Re: [PATCH 3/3] x86, crash: Allocate enough low-mem when crashkernel=high

[Baoquan He <bhe@redhat.com>]

On 01/26/15 at 01:07pm, Joerg Roedel wrote:
> Hi Baoquan,
> 
> thanks for your reply.
> 
> On Fri, Jan 23, 2015 at 04:44:53PM +0800, Baoquan He wrote:
> > 2) increase low-mem when crashkernel=high. But we have to be careful to
> > do this. We implement crashkernel=high not only for the unhappiness
> > crashkernel reservation is limited below 4G, but dma/dma32 memory space
> > is precious on some systems. If set crashkernel=high still too much low
> > memory has to be reserved by default, it's important to find the
> > balance. So if we have to increase the default low-mem, how much memory
> > is enough, why 256M?  why not 128M/192M/320M/384M?  And if 256M works
> > on your system, what if another person say it does't work because there
> > are more devices on his system?
> > 
> > Anyway, I understand the requirement, but we need find out how much
> > memory can satisfy most of systems.
> 
> Yes, I totally agree that it is tough to find a good default here. I
> used 256MB because this is what was required on the system the  failed
> kdumps were reported on.
> 
> But probably we can agree that 72MB is not enough (given that 64MB are
> taken away by swiotlb already), and increase it to a value we think by
> now is sufficient for most systems.

Yeah, and I got report from user about this issue too. It should be
fixed. Like I said, the 1st suggestion mainly will goes to the area of
initramfs making tools, currently maybe dracut which is used widely.
This may cause many changes. Hence increasing low mem is a better idea.

Before I said 256M may not be a good value, that's because in your patch
cover you said this number comes from experiments on the affected
systems, and 128M was still not enough, then you set it to 256M. This
may be a little rush. I think the step size to increase should be 32M,
after all previously people only take 64M and 8M, enlarge it on a step
size of 128M only one time, it can't be seen as patient and careful.
If it failed on 224M but succeed on 256M, then 256M may be not enough.
I would like to say 32M is better, then we can make a good evaluate.

I will ask user reported this issue to help test and see what value will
be satisfy their system.

Anyway, I think this patch is helpful and necessary.

> 
> Btw, the issue was also reported on machines with only a few devices,
> the reason there is that device drivers allocate more dma memory by
> default on intilization. Maybe we should handle that as a driver
> regression in the future, forcing them to allocate more dma-memory
> on-demand and not on initialization.

Yeah, agree. In that case it shoube be handled as a regression.


Thanks
Baoquan