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

[Andrew Morton]

On Thu, 12 Jun 2025 12:18:40 +0200 Jiri Bohac <jbohac@xxxxxxx> 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 for
DMA and, if that flag is later found to be unset, skip the delay?  Or
something else along these lines?