linux-kernel - Re: [PATCH v2 4/5] kdump: wait for DMA to finish when using CMA

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Message-ID: <Z9Al3DI_PsWPN83k@dwarf.suse.cz>
Date: Tue, 11 Mar 2025 13:00:28 +0100
From: Jiri Bohac <jbohac@...e.cz>
To: Baoquan He <bhe@...hat.com>
Cc: Vivek Goyal <vgoyal@...hat.com>, Dave Young <dyoung@...hat.com>,
	kexec@...ts.infradead.org, Philipp Rudo <prudo@...hat.com>,
	Donald Dutile <ddutile@...hat.com>, Pingfan Liu <piliu@...hat.com>,
	Tao Liu <ltao@...hat.com>, linux-kernel@...r.kernel.org,
	David Hildenbrand <dhildenb@...hat.com>,
	Michal Hocko <mhocko@...e.cz>
Subject: Re: [PATCH v2 4/5] kdump: wait for DMA to finish when using CMA

On Mon, Mar 03, 2025 at 10:02:38AM +0800, Baoquan He wrote:
> On 02/20/25 at 05:55pm, Jiri Bohac wrote:
> > +static void crash_cma_clear_pending_dma(void)
> > +{
> > +	if (!crashk_cma_cnt)
> > +		return;
> > +
> > +	mdelay(CMA_DMA_TIMEOUT_MSEC);
> > +}
> > +
> >  /*
> >   * No panic_cpu check version of crash_kexec().  This function is called
> >   * only when panic_cpu holds the current CPU number; this is the only CPU
> > @@ -116,6 +125,7 @@ void __noclone __crash_kexec(struct pt_regs *regs)
> >  		if (kexec_crash_image) {
> >  			struct pt_regs fixed_regs;
> >  
> > +			crash_cma_clear_pending_dma();
> 
> This could be too ideal, I am not sure if it's a good way. When crash
> triggered, we need do the urgent and necessary thing as soon as
> possible, then shutdown all CPU to avoid further damage. This one second
> of waiting could give the strayed system too much time. My personal
> opinion.

Good point! I think it makes sense to move the call to crash_cma_clear_pending_dma()
past the call of machine_crash_shutdown where all the shutdown
happens, like this:

> >  			crash_setup_regs(&fixed_regs, regs);
> >  			crash_save_vmcoreinfo();
> >  			machine_crash_shutdown(&fixed_regs);

+			crash_cma_clear_pending_dma();

I'll post a v3 with this change included.

-- 
Jiri Bohac <jbohac@...e.cz>
SUSE Labs, Prague, Czechia