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

[Tao Liu]

Hi Jiri,

On Sun, Nov 26, 2023 at 5:22 AM Jiri Bohac <jbohac@xxxxxxx> wrote:
>
> 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 for the explanation! Sorry I don't have any ideas so far...

@Pingfan Liu @Baoquan He Hi, do you have any suggestions for it?

Thanks,
Tao Liu

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