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

[Pingfan Liu]

On Thu, Nov 30, 2023 at 9:29 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Thu 30-11-23 20:04:59, Baoquan He wrote:
> > On 11/30/23 at 11:16am, Michal Hocko wrote:
> > > On Thu 30-11-23 11:00:48, Baoquan He wrote:
> > > [...]
> > > > Now, we are worried if there's risk if the CMA area is retaken into kdump
> > > > kernel as system RAM. E.g is it possible that 1st kernel's ongoing RDMA
> > > > or DMA will interfere with kdump kernel's normal memory accessing?
> > > > Because kdump kernel usually only reset and initialize the needed
> > > > device, e.g dump target. Those unneeded devices will be unshutdown and
> > > > let go.
> > >
> > > I do not really want to discount your concerns but I am bit confused why
> > > this matters so much. First of all, if there is a buggy RDMA driver
> > > which doesn't use the proper pinning API (which would migrate away from
> > > the CMA) then what is the worst case? We will get crash kernel corrupted
> > > potentially and fail to take a proper kernel crash, right? Is this
> > > worrisome? Yes. Is it a real roadblock? I do not think so. The problem
> > > seems theoretical to me and it is not CMA usage at fault here IMHO. It
> > > is the said theoretical driver that needs fixing anyway.
> > >
> > > Now, it is really fair to mention that CMA backed crash kernel memory
> > > has some limitations
> > >     - CMA reservation can only be used by the userspace in the
> > >       primary kernel. If the size is overshot this might have
> > >       negative impact on kernel allocations
> > >     - userspace memory dumping in the crash kernel is fundamentally
> > >       incomplete.
> >
> > I am not sure if we are talking about the same thing. My concern is:
> > ====================================================================
> > 1) system corrutption happened, crash dumping is prepared, cpu and
> > interrupt controllers are shutdown;
> > 2) all pci devices are kept alive;
> > 3) kdump kernel boot up, initialization is only done on those devices
> > which drivers are added into kdump kernel's initrd;
> > 4) those on-flight DMA engine could be still working if their kernel
> > module is not loaded;
> >
> > In this case, if the DMA's destination is located in crashkernel=,cma
> > region, the DMA writting could continue even when kdump kernel has put
> > important kernel data into the area. Is this possible or absolutely not
> > possible with DMA, RDMA, or any other stuff which could keep accessing
> > that area?
>
> I do nuderstand your concern. But as already stated if anybody uses
> movable memory (CMA including) as a target of {R}DMA then that memory
> should be properly pinned. That would mean that the memory will be
> migrated to somewhere outside of movable (CMA) memory before the
> transfer is configured. So modulo bugs this shouldn't really happen.
> Are there {R}DMA drivers that do not pin memory correctly? Possibly. Is
> that a road bloack to not using CMA to back crash kernel memory, I do
> not think so. Those drivers should be fixed instead.
>
I think that is our concern. Is there any method to guarantee that
will not happen instead of 'should be' ?
Any static analysis during compiling time or dynamic checking method?

If this can be resolved, I think this method is promising.

Thanks,

Pingfan

> > The existing crashkernel= syntax can gurantee the reserved crashkernel
> > area for kdump kernel is safe.
>
> I do not think this is true. If a DMA is misconfigured it can still
> target crash kernel memory even if it is not mapped AFAICS. But those
> are theoreticals. Or am I missing something?
> --
> Michal Hocko
> SUSE Labs
>