Re: [RFC Patch 2/2] kexec: allow to shrink reserved memory

[Amerigo Wang]

Eric W. Biederman wrote:
> Amerigo Wang <amwang@xxxxxxxxxx> writes:
>
>   
> > Eric W. Biederman wrote:
> >     
> > > Amerigo Wang <amwang@xxxxxxxxxx> writes:
> > >
> > >   
> > >       
> > > > Eric W. Biederman wrote:
> > > >     
> > > >         
> > > > > Amerigo Wang <amwang@xxxxxxxxxx> writes:
> > > > >
> > > > >         
> > > > >           
> > > > > > This patch implements shrinking the reserved memory for crash kernel,
> > > > > > if it is more than enough.
> > > > > >
> > > > > > For example, if you have already reserved 128M, now you just want 100M,
> > > > > > you can do:
> > > > > >
> > > > > > # echo $((100*1024*1024)) > /sys/kernel/kexec_crash_size
> > > > > >             
> > > > > >             
> > > > > This patch looks like a reasonable start.
> > > > >
> > > > > However once a crash kernel image is loaded we have already told that
> > > > > image about the memory that is available and what you are doing here
> > > > > will go and stop on the memory that is reserved but not yet used,
> > > > > totally breaking the DMA protections.  AKA we know the memory is safe
> > > > > from ongoing DMAs because it has lain fallow since boot up.
> > > > >
> > > > > The only safe thing to do is to reduce the memory size before (possibly
> > > > > just before) we load the crash kernel.  Which means we should only
> > > > > be allowed to shrink the size when nothing is loaded, exactly the
> > > > > opposite of what you have implemented.
> > > > >
> > > > >         
> > > > >           
> > > > Confused, why just loading the crash kernel makes it unsafe?
> > > > DMA should be avoided when reserving that memory during boot, shouldn't it?
> > > >     
> > > >         
> > > Yes.  But you are removing the reservation and starting DMA on memory
> > > we have told the crash kernel it can use.
> > >   
> > >       
> > We can modify the info given to the crash kernel.
> >     
>
> Only by unloading and reloading.
>
>   
> > > > I know I missed the part that freeing memory before loading, but if it is safe
> > > > before loading, how can it be unsafe after that?
> > > >     
> > > >         
> > > We tell the crash kernel when loading it, it can use all of the reserved memory.
> > >   
> > >       
> > Yeah, but we should reload the kernel after shrinking the memory, it is not
> > surprised that doing this is necessary...
> >     
>
> So unload the crash kernel first.  If you don't you open a race where
> many of the guarantees we make for the crash kernel about the state of
> the memory it might be using are not true.
>
> In general I expect we will be able to do this all before we load the
> crash kernel the first time.  But at least we should not need to reboot
> things if there is a problem.
>   

Ok, thanks for your explanation.

I will drop patch 1/2, since we will move it to kexec.
I will put patch 2/2 into my "crashkernel=auto" patch set, and resend 
them all.

Thanks.


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