Re: [PATCH v14 02/11] x86: kdump: make the lower bound of crash kernel reservation consistent
From: Catalin Marinas
Date: Thu Feb 25 2021 - 09:43:36 EST
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