Re: [PATCH v2 0/5] kdump: crashkernel reservation from CMA

[Baoquan He <bhe@redhat.com>]

On 02/20/25 at 05:48pm, Jiri Bohac wrote:
......snip... 
> ---
> Changes since v1:
> 
> The key concern raised in the v1 discussion was that pages in the
> CMA region may be pinned and used for a DMA transfer, potentially
> corrupting the new kernel's memory. When the cma suffix is used, kdump
> may be less reliable and the corruption hard to debug
> 
> This v2 series addresses this concern in two ways:
> 
> 1) Clearly stating the potential problem in the updated
> Documentation and setting the expectation (patch 3/5)
> 
> Documentation now explicitly states that:
> - the risk of kdump failure is increased
> - the CMA reservation is intended for users who can not or don't
>   want to sacrifice enough memory for a standard crashkernel reservation
>   and who prefer less reliable kdump to no kdump at all
> 
> This is consistent with the documentation of the
> crash_kexec_post_notifiers option, which can also increase the
> risk of kdump failure, yet may be the only way to use kdump on
> some systems. And just like the crash_kexec_post_notifiers
> option, the cma crashkernel suffix is completely optional:
> the series has zero effect when the suffix is not used.

Thanks for the effort to investigate and add clear note about the
potential risk in document. Except of the 1 second waiting for short
term pined page for DMA, the whole series looks good to me. Hope other
people can also give comment to evaluate the risk of waiting, I will
wait another week to add my personal ACK.

Thanks
Baoquan

> 
> 2) Giving DMA time to finish before booting the kdump kernel
>    (patch 4/5)
> 
> Pages can be pinned for long term use using the FOLL_LONGTERM
> flag. Then they are migrated outside the CMA region. Pinning
> without this flag shows that the intent of their user is to only
> use them for short-lived DMA transfers. 
> 
> Delay the boot of the kdump kernel when the CMA reservation is
> used, giving potential pending DMA transfers time to finish.
> 
> Other minor changes since v1:
> - updated for 6.14-rc2
> - moved #ifdefs and #defines to header files
> - added __always_unused in parse_crashkernel() to silence a false
>   unused variable warning
>  
> 
> -- 
> Jiri Bohac <jbohac@suse.cz>
> SUSE Labs, Prague, Czechia
> 
>