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

[David Hildenbrand]

On 03.06.25 17:59, Jiri Bohac wrote:
> On Tue, Jun 03, 2025 at 03:15:03PM +0200, David Hildenbrand wrote:
> > On 30.05.25 22:29, Jiri Bohac 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 that are pinned long-term are migrated away from CMA, so these are
> > > not a concern. Pages pinned without FOLL_LONGTERM remain in the CMA and may
> > > possibly be the source or destination of a pending DMA transfer.
> >
> > I'll note that we right now do have an upstream BUG where that is sometimes
> > not the case. I mentioned it previously that such bugs will be a problem :(
> >
> > https://lkml.kernel.org/r/20250523023709epcms1p236d4f55b79adb9366ec1cf6d5792b06b@epcms1p2
>
> I'll just reitarate the whole purpose of this patchset, as
> added to Documentation:

I know, but stating "these are not a concern", when they are currently a 
concern upstream is a bit suboptimal. :)

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."

Or sth like that.

> > > +static void crash_cma_clear_pending_dma(void)
> > > +{
> > > +	unsigned int s = cma_dma_timeout_sec;
> > > +
> > > +	if (!crashk_cma_cnt)
> > > +		return;
> > > +
> > > +	while (s--)
> > > +		mdelay(1000);
> >
> > Any reason we cannot do it in a single mdelay() invocation?
> >
> > mdelay() already is a loop around udelay on larger values IIUC.
>
> No good reasons ;)
> I just wanted to prevent a totally theoretical overflow (if cma_dma_timeout_sec was made configurable;
> I also anticipated someone might want to add some progress printks into the cycle (without verifying if
> that's even possible in this context).
>
> 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. :)

--
Cheers,

David / dhildenb