Re: [PATCH 0/4] kdump: crashkernel reservation from CMA

[Philipp Rudo <prudo@redhat.com>]

On Fri, 1 Dec 2023 17:59:02 +0100
Michal Hocko <mhocko@suse.com> wrote:

> On Fri 01-12-23 16:51:13, Philipp Rudo wrote:
> > On Fri, 1 Dec 2023 12:55:52 +0100
> > Michal Hocko <mhocko@suse.com> wrote:
> >   
> > > On Fri 01-12-23 12:33:53, Philipp Rudo wrote:
> > > [...]  
> > > > And yes, those are all what-if concerns but unfortunately that is all
> > > > we have right now.    
> > > 
> > > Should theoretical concerns without an actual evidence (e.g. multiple
> > > drivers known to be broken) become a roadblock for this otherwise useful
> > > feature?   
> > 
> > Those concerns aren't just theoretical. They are experiences we have
> > from a related feature that suffers exactly the same problem regularly
> > which wouldn't exist if everybody would simply work "properly".  
> 
> What is the related feature?

kexec

> > And yes, even purely theoretical concerns can become a roadblock for a
> > feature when the cost of those theoretical concerns exceed the benefit
> > of the feature. The thing is that bugs will be reported against kexec.
> > So _we_ need to figure out which of the shitty drivers caused the
> > problem. That puts additional burden on _us_. What we are trying to
> > evaluate at the moment is if the benefit outweighs the extra burden
> > with the information we have at the moment.  
> 
> I do understand your concerns! But I am pretty sure you do realize that
> it is really hard to argue theoreticals.  Let me restate what I consider
> facts. Hopefully we can agree on these points
> 	- the CMA region can be used by user space memory which is a
> 	  great advantage because the memory is not wasted and our
> 	  experience has shown that users do care about this a lot. We
> 	  _know_ that pressure on making those reservations smaller
> 	  results in a less reliable crashdump and more resources spent
> 	  on tuning and testing (especially after major upgrades).  A
> 	  larger reservation which is not completely wasted for the
> 	  normal runtime is addressing that concern.
> 	- There is no other known mechanism to achieve the reusability
> 	  of the crash kernel memory to stop the wastage without much
> 	  more intrusive code/api impact (e.g. a separate zone or
> 	  dedicated interface to prevent any hazardous usage like RDMA).
> 	- implementation wise the patch has a very small footprint. It
> 	  is using an existing infrastructure (CMA) and it adds a
> 	  minimal hooking into crashkernel configuration.
> 	- The only identified risk so far is RDMA acting on this memory
> 	  without using proper pinning interface. If it helps to have a
> 	  statement from RDMA maintainers/developers then we can pull
> 	  them in for a further discussion of course.
> 	- The feature requires an explicit opt-in so this doesn't bring
> 	  any new risk to existing crash kernel users until they decide
> 	  to use it. AFAIU there is no way to tell that the crash kernel
> 	  memory used to be CMA based in the primary kernel. If you
> 	  believe that having that information available for
> 	  debugability would help then I believe this shouldn't be hard
> 	  to add.  I think it would even make sense to mark this feature
> 	  experimental to make it clear to users that this needs some
> 	  time before it can be marked production ready.
> 
> I hope I haven't really missed anything important. The final

If I understand Documentation/core-api/pin_user_pages.rst correctly you
missed case 1 Direct IO. In that case "short term" DMA is allowed for
pages without FOLL_LONGTERM. Meaning that there is a way you can
corrupt the CMA and with that the crash kernel after the production
kernel has panicked.

With that I don't see a chance this series can be included unless
someone can explain me that that the documentation is wrong or I
understood it wrong.

Having that said
NAcked-by: Philipp Rudo <prudo@redhat.com>

> cost/benefit judgment is up to you, maintainers, of course but I would
> like to remind that we are dealing with a _real_ problem that many
> production systems are struggling with and that we don't really have any
> other solution available.