Re: [PATCH v14 02/11] x86: kdump: make the lower bound of crash kernel reservation consistent

[Baoquan He <bhe@redhat.com>]

On 02/24/21 at 02:35pm, Catalin Marinas wrote:
> On Sat, Jan 30, 2021 at 03:10:16PM +0800, Chen Zhou wrote:
> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > index da769845597d..27470479e4a3 100644
> > --- a/arch/x86/kernel/setup.c
> > +++ b/arch/x86/kernel/setup.c
> > @@ -439,7 +439,8 @@ static int __init reserve_crashkernel_low(void)
> >  			return 0;
> >  	}
> >  
> > -	low_base = memblock_phys_alloc_range(low_size, CRASH_ALIGN, 0, CRASH_ADDR_LOW_MAX);
> > +	low_base = memblock_phys_alloc_range(low_size, CRASH_ALIGN, CRASH_ALIGN,
> > +			CRASH_ADDR_LOW_MAX);
> >  	if (!low_base) {
> >  		pr_err("Cannot reserve %ldMB crashkernel low memory, please try smaller size.\n",
> >  		       (unsigned long)(low_size >> 20));
> 
> Is there any reason why the lower bound can't be 0 in all low cases
> here? (Sorry if it's been already discussed, I lost track)

Seems like a good question.

This reserve_crashkernel_low(), paired with reserve_crashkernel_high(), is
used to reserve memory under 4G so that kdump kernel owns memory for dma
buffer allocation. In that case, kernel usually is loaded in high
memory. In x86_64, kernel loading need be aligned to 16M because of
CONFIG_PHYSICAL_START, please see commit 32105f7fd8faa7b ("x86: find
offset for crashkernel reservation automatically"). But for crashkernel
low memory, there seems to be no reason to ask for 16M alignment, if
it's taken as dma buffer memory.

So we can make a different alignment for low memory only, e.g 2M. But
16M alignment consistent with crashkernel,high is also fine to me. The
only affect is smaller alignment can increase the possibility of
crashkernel low reservation.

Thanks
Baoquan