Re: [PATCH 3/4] x86, kdump: Retore crashkernel= to allocate low

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Apr 02, 2013 at 11:42:09AM -0700, Yinghai Lu wrote:
> On Tue, Apr 2, 2013 at 11:06 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
> > On Tue, Apr 02, 2013 at 10:19:42AM -0700, Yinghai Lu wrote:
> >
> > [..]
> >> Index: linux-2.6/Documentation/kernel-parameters.txt
> >> ===================================================================
> >> --- linux-2.6.orig/Documentation/kernel-parameters.txt
> >> +++ linux-2.6/Documentation/kernel-parameters.txt
> >> @@ -603,9 +603,13 @@ bytes respectively. Such letter suffixes
> >>                       a memory unit (amount[KMG]). See also
> >>                       Documentation/kdump/kdump.txt for an example.
> >>
> >> +     crashkernel_high=size[KMG]
> >> +                     [KNL, x86_64] range could be above 4G. Allow kernel
> >> +                     to allocate physical memory region from top, so could
> >> +                     be above 4G if system have more than 4G ram installed.
> >>       crashkernel_low=size[KMG]
> >> -                     [KNL, x86_64] range under 4G. When crashkernel= is
> >> -                     passed, kernel allocate physical memory region
> >> +                     [KNL, x86_64] range under 4G. When crashkernel_high= is
> >> +                     passed, kernel could allocate physical memory region
> >>                       above 4G, that cause second kernel crash on system
> >>                       that need swiotlb later. Kernel would try to allocate
> >>                       some region below 4G automatically. This one let
> >
> > Hi Yinghai,
> >
> > I think there are still some issues with crashkernel= semantics.
> >
> > What if I specify both crashkernel_high= as well as crashkernel_low=.
> > Looks like crashkernel_low will be parsed only if crashkernel_high
> > reserved memory above 4G.
> >
> > So if one gives following command line.
> >
> > crashkernel=256M;high crashkernel=100M;low
> >
> > Final outcome will vary across systems. If system has all RAM below 4G
> > we will see only one 256M chunk reserved otherwise we will see one 256M
> > and one 100M chunk reserved. And a user might think that I asked you to
> > reserve two chunks. One 256M and otherr 100M.
> 
> Yes, that is intentional.

Why it is intentional. It seems be to aberration from user's point of
view.

> 
> If you like, I could remove that checking, just add the low.
> 
> >
> > Also interesting is, what if user specifies both crashkernel=X and
> > crashkernel=Y;high. Looks like we will ignore crashkernel=X and honor
> > only crashkernel=Y;high.
> 
> Yes, that is intentional.

Again, it is not clear that why are we prefering crashkernel=Y;high
over crashkernel=X. There needs to be clearly defined behavior.

> 
> >
> > So the problem here is, do we want to parse multiple crashkernel=
> > command line and support reserving multiple ranges? Till 3.8 kernel
> > we did not do that.  If we want to do that, then parsing crashkernel=
> > logic needs to be more generic.
> >
> > - I would say that to keep things simple, we can stick to semantics
> >   of 3.8 kernel and say only first crashkernel= option is parsed and
> >   acted upon. Rest are ignored. Trying to support multiple ranges will
> >   require much more work.
> 
> we could do that, but that is not necessary.
> 
> >
> > - If we say that we will only parse first crashkernel= option, then
> >   crashkernel=X;high and crashkernel0;low can not co-exist. I would say
> >   use a new option to disable automatically reserved low memory. Say,
> >   crashkernel_no_auto_low; That way it can co-exist with other
> >   crashkernel= options without any conflict.
> 
> I don't see any reason to make them co-exist.

We still need to define a clear behavior. What happens if user specifies
multiple crashkernel= options.

> 
> aka:
> old kexec-tools stay with "crashkernel=X"
> new kexec-tools stay with
> 1. like old kexec tools
> 2. or "crashkernel=X,high" or "crashkernel=X,high crashkernel=Y,low",
> Y could be 100M or 0 etc.

You are thinking that user will specify only the options you are looking
for. But a user is free to specify all the possible inputs and we need
to define very clearly what happens in those cases.

> 
> >
> >   In fact this will also work with crashkernel=X, if we decide to extend
> >   crashkernel=X to look for memory below 4G and look beyond 4G.
> >
> > - Support crashkernel=X;high as a new crashkernel= option.
> 
> Actually we still support only one region that is could be high or low,
> and that extra low is just for workaround
> buggy system that does not support iommu with kdump.

Well, crashkernel=X;high crashkernel=Y;low will reserve two ranges (one
high and one low). So in some cases we are supporting 2 and in some
cases we are supporting 1 range.

So I still think that let us stick to old behavior of supporting one
crashkernel= option. Last crashkernel= option on command line will be
acted upon.

Thanks
Vivek