Re: [PATCH 2/3] X86/kdump: crashkernel=X try to reserve below 896M first then below 4G and MAXMEM

[Dave Young]

On 11/15/17 at 01:47pm, Baoquan He wrote:
> Hi Dave,
> 
> Thanks for your effort to push this into upstream. While I have one
> concern, please see the inline comments.
> 
> On 10/24/17 at 01:31pm, Dave Young wrote:
> > Now crashkernel=X will fail if there's not enough memory at low region
> > (below 896M) when trying to reserve large memory size.  One can use
> > crashkernel=xM,high to reserve it at high region (>4G) but it is more
> > convinient to improve crashkernel=X to: 
> > 
> >  - 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.
> > 
> > It's more transparent and user-friendly.
> > 
> > If crashkernel is large and the reserved is beyond 896M, old kexec-tools
> > is not compatible with new kernel because old kexec-tools can not load
> > kernel at high memory region, there was an old discussion below:
> > https://lkml.org/lkml/2013/10/15/601
> > 
> > But actually the behavior is consistent during my test. Suppose
> > old kernel fail to reserve memory at low areas, kdump does not
> > work because no meory reserved. With this patch, suppose new kernel
> > successfully reserved memory at high areas, old kexec-tools still fail
> > to load kdump kernel (tested 2.0.2), so it is acceptable, no need to
> > worry about the compatibility.
> > 
> > Here is the test result (kexec-tools 2.0.2, no high memory load
> > support):
> > Crashkernel over 4G:
> > # cat /proc/iomem|grep Crash
> >   be000000-cdffffff : Crash kernel
> >   213000000-21effffff : Crash kernel
> > # ./kexec  -p /boot/vmlinuz-`uname -r`
> > Memory for crashkernel is not reserved
> > Please reserve memory by passing "crashkernel=X@Y" parameter to the kernel
> > Then try loading kdump kernel
> > 
> > crashkernel: 896M-4G:
> > # cat /proc/iomem|grep Crash
> >   96000000-cdefffff : Crash kernel
> > # ./kexec -p /boot/vmlinuz-4.14.0-rc4+
> > ELF core (kcore) parse failed
> > Cannot load /boot/vmlinuz-4.14.0-rc4+
> > 
> > Signed-off-by: Dave Young <dyoung@xxxxxxxxxx>
> > ---
> >  arch/x86/kernel/setup.c |   16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> > 
> > --- linux-x86.orig/arch/x86/kernel/setup.c
> > +++ linux-x86/arch/x86/kernel/setup.c
> > @@ -568,6 +568,22 @@ static void __init reserve_crashkernel(v
> >  						    high ? CRASH_ADDR_HIGH_MAX
> >  							 : CRASH_ADDR_LOW_MAX,
> >  						    crash_size, CRASH_ALIGN);
> > +#ifdef CONFIG_X86_64
> > +		/*
> > +		 * crashkernel=X reserve below 896M fails? Try below 4G
> > +		 */
> > +		if (!high && !crash_base)
> > +			crash_base = memblock_find_in_range(CRASH_ALIGN,
> > +						(1ULL << 32),
> > +						crash_size, CRASH_ALIGN);
> > +		/*
> > +		 * crashkernel=X reserve below 4G fails? Try MAXMEM
> > +		 */
> > +		if (!high && !crash_base)
> > +			crash_base = memblock_find_in_range(CRASH_ALIGN,
> > +						CRASH_ADDR_HIGH_MAX,
> > +						crash_size, CRASH_ALIGN);
> 
> For kdump, most of systems are x86 64. If both Yinghai and Vivek have no
> objection to search an available region of crash_size above 896M
> naturely, why don't we search it with function
> __memblock_find_range_bottom_up(). It can search from below 896M to
> above 4G, almost the same as the change you have made currently. Mainly
> the code will be much simpler.
> 
> The several times of searching looks not good and a little confusing.
> 
> What do you think?

Bao, thanks for the comment, it might be a good idea, will explore this
way see if there are risks to go with your suggestion.

> 
> Thanks
> Baoquan
> 
> > +#endif
> >  		if (!crash_base) {
> >  			pr_info("crashkernel reservation failed - No suitable area found.\n");
> >  			return;
> > 
> >