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

[Catalin Marinas]

On Thu, Feb 25, 2021 at 03:08:46PM +0800, Baoquan He wrote:
> 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.

I don't mind the 16M alignment in both low and high base. But is there
any reason that the lower bound (third argument) cannot be 0 in both
reserve_crashkernel() (the low attempt) and reserve_crashkernel_low()
cases? The comment in reserve_crashkernel() only talks about the 4G
upper bound but not why we need a 16M lower bound.

-- 
Catalin