linux-kernel - Re: [PATCH 3/3] kdump: round up the total memory size to 128M for crashkernel reservation

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Message-ID: <20171024060944.GB8585@dhcp-128-65.nay.redhat.com>
Date:   Tue, 24 Oct 2017 14:09:44 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, vgoyal@...hat.com, yinghai@...nel.org,
        corbet@....net
Subject: Re: [PATCH 3/3] kdump: round up the total memory size to 128M for
 crashkernel reservation

Hi Baoquan,

On 10/24/17 at 01:57pm, Baoquan He wrote:
> Hi Dave,
> 
> On 10/24/17 at 01:31pm, Dave Young wrote:
> > The total memory size we get in kernel is usually slightly less than 2G with a
> > 2G memory module machine. The main reason is bios/firmware reserve some area
> > it will not export all memory as usable to Linux.
> > 
> > 2G memory X86 kvm guest test result of the total_mem value:
> > UEFI boot with ovmf: 0x7ef10000
> > Legacy boot kvm guest: 0x7ff7cc00
> > 
> > An option is to use dmi/smbios to get physical memory size, but it's not
> > reliable as well. According to Prarit hardware vendors sometimes screw this up.
> > Thus we choose to round up total size to 128M to workaround this problem.
> > This is a best effort workaround, will improve it when we have better way
> > in the future. 
> 
> Thanks for this posting. While I don't get the point of this patch. So
> firmware take piece of memory, then why we need to count it into the
> total memory which we want to calculate a crashkernel memory based on.
> 
> Not counting that, is there anyting incorrect?

Yes, considering crashkernel=1G-2G:128M,  if we have a 1G memory
machine, we get total size 1023M from firmware then it will not fall
into 1G-2G thus no memory reserved.  User will never know that, it is
hard to let user to know the exact total value we get in kernel..

> 
> Thanks
> Baoquan
> 
> > 
> > Signed-off-by: Dave Young <dyoung@...hat.com>
> > ---
> >  kernel/crash_core.c |   17 +++++++++++++----
> >  1 file changed, 13 insertions(+), 4 deletions(-)
> > 
> > --- linux.orig/kernel/crash_core.c
> > +++ linux/kernel/crash_core.c
> > @@ -42,6 +42,15 @@ static int __init parse_crashkernel_mem(
> >  {
> >  	char *cur = cmdline, *tmp;
> >  	bool infinite_end = false;
> > +	unsigned long long total_mem = system_ram;
> > +
> > +	/*
> > +	 * Firmware usually reserves some memory regions for it's own use.
> > +	 * so we get less than actual system memory size.
> > +	 * We workaround this by round up the total size to 128M which is
> > +	 * enough for most test cases.
> > +	 */
> > +	total_mem = roundup(total_mem, 0x8000000);
> >  
> >  	/* for each entry of the comma-separated list */
> >  	do {
> > @@ -86,13 +95,13 @@ static int __init parse_crashkernel_mem(
> >  			return -EINVAL;
> >  		}
> >  		cur = tmp;
> > -		if (size >= system_ram) {
> > +		if (size >= total_mem) {
> >  			pr_warn("crashkernel: invalid size\n");
> >  			return -EINVAL;
> >  		}
> >  
> >  		/* match ? */
> > -		if (system_ram >= start && system_ram < end) {
> > +		if (total_mem >= start && total_mem < end) {
> >  			*crash_size = size;
> >  			if (end == ULLONG_MAX)
> >  				infinite_end = true;
> > @@ -126,9 +135,9 @@ static int __init parse_crashkernel_mem(
> >  				pr_warn("Memory reservation scale order expected after '^'\n");
> >  				return -EINVAL;
> >  			}
> > -			size = (system_ram - *crash_size) >> shift;
> > +			size = (total_mem - *crash_size) >> shift;
> >  			size = *crash_size + roundup(size, 1ULL << 20);
> > -			if (size < system_ram)
> > +			if (size < total_mem)
> >  				*crash_size = size;
> >  			cur = tmp;
> >  		} else
> > 
> > 

Thanks
Dave