Re: [PATCH v5 4/5] kdump: wait for DMA to finish when using CMA

[David Hildenbrand <david@redhat.com>]

On 13.06.25 01:47, Andrew Morton wrote:
> On Thu, 12 Jun 2025 12:18:40 +0200 Jiri Bohac <jbohac@suse.cz> wrote:
> 
>> 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.
> 
> Oh.  10s sounds a lot.  How long does this process typically take?
> 
> It's sad to add a 10s delay for something which some systems will never
> do.  I wonder if there's some simple hack we can add.  Like having a
> global flag which gets set the first time someone pins a CMA page

We would likely have to do that for any GUP on such a page (FOLL_GET | 
FOLL_PIN), both from gup-fast and gup-slow.

Should work, but IMHO can be optimized later, on top of this series.

-- 
Cheers,

David / dhildenb