linux-kernel - Re: [PATCH] x86, kdump: crashkernel=X try to reserve below 896M first, then try below 4G, then MAXMEM

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Date:	Tue, 15 Oct 2013 11:00:18 +0800
From:	WANG Chao <chaowang@...hat.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Pekka Enberg <penberg@...nel.org>,
	Jacob Shin <jacob.shin@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vivek Goyal <vgoyal@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, kdump: crashkernel=X try to reserve below 896M
 first, then try below 4G, then MAXMEM

On 10/14/13 at 11:54am, Yinghai Lu wrote:
> On Mon, Oct 14, 2013 at 4:46 AM, WANG Chao <chaowang@...hat.com> wrote:
> > Now crashkernel=X will fail out if there's not enough memory at
> > low (below 896M). What makes sense for crashkernel=X would be:
> >
> >  - First try to reserve X below 896M (for being compatible with old
> >    kexec-tools).
> >  - If fails, try to reserve X below 4G (swiotlb need to stay below 4G).
> >  - If fails, try to reserve X from MAXMEM top down.
> >
> > So that user can easily reserve large memory with crashkernel=X instead
> > of crashkernel=X,high. It's more transparent and user-friendly.
> >
> > If crashkernel is large and the reserved is beyond 896M, old kexec-tools
> > won't be compatible with new kernel for most of time.
> >
> > kexec will fail out immediately in this case. But the failure could be
> > expected, because old kexec users should not try to reserve that large
> > amount of memory at the first place.
> >
> > On the other hand, old kexec also will fail on old kernel when there's
> > not enough low memory to reserve a large crash kernel area. So the
> > failure of old kexec is consistent between old kernel and new kernel.
> >
> > Signed-off-by: WANG Chao <chaowang@...hat.com>
> > ---
> >  arch/x86/kernel/setup.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> > index f0de629..38e6c1f 100644
> > --- a/arch/x86/kernel/setup.c
> > +++ b/arch/x86/kernel/setup.c
> > @@ -593,6 +593,20 @@ static void __init reserve_crashkernel(void)
> >                                         high ? CRASH_KERNEL_ADDR_HIGH_MAX :
> >                                                CRASH_KERNEL_ADDR_LOW_MAX,
> >                                         crash_size, alignment);
> > +               /*
> > +                * crashkernel=X reserve below 896M fails? Try below 4G
> > +                */
> > +               if (!high && !crash_base)
> > +                       crash_base = memblock_find_in_range(alignment,
> > +                                               (1ULL << 32),
> > +                                               crash_size, alignment);
> > +               /*
> > +                * crashkernel=X reserve below 4G fails? Try MAXMEM
> > +                */
> > +               if (!high && !crash_base)
> > +                       crash_base = memblock_find_in_range(alignment,
> > +                                               CRASH_KERNEL_ADDR_HIGH_MAX,
> > +                                               crash_size, alignment);
> >
> >                 if (!crash_base) {
> >                         pr_info("crashkernel reservation failed - No suitable area found.\n");
> > --
> 
> User should change crashkernel=X to crashkernel=X,high.

crashkernel=X is more straightforward. IMHO, It should be a more general
crash kernel reservation parameter and it shouldn't reserve at low only.

> 
> As user could forget to update kexec-tools to utilize ",high" feature, and get
> kdump later fail later.

old kexec-tools should fail to load kernel high anyway, right?

Thanks
WANG Chao
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