[PATCH v4 4/5] kdump: wait for DMA to finish when using CMA

[David Hildenbrand]

On 04.06.25 09:40, Jiri Bohac wrote:
> On Tue, Jun 03, 2025 at 06:25:57PM +0200, David Hildenbrand wrote:
>> On 03.06.25 17:59, Jiri Bohac wrote:
>> I'd phrase it more like "Pages residing in CMA areas can usually not get
>> long-term pinned, so long-term pinning is typically not a concern. BUGs in
>> the kernel might still lead to long-term pinning of such pages if everything
>> goes wrong."
> 
> ...
> 
>>> If you want, I have no problem changing this to:
>>> +	mdelay(cma_dma_timeout_sec * 1000);
>>
>> Probably good enough. Or just hard-code 10s and call it a day. :)
> 
> Thanks for your comments, David. This would be the v5 of this
> patch:
> 
> Subject: [PATCH v5 4/5] kdump: wait for DMA to finish when using CMA
> 
> When re-using the CMA area for kdump there is a risk of pending DMA
> into pinned user pages in the CMA area.
> 
> Pages residing in CMA areas can usually not get long-term pinned and
> are instead migrated away from the CMA area, so long-term pinning is
> typically not a concern. (BUGs in the kernel might still lead to
> long-term pinning of such pages if everything goes wrong.)
> 
> Pages pinned without FOLL_LONGTERM remain in the CMA and may possibly
> be the source or destination of a pending DMA transfer.
> 
> Although there is no clear specification how long a page may be pinned
> without FOLL_LONGTERM, pinning without the flag shows an intent of the
> caller to only use the memory for short-lived DMA transfers, not a transfer
> initiated by a device asynchronously at a random time in the future.
> 
> Add a delay of CMA_DMA_TIMEOUT_SEC seconds before starting the kdump
> kernel, giving such short-lived DMA transfers time to finish before
> the CMA memory is re-used by the kdump kernel.
> 
> Set CMA_DMA_TIMEOUT_SEC to 10 seconds - chosen arbitrarily as both
> a huge margin for a DMA transfer, yet not increasing the kdump time
> too significantly.
> 
> Signed-off-by: Jiri Bohac <jbohac at suse.cz>
> 
> ---
> Changes since v4:
> - reworded the paragraph about long-term pinning
> - simplified crash_cma_clear_pending_dma()
> 
> ---
> Changes since v3:
> - renamed CMA_DMA_TIMEOUT_SEC to CMA_DMA_TIMEOUT_MSEC, change delay to 10 seconds
> - introduce a cma_dma_timeout_sec initialized to CMA_DMA_TIMEOUT_SEC
>    to make the timeout trivially tunable if needed in the future
> 
> ---
>   include/linux/crash_core.h |  3 +++
>   kernel/crash_core.c        | 15 +++++++++++++++
>   2 files changed, 18 insertions(+)
> 
> diff --git a/include/linux/crash_core.h b/include/linux/crash_core.h
> index 44305336314e..805a07042c96 100644
> --- a/include/linux/crash_core.h
> +++ b/include/linux/crash_core.h
> @@ -56,6 +56,9 @@ static inline unsigned int crash_get_elfcorehdr_size(void) { return 0; }
>   /* Alignment required for elf header segment */
>   #define ELF_CORE_HEADER_ALIGN   4096
>   
> +/* Default value for cma_dma_timeout_sec */
> +#define CMA_DMA_TIMEOUT_SEC 10
> +
>   extern int crash_exclude_mem_range(struct crash_mem *mem,
>   				   unsigned long long mstart,
>   				   unsigned long long mend);
> diff --git a/kernel/crash_core.c b/kernel/crash_core.c
> index 335b8425dd4b..540fd75a4a0d 100644
> --- a/kernel/crash_core.c
> +++ b/kernel/crash_core.c
> @@ -21,6 +21,7 @@
>   #include <linux/reboot.h>
>   #include <linux/btf.h>
>   #include <linux/objtool.h>
> +#include <linux/delay.h>
>   
>   #include <asm/page.h>
>   #include <asm/sections.h>
> @@ -33,6 +34,11 @@
>   /* Per cpu memory for storing cpu states in case of system crash. */
>   note_buf_t __percpu *crash_notes;
>   
> +/* time to wait for possible DMA to finish before starting the kdump kernel
> + * when a CMA reservation is used
> + */
> +unsigned int cma_dma_timeout_sec = CMA_DMA_TIMEOUT_SEC;

Likely no need for that variable?

mdelay(CMA_DMA_TIMEOUT_SEC * 1000);

Then, move the doc over to CMA_DMA_TIMEOUT_SEC

... or rather just move the "#define CMA_DMA_TIMEOUT_SEC 10" over here


With that

Acked-by: David Hildenbrand <david at redhat.com>

-- 
Cheers,

David / dhildenb