages for RDMA, should be pinned with FOLL_LONGTERM and that
would migrate them away from the CMA area.

But you're right that DMA to user pages pinned without
FOLL_LONGTERM would still be possible. Would this be a problem in
practice? Do you see any way around it?

Thanks,

-- 
Jiri Bohac <jbohac@xxxxxxx>
SUSE Labs, Prague, Czechia